The Universe

Three great ages of scientific thinking about the universe can be distinguished. The first began in Greece in the 6th century bc when the Pythagoreans introduced the concept of a spherical Earth and postulated a universe in which the motions of heavenly bodies were governed by natural laws. The infinite atomist universe of Leucippus and Democritus followed, wherein countless worlds, teeming with life, were the result of chance aggregations of atoms. The geocentric Aristotelian universe arose in the 4th century bc. It consisted of a central Earth surrounded by revolving, translucent spheres to which were attached the Sun and the planets; the outermost sphere supported the fixed stars.

The Copernican revolution ushered in the second great age. In the 16th century, Nicolaus Copernicus revived ancient ideas and proposed a heliocentric universe, which during the following century was transformed into the mechanistic, infinite Newtonian universe that flourished until the early 1900s. In the mid-18th century, Thomas Wright proposed the influential notion of a universe composed of numerous galaxies, and William Herschel, followed by many other astronomers, made rapid strides in the study of stars and of the Milky Way Galaxy, of which the Earth is a component.

The third great age began in the early years of the 20th century, with the discovery of special relativity and its development into general relativity by Albert Einstein. These years also saw momentous developments in astronomy: extragalactic redshifts were detected by Vesto Slipher; extragalactic nebulae were shown to be galaxies comparable with the Milky Way; and Edwin Hubble began to estimate the distances of these galactic systems. Such discoveries and the application of general relativity to cosmology eventually gave rise to the view that the universe is expanding. The basic premise of modern thinking on the universe is the principle that asserts that the universe is homogeneous in space (on the average all places are alike at any time) and that the laws of physics are everywhere the same.

Two theories of the origin of the universe have been the most influential during the last century—the steady state theory and the big bang theory. The steady state theory posits that the universe is always expanding but maintains a constant average density, matter being continuously created to form new stars and galaxies at the same rate that old ones become unobservable as a consequence of their increasing distance and velocity of recession. A steady-state universe has no beginning or end in time; and from any point within it the view on the grand scale—i.e., the average density and arrangement of galaxies—is the same. Galaxies of all possible ages are intermingled. Observations since the 1950s have produced much evidence contradictory to the steady-state picture and supportive of the big-bang model.

The essential feature of the widely-held big bang theory is the emergence of the universe from a state of extremely high temperature and density—the so-called big bang that occurred at least 10,000,000,000 years ago. Although this type of universe was proposed by Alexander Friedmann and Abbe Georges LemaTtre in the 1920s, the modern version was developed by George Gamowand colleagues in the 1940s.

One current problem that scientists are studying is the amount of matter in the universe. Based upon such things as the rate of the motion of galaxies, scientists realized that there is some 90% more matter in the universe than can be seen. Scientists refer to the matter that can be observed as “bright matter” and this other 90% is called “dark matter.” Whether dark matter is of a different and exotic nature from the matter with which we are familiar, or whether dark matter isjust like luminous matter (and forsome reason we cannot detect it), is something a large number of scientists are studying.

Astronomical Constants

QUANTITY                         SYMBOL                                                               VALUE astronomical unit   AU  length of the semimajor axis of the Earth’s orbit around the Sun—149,597,870 km (92,955,808 mi)

measures large distances in space; equals the average distance from the Earth to the Sun

parsec pc one parsec equals 3.26 light-years measures the distance at which the radius of the Earth’s orbit subtends an angle of one second of arc

light-year ly 9.46089 x 1012 km (5.8787 x 1012 mi) measures the distance traveled by light moving in a vacuum in the course of one year solar parallax 8.79414 seconds of arc quantifies the angular difference in direction of the Sun as seen from the Earth’s center and a point one Earth radius away

lunar parallax 57 minutes 02.608 seconds of arc quantifies the angular difference in direction of the Moon as seen from the Earth’s center and a point one Earth radius away

general precession 50.29 seconds of arc per year measures the cyclic wobbling in the orientation of the Earth’s axis of rotation with a period of almost 26,000 years

constant of aberration about 20.49 seconds of arc the maximum amount of the apparent yearly aberrational displacement of a star or other celestial body, resulting from the Earth’s orbital motion around the Sun

constant of nutation 9.202 seconds of arc a small irregularity in the Earth’s axial precession of that occurs over a period of 18.6 years

speed of light (in a vacuum) c 2.99792458 x 1010 cm per sec (186,282 mi per sec)

radius of the Sun Sun RQ 6.96 x 108 m (109 times the radius of Earth)

mass of the Sun Sun Ms 1.989 x 1030 kg (330,000 times the mass of the Earth)

Earth’s mean radius 6,378 km (3,963 mi)

sidereal day (on Earth) 23 h 56 min 4.10 sec of mean solar time defined by the period between two successive passages of a star across the same meridian; it is the time required for the Earth to rotate once relative to the distant stars

mean solar day (on Earth) 24 h 3 min 56.55 sec of mean sideral time the interval between two successive passages of the Sun across the same meridian is a solar day; in practice, since the rate of the Sun’s motion varies with the seasons, use is made of a fictitious Sun that always moves across the sky at an even rate

tropical (or solar) year (on Earth) 365.242 days the time required for the Earth’s orbital motion to return the Sun’s position to the spring equinoctial point sidereal year (on Earth) 365.256 days the time required for the Earth in its orbit to return to the longitude of a distant star

synodic month (on Earth) 29.53 days the time required for the Moon to pass through one complete cycle of phases

sidereal month (on Earth) 27.32 days the time required for the Moon to return to the same place in relation to distant stars

The speed of steamboats increased dramatically over the years; the run from New Orleans to Louisville KY, which took 25 days in 1816, required only 4 days by 1853. The average life span of a steamboat was only four to five years because of poor construction and maintenance, exploding boilers, and sinkings due to river construction. Spontaneous races were common and contributed greatly to the approximately 4,000 deaths in steamboat disasters between 1810 and 1850.

Did  you knows

The speed of steamboats increased dramatically over the years; the run from New Orleans to Louisville KY, which took 25 days in 1816, required only 4 days by 1853. The average life span of a steamboat was only four to five years because of poor construction and maintenance, exploding boilers, and sinkings due to river construction. Spontaneous races were common and contributed greatly to the approximately 4,000 deaths in steamboat disasters between 1810 and 1850.

A conjunction is an apparent meeting or passing of two or more celestial bodies. For example, the Moon is in conjunction with the Sun at the phase of new Moon, when it moves between the Earth and Sun and the side turned toward the Earth is dark. Inferior planets—those with orbits smaller than the Earth’s (namely, Venus and Mercury)—have two kinds of conjunctions with the Sun. An inferior conjunction occurs when the planet passes approximately between Earth and Sun; if it passes exactly between them, moving across the Sun’s face as seen from Earth, it is said to be in transit (see below). A superior conjunction occurs when Earth and the other planet are on opposite sides of the Sun, but all three bodies are again nearly in a straight line. Superior planets, those having orbits larger than the Earth’s can have only superior conjunctions with the Sun.

When celestial bodies appear in opposite directions in the sky they are said to be in opposition. The Moon, when full, is said to be in opposition to the Sun (the Earth is then approximately between them). A superior planet (one with an orbit farther from the Sun than Earth’s) is in opposition when Earth passes between it and the Sun. The opposition of a planet is a good time to observe it, because the planet is then at its nearest point to the Earth and in its full phase. The inferior planets, Venus and Mercury, can never be in opposition to the Sun.

When a celestial body as seen from the Earth makes a right angle with the direction of the Sun it is said to be in quadrature. The Moon at first or last quarter is said to be at east or west quadrature, respectively. A superior planet is at west quadrature when its position is 90° west of the Sun.

The east-west coordinate by which the position of a celestial body is ordinarily measured is known as the right ascension. Right ascension in combination with declination defines the position of a celestial object. Declination is the angular distance of a body north or south of the celestial equator. North declination is considered positive and south, negative. Thus, +90° declination marks the north celestial pole, 0° the celestial equator, and -90° the south celestial pole. The symbol for right ascension is the Greek letter a (alpha) and for declination the lowercase Greek letter A (delta).

The angular distance in celestial longitude separating the Moon or a planet from the Sun is known as elongation. The greatest elongation possible for the two inferior planets is about 48° in the case of Venus and about 28° in that of Mercury. Elongation may also refer to the angular distance of any celestial body from another around which it revolves or from a particular point in the sky; e.g., the extreme east or west position of a star with reference to the north celestial pole.

The point at which a planet is closest to the Sun is called the perihelion, and the most distant point in that planet’s orbit is the aphelion. The term helion refers specifically to the Sun as the primary body about which the planet is orbiting.

Occultation refers to the obscuring of the light of an astronomical body, most commonly a star, by another astronomical body, such as a planet or a satellite. Hence, a solar eclipse is the occultation of the Sun by the Moon. From occultations of stars by planets, asteroids, and satellites, astronomers are able to determine the precise sizes and shapes of the latter bodies in addition to the temperatures of planetary atmospheres. For example, astronomers unexpectedly discovered the rings of Uranus during a stellar occultation on 10 Mar 1977.

A complete or partial obscuring of a celestial body by another is an eclipse; these occur when three celestial objects become aligned. The Sun is eclipsed when the Moon comes between it and the Earth; the Moon is eclipsed when it moves into the shadow of the Earth cast by the Sun. Eclipses of natural or artificial satellites of a planet occur as the satellites move into the planet’s shadow. When the apparent size of the eclipsed body is much smaller than that of the eclipsing body, the phenomenon is known as an occultation (see above). Examples are the disappearance of a star, nebula, or planet behind the Moon, or the vanishing of a natural satellite or space probe behind some body of the solar system. A transit (see above) occurs when, as viewed from the Earth, a relatively small body passes across the disk of a larger body, usually the Sun or a planet, eclipsing only a very small area: Mercury and Venus periodically transit the Sun, and a satellite may transit its planet.

When an object orbiting the Earth is at the point in its orbit that is the greatest distance from the center of the Earth, this point is known as apogee; the term is also used to describe the point farthest from a planet or a satellite (as the Moon) reached by an object orbiting it. Perigee is the opposite of apogee.

The difference in direction of a celestial object as seen by an observer from two widely separated points is termed parallax. The measurement of parallax is used directly to find the distance of the body from the Earth (geocentric parallax) and from the Sun (heliocentric parallax). The two positions of the observer and the position of the object form a triangle; if the base line between the two observing points is known and the direction of the object as seen from each has been measured, the apex angle (the parallax) and the distance of the object from the observer can be determined.

An hour angle is the angle between an observer’s meridian (a great circle passing over his head and through the celestial poles) and the hour circle (any other great circle passing through the poles) on which some celestial body lies. This angle, when expressed in hours and minutes, is the time elapsed since the celestial body’s last transit of the observer’s meridian. The hourangle can also be expressed in degrees, 15° of arc being equal to one hour.

Constellations

Constellations are certain groupings of stars that were imagined—at least by those who named them—to form conspicuous configurations of objects or creatures in the sky. Constellations are useful in tracking artificial satellites and in assisting astronomers and navigators to locate certain stars.

From the earliest times the star groups known as constellations, the smaller groups (parts of constellations) known as asterisms, and, also, individual stars have received names connoting some meteorological phenomena or symbolizing religious or mythological beliefs. At one time it was held that the constellation names and myths were of Greek origin; this view has now been disproved. It is now thought that the Greek constellation system and the cognate legends are primarily of Semitic or even pre-Semitic origin and that they came to the Greeks through the Phoenicians.

The Alexandrian astronomer Ptolemy lists the names and orientation of the 48 constellations in his Almagest, and, with but few exceptions, they are identical with those used at the present time. The majority of the remaining 40 constellations that are now accepted were added by European astronomers in the 17th and 18th centuries. In the 20th century the delineation of precise boundaries for all the 88 constellations was undertaken by a committee of the International Astronomical Union. By 1930 it was possible to assign any star to a constellation.

NAME GENITIVE MEANING Constellations described by Ptolemy: the zodiac			NOTES (First-magnitude stars are given in italics in this column)
Aries	Arietis	Ram
Taurus	Tauri	Bull	Aldebaran is the constellation’s brightest star. Taurus also contains the Pleiades star cluster and the Crab Nebula.
Gemini	Geminorum	Twins	The brightest stars in Gemini are Castor and
			Pollux.
Cancer	Cancri	Crab	Cancer contains the well-known star cluster Praesepe.
Leo	Leonis	Lion	Regulus is the brightest star in Leo.
Virgo	Virginis	Virgin	Spica is the brightest star in Virgo.
Libra	Librae	Balance
Scorpius	Scorpii	Scorpion	Antares is the brightest star of Scorpius, which
			also contains many star clusters.
Sagittarius	Sagittarii	Archer	The center of the Milky Way Galaxy lies in Sagit
			tarius, with the densest star clouds of the
			galaxy.
Capricornus	Capricorni	Sea-goat
Aquarius	Aquarii	Water-bearer
Pisces	Piscium	Fishes
Other Ptolemaic constellations
Andromeda	Andromedae	Andromeda (an Ethiopian princess of Greek legend, daughter of Cepheus and Cas	The constellation’s most notable feature is the great spiral galaxy Andromeda (also called M31).
		siopeia)
Aquila Ara	Aquilae Arae	Eagle Altar	The brightest star in Aquila is Altair.
Argo Navis	Argus Navis	the ship Argo	Argo Navis is now divided into smaller constellations that include Carina, Puppis, Pyxis, and Vela.
Auriga	Aurigae	Charioteer	The brightest star in Auriga is Capella. The con-
			stellation also contains open star clusters
			M36, M37, and M38.
Bootes	Bootis	Herdsman	Arcturus is the brightest star in Bootes.
Canis Major	Canis Majoris	Greater Dog	Sirius is the brightest star in Canis Major.
Canis Minor	Canis Minoris	Smaller Dog	Procyon is the brightest star in Canis Minor.
Cassiopeia	Cassiopeiae	Cassiopeia was a	Tycho’s nova, one of the few recorded super-
		legendary queen of Ethiopia	novae in the Galaxy, appeared in Cassiopeia in 1572.
Centaurus	Centauri	Centaur (possibly	Alpha Centauri in Centaurus contains Proxima,
		represents Chiron)	the nearest star to the Sun.
Cepheus	Cephei	Cepheus (legendary king of Ethiopia)	Delta Cephei was the prototype for cepheid variables (a class of variable stars).
Cetus	Ceti	Whale	Mira Ceti was the first recognized variable star.
Corona Austrina	Coronae Austrinae	Southern Crown
Corona Borealis	Coronae Borealis	Northern Crown
Corvus	Corvi	Raven
Crater	Crateris	Cup
Cygnus	Cygni	Swan	Cygnus contains the asterism (grouping of stars) known as the Northern Cross; the constellation’s brightest star is Deneb.
Delphinus	Delphini	Dolphin	Delphinus contains the asterism known as Job’s Coffin.
Draco	Draconis	Dragon	Draco contains the star Thuban, which was the polestar in 3000 bc.

NAME	GENITIVE	MEANING	NOTES
Other Ptolemaic constellations (continued)
Equuleus	Equulei	Little Horse
Eridanus	Eridani	River Eridanus or	Achernar is the brightest star in Eridanus.
		river god
Hercules	Herculis	Hercules (Greek	Hercules contains the great globular star cluster
		hero)	M13.
Hydra	Hydrae	Water Snake
Lepus	Leporis	Hare
Lupus	Lupi	Wolf
Lyra	Lyrae	Lyre	The brightest star in Lyra is Vega. In some
			10,000 years, Vega will become the polestar.
			Lyra also contains the Ring Nebula (M57).
Ophiuchus	Ophiuchi	Serpent-bearer	When the Zodiac was conceived of, Ophiuchus
			was not in the Sun’s path, but the Sun does
			now pass through Ophiuchus each December.
Orion	Orionis	Hunter	Rigel is the brightest star in Orion, followed
			closely by Betelgeuse; M42 (the Great Nebula)
			resides in Orion.
Pegasus	Pegasi	Pegasus (winged	The constellation contains stars of the Great
		horse)	Square of Pegasus.
Perseus	Persei	Perseus (legendary
		Greek hero)
Piscis Austrinus	Piscis Austrini	Southern Fish	The brightest star in Piscis Austrinus is Fomal-haut.
Sagitta	Sagittae	Arrow
Serpens	Serpentis	Serpent
Triangulum	Trianguli	Triangle	The constellation contains M33, a nearby spiral
			galaxy.
Ursa Major	Ursae Majoris	Great Bear	The seven brightest stars of this constellation
			are the Big Dipper (also called the Plough).
Ursa Minor	Ursae Minoris	Lesser Bear	Ursa Minor contains Polaris (the north polestar).
Southern constellations, added c. 1600
Apus	Apodis	Bird of Paradise
Chamaeleon	Chamaeleontis	Chameleon
Dorado	Doradus	Swordfish	The most notable object in Dorado is the Large
			Magellanic Cloud.
Grus	Gruis	Crane
Hydrus	Hydri	Water Snake
Indus	Indi	Indian
Musca	Muscae	Fly
Pavo	Pavonis	Peacock
Phoenix	Phoenicis	Phoenix (mythical
		bird)
Triangulum Aus-	Trianguli Australis	Southern Triangle
trale
Tucana	Tucanae	Toucan	The most notable object in Tucana is the Small
			Magellanic Cloud.
Volans	Volantis	Flying Fish
Constellations of	Bartsch, 1624
Camelopardalis	Camelopardalis	Giraffe
Columba	Columbae	Dove	The constellation was formed by Petrus Plancius
			in the early 1600s.
Monoceros	Monocerotis	Unicorn
Constellations of	Hevelius, 1687
Canes Venatici	Canum Venatico-	Hunting Dogs	The constellation contains M51 (the Whirlpool
	rum		Galaxy).
Lacerta	Lacertae	Lizard
Leo Minor	Leonis Minoris	Lesser Lion
Lynx	Lyncis	Lynx
Scutum	Scuti	Shield	Scutum contains the Scutim star cloud in the
			Milky Way.
Sextans	Sextantis	Sextant
Vulpecula	Vulpeculae	Fox	Vulpecula contains M27 (the Dumbbell Nebula).

NAME	GENITIVE	MEANING	NOTES
Ancient asterisms that are now separate constellations
Carina	Carinae	Keel [of the leg	The brightest star in Carina is Canopus.
		endary ship the
		Argo]
Coma Berenices	Comae Berenices	Berenice’s Hair	The constellation contains both a coma (star cluster) and the north galactic pole (a point that lies perpendicular to the Milky Way).
Crux	Crucis	[Southern] Cross
Puppis	Puppis	Stern [of the Argo]
Pyxis	Pyxidis	Compass [of the
		Argo]
Vela	Velorum	Sails [of the Argo]
Southern constellations of Lacaille, c.		1750
Antlia	Antliae	Pump
Caelum	Caeli	[Sculptor's] Chisel
Circinus	Circini	Drawing Compasses
Fornax	Fornacis	[Chemical] Furnace
Horologium	Horologii	Clock
Mensa	Mensae	Table [Mountain]
Microscopium	Microscopii	Microscope
Norma	Normae	Square
Octans	Octantis	Octant	Octans contains the south celestial pole.
Pictor	Pictoris	Painter’s [Easel]
Reticulum	Reticuli	Reticle
Sculptor	Sculptoris	Sculptor’s [Work-	Sculptor contains the south galactic pole.
		shop]
Telescopium	Telescopii	Telescope

Astrology: The Zodiac

Signs of the zodiac are popularly used for divination as well as for designation of constellations.

NAME	SYMBOL	DATES	SEX/NATURE	TRIPLICITY	HOUSE	EXALTATION
Aries the Ram	r	21 Mar-19 Apr	masculine/moving	fire	Mars	Sun (19°)
Taurus the Bull	v	20 Apr-20 May	feminine/fixed	earth	Venus	Moon (3°)
Gemini the Twins	n	21 May-21 Jun	masculine/common	air	Mercury
Cancer the Crab	s	22Jun-22 Jul	feminine/moving	water	Moon	Jupiter (15°)
Leo the Lion		23Jul-22 Aug	masculine/fixed	fire	Sun
Virgo the Virgin		23 Aug-22 Sep	feminine/common	earth	Mercury	Mercury (15°)
Libra the Balance		23 Sep-23 Oct	masculine/moving	air	Venus	Saturn (21°)
Scorpius the Scorpion		24 Oct-21 Nov	feminine/fixed	water	Mars
Sagittarius the Archer	*	22 Nov-21 Dec	masculine/common	fire	Jupiter
Capricorn the Goat		22 Dec-19Jan	feminine/moving	earth	Saturn	Mars (28°)
Aquarius the Water Bearer		20 Jan-18 Feb	masculine/fixed	air	Saturn
Pisces the Fish	H	19 Feb-20 Mar	feminine/common	water	Jupiter	Venus (27°)

Classification of Stars

The spectral sequence O-M represents stars of essentially the same chemical composition but of different temperatures and atmospheric pressures. Stars belonging to other, more rare types of spectral classifications differ in chemical composition from those stars classified under the O-M scheme.

Each spectral class is additionally subdivided into 10 spectral types. For example, spectral class A is subdivided into spectral types A0-A9 with 0 being the hottest and 9 the coolest. (Spectral class O is unusual in that it is subdivided into O4-O9.) Between two stars of the same spectral type, the more luminous star will also be larger in diameter. Thus the Yerkes system of luminosity also tells something of a star’s radius, with la being the largest and V the smallest. Approximately 90% of all stars are main-sequence, or type V, stars.

Based upon these systems, the Sun would be a G2 V star (a yellow, relatively hot dwarf star).

This table lists the stars in descending order from brightest to least bright, based on apparent visual magnitude. Formal names of stars, such as Alpha Carinae, refer to the constellation in which the star appears (Carina) and to which star appears the brightest in that constellation; the second highest would be designated Beta, etc. Some anomalies exist within the naming convention: Betelgeuse, for example, is the Alpha star of Orion, though Rigel appears brighter.

On the scale of brightness, negative magnitudes are brightest, and one magnitude difference corresponds to a difference in brightness of 2.5 times; e.g., a star of magnitude -1 is 10 times brighter than one of magnitude +1.5.

Classification of Stars (continued)

	APPROXIMATE
SPECTRAL CLASS COLOR	SURFACE TEMP (°C)	EXAMPLES
G yellow	6,000 to 7,000	Sun
K orange	4,500 to 6,000	Arcturus, Aldebaran
M red	3,000 to 4,500	Betelgeuse, Antares
LUMINOSITY CLASSES (BASED UPON THE YERKES SYSTEM)
Ia most luminous supergiants
Ib luminous supergiants
II bright giants
III normal giants
IV subgiants
V main-sequence stars (dwarfs)

Apparent magnitude is a measure of how bright a star appears to a viewer on Earth. Absolute magnitude, another designation used by astronomers, represents the brightness one would perceive if all stars were located 10 parsecs (about 32.6 light-years; one light-year equals about 9.46 x 1012 km) from Earth. The Sun, for purposes of comparison with the stars in the table, has an apparent magnitude of -26.8; it is a yellow dwarf star that is 8.3 light-minutes (one light-minute equals about 18 million km) from Earth.

DISTANCE FROM APPARENT VISUAL THE SOLAR STAR MAGNITUDE SYSTEM (LIGHT-YEARS) CONSTELLATION

Sirius (Alpha Canis -1.44 8.6 Canis Major Majoris, or Dog Star) Sirius is a blue-white dwarf with a white-dwarf companion; among the ancient Romans, the hottest part of the year was associated with the time in which the Dog Star rose just before dawn; this connection survives in the expression “dog days.”

Canopus (Alpha Carinae) -0.73 (reported 312.0 (reported Carina values vary) values vary) A yellow-white supergiant, Canopus is sometimes used as a guide in the attitude control of spacecraft because of its angular distance from the Sun and the contrast of its brightness among nearby celestial objects.

Arcturus (Alpha Bootis) -0.05 36.7 Bootes An orange-colored giant, Arcturus lies in an almost direct line with the tail of Ursa Major (the Great Bear), hence its name, derived from the Greek words for “bear guard.”

Alpha Centauri 0.00 4.4 Centaurus (Rigel Kentaurus) Alpha Centauri is a triple star—a binary yellow dwarf circled by a red dwarf with a much smaller red dwarf; the faintest of Alpha Centauri’s three stars, Proxima, is the star closest to the Sun.

Vega (Alpha Lyrae) +0.03 25.3 Lyra A blue dwarf, Vega will become the northern polestar by about AD 14,000 because of the precession of the equinoxes.

Capella (Alpha Aurigae) +0.08 42.2 Auriga Capella is actually four stars, two yellow giants and two red-dwarf companion stars. Scientists are studying Capella to determine why it emits more X-rays than other stars of its type.

Rigel (Beta Orionis) +0.18 (reported 773.0 Orion values vary) Rigel is a blue-white supergiant with two smaller companion stars. The name Rigel derives from an Arabic term meaning “the left leg of the giant,” referring to the figure of Orion.

The 20 Brightest Stars in the Night Sky

DISTANCE FROM APPARENT VISUAL THE SOLAR STAR MAGNITUDE SYSTEM (LIGHT-YEARS) CONSTELLATION Procyon +0.40 11.4 Canis Minor

(Alpha Canis Minoris) Procyon is a yellow-white subgiant with a faint white-dwarf companion. The name Procyon apparently derives from Greek words for “before the dog,” as in northern latitudes the star rises just before Sirius, the Dog Star.

Achernar (Alpha Eridani) +0.45 144.0 Eridanus Achernar is a blue dwarf. The name Achernar probably derives from an Arabic phrase meaning “the end of the river,” in which the river referred to is the constellation.

Betelgeuse (Alpha Orionis) +0.45 (reported 427.0 Orion values vary) A red supergiant, Betelgeuse has a diameter that varies between 430 and 625 times the diameter of the Sun over a period of 5.8 years.

Beta Centauri (Hadar) +0.58 526.0 Centaurus Beta Centauri is a blue-white supergiant with two smaller companion stars; the constellation Centaurus most likely is meant to represent the centaur Chiron. In Greek mythology Chiron was renowned for his wisdom and knowledge of medicine. He renounced his immortality to escape a painful wound, and Zeus placed him in the Southern sky.

Altair (Alpha Aquilae) +0.76 16.8 Aquila A blue dwarf, Altair spins nearly 760,000 km/h (470,000 mph), as compared with Earth, which spins some 1,600 km/h (1,000 mph). This rapid spinning flattens Altair from a spherical into an oblate shape.

Aldebaran (Alpha Tauri) +0.87 65.1 Taurus A red giant, Aldebaran has a name derived from the Arabic for “the follower,” perhaps because it rises after the Pleiades cluster of stars.

Spica (Alpha Virginis) +0.98 262.0 Virgo A binary blue-white dwarf with a nonvisible companion, Spica has a name derived from the Latin for “ear of wheat”; the star is said to represent the wheat being held by the Virgin/fertility goddess (for whom Virgo is named).

Antares (Alpha Scorpii) +1.06 (reported 604.0 Scorpio values vary) Antares is a red supergiant. The name Antares seems to come from a Greek phrase meaning “rival of Ares” (i.e., rival of the planet Mars) and was probably given because of the star’s color and brightness.

Pollux (Beta Geminorum) +1.16 96.7 Gemini A red giant, Pollux is named for one of the twins of ancient Greek mythology (the other is Castor).

Fomalhaut +1.17 25.1 Piscis (Alpha Piscis Austrini) Austrinus The blue-white dwarf Formalhaut’s name is derived from the Arabic for “mouth of the fish.”

Becrux +1.25 352.0 Crux (The (Beta Crucis, or Mimosa) Southern

Cross) A blue-white giant, Becrux forms the eastern tip of the Southern Cross. Deneb (Alpha Cygni) +1.25 3,230.0 Cygnus A blue-white supergiant, Deneb gained its name from an Arabic word meaning “tail,” as it is considered the tail of the swan Cygnus.

Acrux (Alpha Crucis) +1.40 321.0 Crux (The Southern Cross) Acrux is a double star that stands at the foot of the Southern Cross.

Astronomical Phenomena for 2009

		HOUR				HOUR
MONTH	DAY	(GMT)	EVENT	MONTH	DAY	(GMT)	EVENT
January	1	20	Saturn stationary	March	20	12	equinox
	2	17	Uranus 5° S of Moon	(	22	21	Jupiter 1 ?5 S of Moon
	4	12	first quarter		23	14	Neptune 2 S of Moon
	4	14	Mercury greatest		24	14	Mars 4 S of Moon
	\		elongation E (19°)		26	16	new moon
	4	15	Earth at perihelion		27	19	Venus in inferior
	10	11	Moon at perigee				conjunction
	11	03	full moon		31	03	Mercury in superior
	11	07	Mercury stationary				conjunction
\	14	21	Venus greatest
			elongation E (47°)	April	2	02	Moon at perigee
	15	12	Saturn 6° N of Moon		2	15	first quarter
	17	18	Ceres stationary		4	16	Pluto stationary
	18	03	last quarter		7	07	Saturn 6 ° N of Moon
	18	22	Juno in conjunction with		9	15	full moon
			Sun		13	13	Antares 0 °4 S of Moon1
	20	16	Mercury in inferior		15	04	Mars 0 ?5 S of Uranus
			conjunction		15	08	Venus stationary
	21	13	Antares 0°02 S of		16	09	Moon at apogee
			Moon1		17	14	last quarter
	21	13	Pallas stationary		17	15	Ceres stationary
	23	00	Moon at apogee		18	17	Venus 6 N of Mars
	23	16	Venus 1°4 N of Uranus		19	16	Jupiter 2 ° S of Moon
	24	06	Jupiter in conjunction		20	00	Neptune 2° S of Moon
			with Sun		22	08	Uranus 5° S of Moon
	26	08	new moon2		22	14	Venus 1°1Sof Moon1
	27	18	Neptune 1°8 S of Moon		22	19	Mars 6° S of Moon
	30	01	Uranus 5° S of Moon		25	03	new moon
	30	12	Venus 3° S of Moon		26	08	Mercury greatest
							elongation E (20°)
February	1	02	Mercury stationary		26	16	Mercury 1°9 S of Moon
	2	23	first quarter		28	06	Moon at perigee
	7	20	Moon at perigee
	9	15	full moon3	May	1	21	first quarter
\	11	20	Saturn 6° N of Moon		2	15	Venus greatest
	12	13	Neptune in conjunction				illuminated extent
			with Sun	\	4	11	Saturn 6° N of Moon
	13	21	Mercury greatest		7	16	Mercury stationary
			elongation W (26°)		9	04	full moon
	16	22	last quarter		10	21	Antares 0?6S of Moon1
	17	10	Mars 0°6 S of Jupiter		14	03	Moon at apogee
	17	21	Antares 0°04S of Moon1		17	07	last quarter
	19	15	Venus greatest		17	08	Jupiter 3° S of Moon
			illuminated extent		17	09	Neptune 3° S of Moon
	19	17	Moon at apogee		17	19	Saturn stationary
	22	22	Mercury 1°1 S of Moon1		18	10	Mercury in inferior
	23	01	Jupiter 0?7 N of Moon1				conjunction
	23	08	Mars 1?7 S of Moon		19	20	Uranus 5° S of Moon
	24	03	Mercury 0 ?6 S of Jupiter		21	08	Venus 7° S of Moon
	25	02	new moon		21	20	Mars 7° S of Moon
	25	14	Ceres at opposition		24	12	new moon
	27	23	Venus 1 ?3 N of Moon1		25	13	Jupiter 0°4S of Neptune
					26	04	Moon at perigee
March	1	20	Mercury 0 ?6 S of Mars		29	11	Neptune stationary
	4	08	first quarter		30	16	Mercury stationary
	5	01	Venus stationary		31	03	first quarter
	7	15	Moon at perigee		31	17	Saturn 6° N of Moon
	8	04	Mars 0 ?8 S of Neptune
	8	20	Saturn at opposition	June	5	21	Venus greatest
	11	03	full moon				elongation W (46°)
	11	03	Saturn 6 N of Moon		7	04	Antares 0?6 S of Moon1
	13	01	Uranus in conjunction		7	18	full moon
			with Sun		10	16	Moon at apogee
	17	05	Antares 0 ?2 S of Moon1		13	12	Mercury greatest
	18	18	last quarter				elongation W (23°)
	19	1	Moon at apogee		13	16	Neptune 3° S of Moon

	HOUR				HOUR
MONTH DAY	(GMT)	EVENT	MONTH	DAY	(GMT)	EVENT
June 13	18	Jupiter 3° S of Moon	September	2	21	Jupiter 3° S of Moon
(continued) 15	20	Jupiter stationary		3	07	Neptune 3° S of Moon
15	22	last quarter		4	16	full moon
15	23	Juno 0°4 N of Moon1		5	21	Uranus 6° S of Moon
16	06	Uranus 6° S of Moon		6	20	Mercury stationary
19	14	Venus 2° S of Mars		11	16	Pluto stationary
19	17	Mars 6° Sof Moon		12	02	last quarter
19	17	Venus 8° S of Moon		13	00	Pallas in conjunction
21	06	solstice				with Sun
21	09	Mercury 7° S of Moon		13	16	Mars 1°1 Sof Moon1
22	12	Vesta in conjunction		16	08	Moon at perigee
		with Sun		16	18	Venus 3° N of Moon
2	14	Mercury 3° N of		17	10	Uranus at opposition
		Aldebaran		17	18	Saturn in conjunction
22	20	new moon				with Sun
23	08	Pluto at opposition		18	19	new moon
23	11	Moon at perigee		20	1	Mercury in inferior
28	02	Saturn 7° N of Moon				conjunction
29	11	first quarter		20	10	Venus 0°5 N of Regulus
				21	08	Juno at opposition
July 1	16	Uranus stationary		22	21	equinox
4	02	Earth at aphelion		24	06	Antares 0?8 S of Moon1
4	10	Antares 0°5 S of Moon1		26	05	first quarter
7	09	full moon3		28	04	Moon at apogee
7	22	Moon at apogee		28	18	Mercury stationary
10	22	Jupiter 4° S of Moon		30	00	Jupiter 3° S of Moon
10	22	Neptune 3° S of Moon		30	13	Neptune 3° S of Moon
13	12	Uranus 6° S of Moon	\
13	19	Jupiter 0°6S of Neptune	October	3	02	Uranus 6° S of Moon
14	02	Mercury in superior		4	06	full moon
		conjunction		5	22	Mars 6° S of Pollux
14	18	Venus 3° N of Aldebaran		6	02	Mercury greatest
15	10	last quarter				elongation W (18°)
18	12	Mars 5° S of Moon		8	09	Mercury 0? 3 S of Saturn
19	05	Venus 6° S of Moon		11	09	last quarter
21	20	Moon at perigee		12	01	Mars 1°. 2 N of Moon1
22	03	new moon2		13	09	Jupiter stationary
25	15	Saturn 7° N of Moon		13	12	Moon at perigee
27	11	Mars 5° N of Aldebaran		13	16	Venus 0.6 S of Saturn
28	22	first quarter		16	13	Saturn 7° N of Moon
31	16	Antares 0°5 S of Moon1		16	19	Venus 7° N of Moon
				18	06	new moon
August 2	19	Mercury 0.6 N of		21	15	Antares 1.0 S of Moon1
		Regulus		25	23	Moon at apogee
4	01	Moon at apogee		26	01	first quarter
6	01	full moon3		27	09	Jupiter 3° S of Moon
6	22	Jupiter 3° S of Moon		27	21	Neptune 3° S of Moon
7	02	Neptune 3° S of Moon		30	09	Uranus 6° S of Moon
9	17	Uranus 6° S of Moon		31	10	Juno stationary
13	19	last quarter		31	15	Ceres in conjunction
14	18	Jupiter at opposition				with Sun
15	18	Juno stationary
16	03	Mars 3° S of Moon	November	2	02	Venus 4° N of Spica
17	21	Neptune at opposition		2	19	full moon
17	21	Venus 1°7 S of Moon		4	19	Neptune stationary
18	07	Vesta 0°4S of Moon1		5	08	Mercury in superior
18	21	Mercury 3° S of Saturn				conjunction
19	05	Moon at perigee		7	07	Moon at perigee
20	10	new moon		9	06	Mars 3° N of Moon
22	04	Venus 7° S of Pollux		9	16	last quarter
22	06	Saturn 7° N of Moon		13	01	Saturn 8° N of Moon
22	12	Mercury 3° N of Moon		16	19	new moon
24	16	Mercury greatest		22	20	Moon at apogee
		elongation E (27°)		23	22	Jupiter 4° S of Moon
27	12	first quartr		24	06	Neptune 3° S of Moon
27	22	Antares 0°6 Sof Moon1		24	22	first quarter
31	11	Moon at apogee		26	18	Uranus 6° S of Moon

	HOUR				HOUR
MONTH DAY	(GMT)	EVENT	MONTH	DAY	(GMT)	EVENT
December 2	05	Uranus stationary	December	20	15	Moon at apogee
2	08	full moon	(continued)	21	15	Jupiter 4° S of Moon
4	14	Moon at perigee		21	15	Neptune 4° S of Moon
7	03	Mars 6° N of Moon		21	16	Mars stationary
9	00	last quarter		21	18	solstice
10	11	Saturn 8° N of Moon		24	02	Uranus 6° S of Moon
16	12	new moon		24	18	first quarter
18	08	Mercury 1°4 S of Moon		24	18	Pluto in conjunction
18	17	Mercury greatest				with Sun
		elongation E (20°)		26	09	Mercury stationary
20	05	Jupiter 0?6S of Neptune		31	19	full moon2

Did you knows

Uranus was the first planet to be discovered with a telescope. The German-born astronomer William Herschel accidentally discovered the planet in 1781 during a routine sky survey at his observatory in Bath, England. At first he thought it was a comet. When astronomers concluded that the object was really a planet, the German astronomer J.E. Bode suggested that it be called Uranus, in honor of the ancient sky god who was the father of Saturn in Greco-Roman mythology.

Morning and Evening Stars

This table gives the morning and evening stars for autumn 2008 through 2009. The morning and evening stars are actually planets visible to the naked eye during the early morning and at evening twilight.

PLANET Mercury	MORNING STAR 14 Oct-10 Nov 2008; 27 Jan-22 Mar, 28 May-6 Jul, 28 Sep-23 Oct 2009	EVENING STAR 8 Aug-30 Sep, 13-31 Dec 2008; 1-15 Jan, 9 Apr-9 May, 22 Jul- 14 Sep, 22 Nov-30 Dec 2009
Venus	1 Apr-1 Dec 2009	16 Jul-31 Dec 2008; 1 Jan-24 Mar 2009
Mars	1 Feb-31 Dec 2009	1 Jan-16 Oct 2008
Jupiter	7 Feb-14 Aug 2009	9 Jul-31 Dec 2008; 1-11 Jan, 14Aug-31 Dec 2009
Saturn	22 Sep-31 Dec 2008; 1 Jan-8 Mar, 6 Oct-31 Dec 2009	24 Feb-17 Aug 2008; 8 Mar-31 Aug 2009
Uranus	late March-September 2008; early April-mid-December 2009	late December 2008-mid-February 2009, mid-December 2009
Neptune	early March-August 2008; early March-mid-November 2009	mid-November-31 Dec 2008; January 2009 mid-November-31 Dec 2009

Meteors, Meteorites, and Meteor Showers

A meteor (also called a shooting star or falling star) is a streak of light in the sky that results when a particle or small chunk of stony or metallic matter enters the Earth’s atmosphere and vaporizes. The term is sometimes applied to the falling object itself, but the latter is properly called a meteoroid. The vast majority of meteoroids burn up in the upper atmosphere, but occasionally one of relatively large mass survives its fiery plunge and reaches the surface as a solid body. Such an object is known as a meteorite.

On any clear night in the countryside beyond the bright lights of cities, one can observe with the naked eye several meteors per hour as they streak through the sky. Quite often they vary in brightness along the path of their flight, appear to emit “sparks” or flares, and sometimes leave a luminous train that lingers after their flight has ended. These meteors are the result of the high-velocity collision of meteoroids with the Earth’s atmosphere. Nearly all such interplanetary bodies are small fragments derived from comets or asteroids.

The brightest meteor (possibly of cometary origin) for which historical documentation exists—called the Tunguska event—struck on 30 Jun 1908 in central Siberia and rivaled the Sun in brightness. The energy delivered to the atmosphere by this impact was roughly equivalent to that of a 10-megaton thermonuclear explosion and caused the destruction of forest over an area of about 2,000 sq km (772.2 sq mi). The geologic record of cratering attests to the impact of much more massive meteoroids. Fortunately, impacts of this magnitude occur only once or twice every 100 million years. It is hypothesized that large impacts of this kind may have played a major role in determining the course of biological evolution by causing simultaneous mass extinctions of many species of organisms, possibly including the dinosaurs some 65 million years ago. If so, the replacement of reptiles by mammals as the dominant land animals, the eventual consequence of which was the rise of the human species, would be the result of a grand example of a phenomenon observable every clear night.

The visibility of meteors is a consequence of the high velocity of meteoroids in interplanetary space. Before entering the region of the Earth’s gravitational influence, their velocities range from a few kilometers per second up to as high as 72 km (44.7 mi) per second. As they approach the Earth, the Earth’s gravitational field accelerates them to even higher velocities. This great release of energy destroys meteoroids of small mass—particularly those with relatively high velocities—very quickly. Numerous meteors end their observed flight at altitudes above 80 km (49.7 mi), and penetration to as low as 50 km (31 mi) is unusual.

“Showers” of meteors have been known since ancient times. On rare occasions, these showers are very dramatic, with thousands of meteors falling per hour. More often, the background hourly rate of roughly 5 observed meteors increases up to about 10-50. Some of the best-known meteor showers are listed below, with their average date of maximum strength and associated comet, if known: Quadrantid (3 January); Lyrid (22 April; 1861 I [Thatcher]); Eta Aquarid (3 May; Halley); S. Delta Aquarid (29 July); Capricornid (30 July); Perseid (12 August; Swift-Tut-tle); Andromedid (3 October; Biela); Draconid (9 October; Giacobini-Zinner); Orionid (21 October; Halley); Taurid (8 November; Encke); Leonid (17 November; Temple-Tuttle); Germinid (14 December; 3200 Phaeton [this body exhibits no cometary activity and may be of asteroidal rather than cometary origin]).

Auroras

Auroras are luminous phenomena of the upper atmosphere that occur primarily in high latitudes of both hemispheres; auroras in the Northern Hemisphere are called aurora borealis, or northern lights; in the Southern Hemisphere, aurora australis, or southern lights.

Auroras are caused by the interaction of energetic particles (electrons and protons) from outside the atmosphere with atoms of the upper atmosphere. Such interaction occurs in zones surrounding the Earth’s magnetic poles. During periods of intense solar activity, auroras occasionally extend to the middle latitudes; for example, the aurora borealis has been seen at latitudes as far south as 40° in the US.

Auroras take many forms, including luminous curtains, arcs, bands, and patches. The uniform arc is the most stable form of aurora, sometimes persisting for hours without noticeable variation. In a great display, however, other forms appear, commonly undergoing dramatic variation. The lower edges of the arcs and folds are usually much more sharply defined than the upper parts. Greenish rays may cover most of the sky poleward of the magnetic zenith, ending in an arc that is usually folded and sometimes edged with a lower red border that may ripple like drapery. The display ends with a poleward retreat of the auroral forms, the rays gradually degenerating into diffuse areas of white light.

The mechanisms that produce auroral displays are not completely understood. It is known, however, that charged particles arriving in the vicinity of Earth as part of the solar wind are captured by the Earth’s magnetic field and conducted downward toward the magnetic poles. They collide with oxygen and nitrogen atoms, knocking away electrons to leave ions in excited states. These ions emit radiation at various wavelengths, creating the characteristic colors (red or greenish blue) of the aurora.

Eclipses

An eclipse is a complete or partial obscuring of one celestial body by another; this event occurs when three celestial objects become aligned.

The Sun is eclipsed when the Moon comes between it and the Earth. (Hence, a solar eclipse can only occur during a new moon.) The Moon’s shadow sweeps across the Earth, darkening the sky, while the Moon blocks out some portion of the view of the Sun. During a total eclipse of the Sun, the Moon’s elliptical orbit brings the satellite closer to Earth and causes it to appear larger than the Sun. When the Moon’s orbit places it at its farthest distance from Earth, the Moon appears smaller than the Sun and the eclipse will appear as a ring, or “annulus,” of brightsunlight around the Moon.

A lunar eclipse occurs when the Moon moves into the shadow of the Earth cast by the Sun. A lunar eclipse can only occur during a full moon. Lunar eclipses can be penumbral, partial, or total. The first type is of interest to astronomers but is difficult to detect because the Moon’s dimming is so slight. With the next two types either a portion of the Moon or the entire Moon passes through Earth’s umbral shadow.

It is safe to watch a lunar eclipse, but solar eclipses must be viewed via a projection onto another surface or through protective filters designed specially for eclipses.

The eclipses for 2009 are given in the table below. Penumbral eclipses are not included.

Sun

diameter (at equator): 1,390,000 km (863,705 mi)

mass (in 1020kg): 19.8 billion

density (mass/volume, in kg/m3): 1,408

mean orbital velocity: the Sun orbits the Milky Way’s

center at around 220 km/sec (136.7 mi/sec) orbital period: the Sun takes approximately 250 million Earth years to complete its orbit around the Milky Way’s center rotation period: 25-36 Earth days gravitational acceleration: 275 m/sec2 (902.2 ft/sec2)

escape velocity: 618.02 km/sec (384.01 mi/sec) mean temperature at visible surface: 5,527 °C (9,980 °F)

probes and space missions: US—Pioneer 5-9, launched 1959-87; Skylab, launched 1973; Genesis, 2001; Japan—Yohkoh, 1991; US/European Space Agency (ESA)—Ulysses, 1990; SOHO, 1995.

Mercury

average distance from Sun: 58 million km (36 million mi)

diameter (at equator): 4,879 km (3,032 mi)

mass (in 1020kg): 3,300

density (mass/volume, in kg/m3): 5,427

eccentricity of orbit: 0.205

mean orbital velocity: 47.9 km/sec (29.7 mi/sec)

inclination of orbit to ecliptic: 7.0°

orbital period: 88 Earth days

rotation period: 58.6 Earth days

inclination of equator to orbit: probably 0°

gravitational acceleration: 3.7 m/sec2 (12.1 ft/sec2)

escape velocity: 4.3 km/sec (2.7 mi/sec)

mean temperature at surfacef: 167 °C(333 °F)

satellites: none known

probes and space missions: US—Mariner 10, 1973; Messenger, 2004.

Venus

average distance from Sun: 108.2 million km (67.2 million mi)

diameter (at equator): 12,104 km (7,521 mi)

mass (in 1020kg): 48,700

density (mass/volume, in kg/m3): 5,243

eccentricity of orbit: 0.007

mean orbital velocity: 35.0 km/sec (21.8 mi/sec)

inclination of orbit to ecliptic: 3.4°

orbital period: 224.7 Earth days

rotation period: 243.0 Earth days (retrograde)

inclination of equator to orbit: 177.4°

gravitational acceleration: 8.9 m/sec2 (29.1 ft/sec2)

escape velocity: 10.4 km/sec (6.4 mi/sec)

mean temperature at surfacef: 464 °C(867 °F)

satellites: none known

probes and space missions: USSR—Venera 1-16, 1961-83; Vega 1 and 2, 1984; US—Mariner 2, 5, and 10, 1962, 1967, and 1973; Pioneer Venus 1 and 2, 1978; Galileo, 1989; Magellan, 1989; Venus Express, 2005.

Earth

average distance from Sun: 149.6 million km (93 million mi)

diameter (at equator): 12,756 km (7,926 mi)

mass (in 1020kg): 59,700

density (mass/volume, in kg/m3): 5,515

eccentricity of orbit: 0.017

mean orbital velocity: 29.8 km/sec (18.5 mi/sec)

inclination of orbit to ecliptic: 0.00°

orbital period: 365.25 days

rotation period: 23 hours, 56 minutes, and 4

seconds of mean solar time inclination of equator to orbit: 23.5° gravitational acceleration: 9.8 m/sec2 (32.1 ft/sec2) escape velocity: 11.2 km/sec (7.0 mi/sec) mean temperature at surfacef: 15 °C(59 °F) satellites: 1 known—the Moon.

Moon (of Earth)

average distance from Earth: 384,401 km

(238,855.7 mi) diameter (at equator): 3,475 km (2,159 mi) mass (in 1020kg): 730 density (mass/volume, in kg/m3): 3,340 eccentricity of orbit: orbital eccentricity of Moon

around Earth is 0.055 mean orbital velocity: the Moon orbits Earth at 1.0

km/sec (0.64 mi/sec) inclination of orbit to ecliptic: 5.1° orbital period: the Moon revolves around the Earth in

27.32 Earth days rotation period: the Moon rotates on its axis every 27.32 Earth days (synchronous with orbital period) inclination of equator to orbit: 6.7° gravitational acceleration: 1.6 m/sec2 (5.3 ft/sec2) escape velocity: 2.4 km/sec (1.5 mi/sec) mean temperature at surfacef: daytime: 107 °C

(224.6 °F); nighttime: -153 °C (-243.4 °F) probes and space missions: USSR, US, ESA, Japan— collectively about 70 missions since 1959, including 9 manned missions by the US. On 20 Jul 1969 humans first set foot on the Moon, from NASA’s Apollo 11.

Mars

average distance from Sun: 227.9 million km (141.6 million mi)

diameter (at equator): 6,794 km (4,222 mi)

mass (in 1020kg): 6,420

density (mass/volume, in kg/m3): 3,933

eccentricity of orbit: 0.094

mean orbital velocity: 24.1 km/sec (15 mi/sec)

inclination of orbit to ecliptic: 1.9°

orbital period: 687 Earth days (1.88 Earth years)

rotation period: 24.6 Earth hours

inclination of equator to orbit: 24.9°

gravitational acceleration: 3.7 m/sec2 (12.1 ft/sec2)

escape velocity: 5.0 km/sec (3.1 mi/sec)

mean temperature at surfacef: -65 °c(-85 °F)

satellites: 2 known—Phobos and Deimos

Characteristics of Celestial Bodies

Mean orbital velocity indicates the average speed with which a planet orbits the Sun unless otherwise specified. Inclination of orbit to ecliptic indicates the angle of tilt between a planet’s orbit and the plane of the Earth’s orbit (essentially the plane of the solar system). Orbital period indicates the planet’s sidereal year (in Earth days except where noted). Rotation period indicates the planet’s sidereal day (in Earth days except where noted). Inclination of equator to orbit indicates the angle of tilt between a planet’s orbit and its equator. Gravitational acceleration is a measure of the body’s gravitational pull on other objects. Escape velocity is the speed needed at the surface to escape the planet’s gravitational pull. Eccentricity of orbit is a measure of the circularity or elongation of an orbit; 0 indicates circular orbits, and closer to 1 more elliptical ones.

Probes and space missions: US—Mariner 4, 6, 7, and 9, 1964-71; Viking 1 and 2, 1975; Mars Global Surveyor, 1996; Mars Pathfinder, 1996; 2001 Mars Odyssey, 2001; Mars Exploration Rovers, 2003; USSR—Mars 2-7, 1971-73; Phobos 1 and 2, 1988; ESA—Mars Express, 2003; Mars Exploration Rovers, 2004; Phoenix, 2007.

asteroids

(several hundred thousand small rocky bodies, about 1,000 km [610 mi] or less in diameter, that orbit the Sun primarily between the orbits of Mars and Jupiter) distance from Sun: between approximately 300 million km (190 million mi) and 600 million km (380 million mi), with notable outlyers estimated mass: 2.3 x 1021 kg probes and space missions: US—Galileo, 1989; Ulysses, 1990; NEAR Shoemaker, 1996; Deep Space 1, 1998; Stardust, 1999; US/ESA/Italy— Cassini-Huygens, 1997; ESA—Rosetta, 2004; Japan—Hayabusa, 2003.

Jupiter

average distance from Sun: 778.6 million km (483.8 million mi)

diameter (at equator): 142,984 km (88,846 mi)

mass (in 1020kg): 18,990,000

density (mass/volume, in kg/m3): 1,326

eccentricity of orbit: 0.049

mean orbital velocity: 13.1 km/sec (8.1 mi/sec)

inclination of orbit to ecliptic: 1.3°

orbital period: 11.86 Earth years

rotation period: 9.9 Earth hours

inclination of equator to orbit: 3.1°

gravitational acceleration: 23.1 m/sec2 (75.9 ft/sec2)

escape velocity: 59.5 km/sec (37.0 mi/sec)

mean temperature at surfacef: -110 °C(-166 °F)

satellites: at least 63 moons—including Callisto,

Ganymede, Europa, and Io—plus rings probes and space missions: US—Pioneer 10 and 11, 1972-73; Voyager 1 and 2, 1977; Galileo, 1989; Ulysses, 1990; US/ESA/Italy—Cassini-Huygens, 1997.

Saturn

average distance from Sun: 1.433 billion km (890.8 million mi)

diameter (at equator): 120,536 km (74,897 mi) mass (in 1020kg): 5,680,000 density (mass/volume, in kg/m3): 687 eccentricity of orbit: 0.057 mean orbital velocity: 9.7 km/sec (6 mi/sec) inclination of orbit to ecliptic: 2.5° orbital period: 29.43 Earth years rotation period: 10.7 Earth hours inclination of equator to orbit: 26.7° gravitational acceleration: 9.0 m/sec2 (29.4ft/sec2) escape velocity: 35.5 km/sec (22.1 mi/sec) mean temperature at surfacef: -140 °C(-220 °F) satellites: at least 60 moons—including Titan—plus rings

probes and space missions: US—Pioneer 11, 1973; Voyager1 and 2, 1977; US/ESA/Italy—Cassini/Huy-gens, 1997.

Uranus

average distance from Sun: 2.872 billion km (1.784 billion miles)

diameter (at equator): 51,118 km (31,763 mi)

mass (in 1020kg): 868,000

density (mass/volume, in kg/m3): 1,270

eccentricity of orbit: 0.046

mean orbital velocity: 6.8 km/sec (4.2 mi/sec)

inclination of orbit to ecliptic: 0.8°

orbital period: 84.01 Earth years

rotation period: 17.2 Earth hours (retrograde)

inclination of equator to orbit: 97.8°

gravitational acceleration: 8.7 m/sec2 (28.5 ft/sec2)

escape velocity: 21.3 km/sec ( 13.2 mi/sec)

mean temperature at surfacef: -195 °C (-320 °F)

satellites: at least 27 moons, plus rings

probes and space missions: US—Voyager 2, 1977.

Neptune

average distance from Sun: 4.495 billion km (2.793 billion mi)

diameter (at equator): 49,528 km (30,775 mi) mass (in 1020kg): 1,020,000 density (mass/volume, in kg/m3): 1,638 eccentricity of orbit: 0.009 mean orbital velocity: 5.4 km/sec (3.4 mi/sec) inclination of orbit to ecliptic: 1.8° orbital period: 164.79 Earth years rotation period: 16.1 Earth hours inclination of equator to orbit: 28.3° gravitational acceleration: 11.0 m/sec2 (36.0 ft/sec2)

escape velocity: 23.5 km/sec (14.6 mi/sec) mean temperature at surfacef: -200 °C(-330 °F) satellites: at least 13 moons, plus rings probes and space missions: US—Voyager 2, 1977.

Pluto

average distance from Sun: 5.910 billion km (3.67 billion mi); Pluto lies within the Kuiper belt and can be considered its largest known member diameter (at equator): 2,344 km (1,485 mi) mass (in 1020kg): 125

density (mass/volume, in kg/m3): about 2,000

eccentricity of orbit: 0.249

mean orbital velocity: 4.72 km/sec (2.93 mi/sec)

inclination of orbit to ecliptic: 17.2°

orbital period: 248 Earth years

rotation period: 6.4 Earth days (retrograde)

inclination of equator to orbit: 122.5°

gravitational acceleration: 0.6 m/sec2 (1.9 ft/sec2)

escape velocity: 1.1 km/sec (0.7 mi/sec)

mean temperature atsurfacef: -225 °C (-375 °F)

satellites: 3 known—including Charon

probes and space missions: US—New Horizons, 2006.

Charon (moon of Pluto)

average distance from Pluto: 19,600 km (12,178.8 mi)

diameter (at equator): 1,250 km (777 mi) mass (in 1020kg): 19

density (mass/volume, in kg/m3): about 1,700 eccentricity of orbit: 0

mean orbital velocity: Charon orbits Pluto at 0.23

km/sec (0.142 mi/sec) inclination of orbit to Pluto’s equator: close to 0° orbital period: 6.3873 Earth days rotation period: 6.3873 Earth days gravitational acceleration: 0.21 m/sec2 (0.69 ft/sec2) escape velocity: 0.58 km/sec (0.36 mi/sec) mean temperature at surfacef: as low as -240 °C (-400 °F).

Comet 1P Halley

distance from Sun at closest point of orbit is 87.8 million km (54 million mi). Farthest distance from Sun is 5.2 billion km (3.2 billion mi). diameter (at equator): 16x8x8km(9.9×4.9×4.9 mi) density (mass/volume, in kg/m3): possibly as low as 200 eccentricity of orbit: 0.967 inclination of orbit to ecliptic: 18° orbital period: 76.1 to 79.3 Earth years. The next appearance will be 2061. The comet’s orbit is retrograde.

rotation period: 52 Earth hours probes and space missions: ESA—Giotto, 1985; USSR—Vega 1 and 2, 1985; Japan—Sakigake and Suisei, 1985.

Comet 2P Encke

distance from Sun at closest point of orbit is 50 million km (31 million mi). Farthest distance from Sun is 658 million km (408 million mi). eccentricity of orbit: 0.847

orbital period: 3.3 Earth years (shortest known for a comet); next closest pass of Sun is on 19 Apr 2007.

Comet 9P Tempel 1

distance from Sun at closest point of orbit is 225 million km (140 million mi). Farthest distance from Sun is 708 million km (440 million mi). eccentricity of orbit: 0.52

orbital period: 5.52 Earth years; next closest pass of

Sun is in January 2011. rotation period: 41 Earth hours probes and space missions: US—Deep Impact, 2005

Comet 81P Wild 2

distance from Sun at closest point of orbit is 236.8 million km (147.1 million mi). Farthest distance from Sun is 10 billion km (6.2 billion mi). eccentricity of orbit: 0.54

orbital period: 6.39 Earth years; next closest pass of

Sun is in February 2010. probes and space missions: US—Stardust, 1999.

Comet Hale-Bopp

distance from Sun at closest point of orbit is 136 million km (84.5 million mi). Farthest distance from Sun is 74.7 billion km (46.4 billion mi).

eccentricity of orbit: 0.995

orbital period: 4,000 Earth years; last closest pass of Sun was on 31 Mar 1997.

Comet Hyakutake

distance from Sun at closest point of orbit is 34 million km (21 million mi). Farthest distance from Sun is 344 billion km (213 billion mi).

eccentricity of orbit: 0.9998

orbital period: about 40,000 Earth years; last closest pass of Sun was on 1 May 1996.

Kuiper belt

(a huge flat ring located beyond Neptune containing residual icy material from the formation of the outer planets)

average distance from Sun (main concentration): 4.5-7.5 billion km (2.8-4.7 billion mi)

mass: Scientists estimate there may be as many as 100,000 icy, cometlike bodies of a size greater than 100 km in the Kuiper belt; the belt is estimated to have a mass of 6,000 x 1020kg.

Oort cloud

(an immense, roughly spherical cloud of icy, cometlike bodies inferred to orbit Sun at distances roughly 1,000 times that of the orbit of Pluto)

average distance from Sun: 3-7 trillion km (1.9-4.3 trillion mi)

mass: some trillions of the cloud’s icy objects have an estimated total mass of at least 600,000 x 1020 kg (10 times the mass of Earth).

Solar System Superlatives

Largest planet in the solar system: Jupiter (142,984 km [88,846 mi] diameter); all of the other planets in the solar system could fit inside Jupiter.

Largest moon in the solar system: Jupiter’s moon Ganymede (5,270 km [3,275 mi]).

Smallest planet in the solar system: Mercury (4,879 km [3,032 mi] diameter).

Smallest moons in the solar system: Saturn and Jupiter both have numerous satellites that are smaller than 10 km (6 mi) in diameter.

Planet closest to the Sun: Mercury (average distance from the Sun 58 million km [36 million mi]).

Planet farthest from the Sun: Neptune (average distance from the Sun 4.50 billion km [2.80 billion mi]); Pluto, demoted to the status of dwarf planet in 2006, was the farthest planet from the Sun for all but 20 years of its 248-year orbital period.

Planet with the most eccentric (least circular) orbit: Mercury (eccentricity of 0.206).

Moon with the most eccentric orbit: Neptune’s moon Nereid (eccentricity of 0.75).

Planet with the least eccentric orbit: Venus (eccentricity of 0.007).

Moon with the least eccentric orbit: Saturn’s moon Tethys (eccentricity of 0.00000).

Planet most tilted on its axis: Uranus (axial tilt of 98° from its orbital plane).

Planet with the most moons: Jupiter (at least 63).

Planets with the fewest moons: Mercury and Venus (no moons).

Planet with the longest day: Venus (1 day on Venus equals 243 Earth days).

Planet with the shortest day: Jupiter (1 day on Jupiter equals 9.9 hours).

Planet with the longest year: Neptune (1 year on Neptune equals 165 Earth years).

Planet with the shortest year: Mercury (1 year on Mercury equals 88 Earth days).

Fastest orbiting planet in the solar system: Mercury (47.9 km per second [29.7 mi per second] average orbital speed).

Slowest orbiting planet in the solar system: Neptune (5.48 km per second [3.40 mi per second] average orbital speed).

Hottest planet in the solar system: Venus (464 °C [867 °F] average temperature); although Mercury is closer to the Sun, Venus is hotter because Mercury has no atmosphere, whereas the atmosphere of Venus traps heat via a strong greenhouse effect.

Coldest planet in the solar system: Neptune (-220 °C [-364 °F] average temperature).

Brightest visible star in the night sky: Sirius (-1.46 apparent visual magnitude).

Brightest planet in the night sky: Venus (apparent visual magnitude -4.5 to -3.77).

Densest planet: Earth (density of 5,515 kg/m3).

Least dense planet: Saturn (density of 687 kg/m3); Saturn in theory would float in water.

Planet with strongest gravity: Jupiter (more than twice the gravitational force of Earth at an altitude at which 1 bar of atmospheric pressure is exerted).

Planet with weakest gravity: Mars (slightly more than % the gravitational force of Earth).

Planet with the largest mountain: Mars (Olympus Mons, an extinct volcano, stands some 21 km [13 mi] above the planet’s mean radius and 540 km [335 mi] across).

Planet with the deepest valley: Mars (Valles Marineris, a system of canyons, is some 4,000 km [2,500 mi] long and from about 2 to 9 km [1 to 5.6 mi] deep).

Largest known impact crater: Valhalla, a crater on Jupiter’s moon Callisto, has a bright central area that is about 600 km (370 mi) across, with sets of concentric ridges extending about 1,500 km (900 mi) from the center. For contrast, the largest crater on Earth believed to be of impact origin is the Vre-defort ring structure in South Africa, which is about 300 km (190 mi) across.

The Sun

The Sun is the star around which the Earth and the other components of the solar system revolve. It is the dominant body of the system, constituting more than 99% of the system’s entire mass. The Sun is the source of an enormous amount of energy, a portion of which provides the Earth with the light and heat necessary to support life. The geologic record of the Earth and Moon reveals that the Sun was formed about 4.5 billion years ago. The energy radiated by the Sun is produced during the conversion of hydrogen atoms to helium. The Sun is at least 90% hydrogen by number of atoms, so the fuel is readily available.

The Sun is classified as a G2 V star, where G2 stands for the second hottest stars of the yellow G class—of surface temperature about 5,500 °C (10,000 °F)—and V represents a main sequence, or dwarf, star, the typical star for this temperature class (see also “Classification of Stars”). The Sun exists in the outer part of the Milky Way Galaxy and was formed from material that had been processed inside other stars and supernovas.

The mass of the Sun is 743 times the total mass of all the planets in the solar system and 330,000 times that of the Earth. All the interesting planetary and interplanetary gravitational phenomena are negligible effects in comparison to the gravitational force exerted by the Sun. Under the force of gravity, the great mass of the Sun presses inward, and to keep the star from collapsing, the central pressure outward must be great enough to support its weight. The Sun’s core, which occupies approximately 25% of the star’s radius, has a density about 100 times that of water (roughly 6 times that at the center of the Earth), but the temperature at the core is at least 15 million °C (27 million °F), so the central pressure is at least 10,000 times greater than that at the center of the Earth. In this environment atoms are completely stripped of their electrons, and at this high temperature the bare nuclei collide to produce the nuclear reactions that are responsible for generating the energy vital to life on Earth.

The temperature of the Sun’s surface is so high that no solid or liquid can exist; the constituent materials are predominantly gaseous atoms, with a very small number of molecules. As a result, there is no fixed surface. The surface viewed from Earth, the photosphere, is approximately 400 km (250 mi) thick and is the layer from which most of the radiation reaches us; the radiation from below the photosphere is absorbed and reradiated, while the emission from overlying layers drops sharply, by about a factor of six every 200 km (124 mi).

While the temperature of the Sun drops from 15 million °C (27 million °F) at the core to around 5,500 °C (10,000 °F)atthe photosphere, a surprising reversal occurs above that point; the temperature begins to rise in the chromosphere, a layer several thousand kilometers thick. Temperatures there range from 4,200 °C (7,600 °F) to 100,000 °C (180,000 °F). Above the chromosphere is a comparatively dim, extended halo called the corona, which has a temperature of 1 million °C (1.8 million °F) and reaches far past the planets. Beyond a distance of around 3.5 million km (2.2 million mi) from the Sun, the corona flows outward at a speed (near the Earth) of 400 km/sec (250 mi/sec); this flow of charged particles is called the solar wind.

The Sun is a very stable source of energy. Superposed on this stability, however, is an interesting 11-year cycle of magnetic activity manifested by regions of transient strong magnetic fields called sunspots. The largest sunspots can be seen on the solar surface even without a telescope.

Mercury

Mercury is the planet closest to the Sun, revolving around it at an average distance of 58 million km (36 million mi). In Sumerian times, some 5,000 years ago, it was already known in the night sky. In classical Greece the planet was called Apollo when it appeared as a morning star and Hermes, for the Greek equivalent of the Roman god Mercury, when it appeared as an evening star.

Mercury’s orbit lies inside the orbit of the Earth and is more elliptical than those of most of the other planets. At its closest approach (perihelion), Mercury is only 46 million km (28.5 million mi) from the Sun, while its greatest distance (aphelion) approaches 70 million km (43.5 million mi). Mercury orbits the Sun in 88 Earth days at an average speed of 48 km per second (29.8 mi per sec), allowing it to overtake and pass Earth every 116 Earth days (synodic period).

Because of its proximity to the Sun, the surface of Mercury can become extremely hot. High temperatures at “noon” may reach 400 °C (755 °F) while the “predawn” lowest temperature is-173 °C(-280 °F). Mercury’s equator is almost exactly in its orbital plane (its spin-axis inclination is nearly zero), and thus Mercury does not have seasons as does the Earth. Because of its elliptical orbit and a peculiarity of its rotational period (see below), however, certain longitudes experience cyclical variations in temperatures on a “yearly” as well as on a “diurnal” basis.

Mercury is about 4,879 km (3,032 mi) in diameter, the smallest of the planets. Mercury is only a bit larger than the Moon. Its mass, as measured by the gravitational perturbation of the path of the Mariner 10 spacecraft during close flybys in 1974-75, is about one-eighteenth of the mass of the Earth. Escape velocity, the speed needed to escape from a planet’s gravitational field, is about 4.3 km per second (2.7 mi per second)—com-pared with 11.2 km per sec (7 mi per sec) for the Earth.

The mean density of Mercury, calculated from its mass and radius, is about 5.43 grams per cubic cm, nearly the same as that of the Earth (5.52 grams per cubic cm).

Photographs relayed by the Mariner 10 spacecraft showed that Mercury spins on its axis (rotates) once every 58.646 Earth days, exactly two-thirds of the orbital period of 87.9694 Earth days. This observation confirmed that Mercury is in a 3:2 spin-orbit tidal resonance—i.e., that tides raised on Mercury by the Sun have forced it into a condition that causes it to rotate three times on its axis in the same time it takes to revolve around the Sun twice. The 3:2 spin-orbit coupling combines with Mercury’s eccentric orbit to create very unusual temperature effects.

Although Mercury rotates on its axis once every 58.646 Earth days, one rotation does not bring the Sun back to the same part of the sky, because during that time Mercury has moved partway around the Sun. A solar day on Mercury (for example, from one sunrise to another, or one noon to another) is 176 Earth days (exactly two Mercurian years).

Mercury’s low escape velocity and high surface temperatures do not permit it to retain a significant atmosphere.

In January 2008 the MESSENGER spacecraft flew by Mercury, revealing previously unseen details in photographs, and scientists approved dozens of new names for surface features such as craters.

Venus

Venus is the second planet from the Sun and the planet whose orbit is closest to that of the Earth. When visible, Venus is the brightest planet in the sky. Viewed through a telescope, it presents a brilliant, yellow-white, essentially featureless face to the observer. The obscured appearance results because the surface of the planet is hidden from sight by a continuous and permanent cover of clouds.

Venus’s orbit is the most nearly circular of that of any planet, with a deviation from perfect circularity of only about 1 part in 150. The period of the orbit—that is, the length of the Venusian year—is 224.7 Earth days. The rotation of Venus is unusual in both its direction and speed. Most of the planets in the solar system rotate in a counterclockwise direction when viewed from above their north poles; Venus, however, rotates in the opposite, or retrograde, direction. Were it not for the planet’s clouds, an observer on Venus’s surface would see the Sun rise in the west and set in the east.

Venus spins on its axis slowly, taking 243 Earth days to complete one rotation. Venus’s spin and orbital periods are nearly synchronized with the Earth’s orbit such that Venus presents almost the same face toward the Earth when the two planets are at their closest.

Venus is nearly the Earth’s twin in terms of size and mass. Venus’s equatorial diameter is about 95% of the Earth’s diameter, while its mass is 81.5% that of the Earth. The similarities to the Earth in size and mass also produce a similarity in density; Venus’s density is 5.24 grams per cubic cm, as compared with 5.52 for the Earth.

In terms of its shape, Venus is more nearly a perfect sphere than are most planets. A planet’s rotation generally causes a slight flattening at the poles and bulging at the equator, but Venus’s very slow rotation rate allows it to maintain its highly spherical shape.

Venus has the most massive atmosphere of all the terrestrial planets (Mercury, Venus, Earth, and Mars). Its atmosphere is composed of 96.5% carbon dioxide and 3.5% nitrogen. The atmospheric pressure at the planet’s surface varies with the surface elevation but averages about 90 bars, or 90 times the atmospheric pressure at the Earth’s surface. This is the same pressure found at a depth of about one kilometer in the Earth’s oceans. Temperatures range between a minimum temperature of-45 °C(-49 °F) and a maximum temperature of 500 °C (932 °F); the average temperature is 464 °C (867 °F).

Earth

The Earth is the third planet in distance outward from the Sun. It is the only planetary body in the solar system that has conditions suitable for life, at least as known to modern science.

The average distance of the Earth from the Sun— 149.6 million km (93 million mi)—is designated as the distance of the unit of measurement known as the AU (astronomical unit). The Earth orbits the Sun at a speed of 29.8 km (18.5 mi) per second, making one complete revolution in 365.25 days. As it revolves around the Sun, the Earth spins on its axis and rotates completely once every 23 hr 56 min 4 sec. The Earth has a single natural satellite, the Moon.

The fifth largest planet of the solar system, the Earth has a total surface area of roughly 509.6 million sq km (197 million sq mi), of which about 29%, or 148 million square km (57 million square mi), is land. Oceans and smaller seas cover the balance of the surface. The Earth is the only planet known to have liquid water. Together with ice, the liquid water constitutes the hydrosphere. Seawater makes up more than 98% of the total mass of the hydrosphere and covers about 71% of the Earth’s surface. Significantly, seawater constituted the environment of the earliest terrestrial life forms.

The Earth’s atmosphere consists of a mixture of gases, chiefly nitrogen (78%) and oxygen (21%). Argon makes up much of the remainder of the gaseous envelope, with trace amounts of water vapor, carbon dioxide, and various other gases also present.

The Earth’s structure consists of an inner core of nearly solid iron, surrounded by successive layers of molten metals and solid rock, and a thin layer at the surface comprising the continental crust.

The Earth is surrounded by a magnetosphere, a region dominated by the Earth’s magnetic field and extending upward from about 140 km (90 mi) in the upper atmosphere. In the magnetosphere, the magnetic field of the Earth traps rapidly moving charged particles (mainly electrons and protons), the majority of which flow from the Sun (as solar wind). If it were not for this shielding effect, such particles would bombard the terrestrial surface and destroy life. High concentrations of the trapped particles make up two doughnut-shaped zones called the Van Allen radiation belts. These belts play a key role in certain geophysical phenomena, such as auroras.

The Moon

The Moon is the sole natural satellite of the Earth. It revolves around the planet from west to east at a mean distance of about 384,400 km (238,900 mi). The Moon is less than one-third the size of the Earth, having a diameter of only about 3,475 km (2,159 mi) at its equator. The Moon shines by reflecting sunlight, but its albedo—i.e., the fraction of light received that is reflected—is only 0.073.

The Moon rotates about its own axis in about 27.32 days, which is virtually identical to the time it takes to complete its orbit around the Earth. As a result, the Moon always presents nearly the same face to the Earth. The rate of actual rotation is uniform, but the arc through which the Moon moves from day to day varies somewhat, causing the lunar globe (as seen by a terrestrial observer) to oscillate slightly over a period nearly equal to that of revolution.

The surface of the Moon has been a subject of continuous telescopic study from the time of Galileo’s first observation in 1609. The Italian Jesuit astronomer Giovanni B. Riccioli designated the dark areas on the Moon as seas (maria), with such fanciful names as Mare Imbrium (“Sea of Showers”) and Mare Nectaris (“Sea of Nectar”). This nomenclature continues to be used even though it is now known that the Moon is completely devoid of surface water. During the centuries that followed the publication of these early studies, more detailed maps and, eventually, photographs were produced. A Soviet space probe photographed the side of the Moon facing away from the Earth in 1959. By the late 1960s the US Lunar Or-biter missions had yielded close-up photographs of the entire lunar surface. On 20 Jul 1969, Apollo 11 astronauts Neil Armstrong and Edwin (“Buzz”) Aldrin set foot on the Moon.

The most striking formations on the Moon are its craters. These features, which measure up to about 200 km (320 mi) or more in diameter, are scattered over the surface in great profusion and often overlap one another. Meteorites hitting the lunar surface at high velocity produced most of the large craters. Many of the smaller ones—those measuring less than 1 km (0.6 mi) across—appear to have been formed by explosive volcanic activity, however. The Moon’s maria have relatively few craters. These lava outpourings spread over vast areas after most of the craters had already been formed.

Various theories for the Moon’s origin have been proposed. At the end of the 19th century, the English astronomer Sir George H. Darwin advanced a hypothesis stating that the Moon had been originally part of the Earth but had broken away as a result of tidal gravitational action and receded from the planet. This was proved unlikely in the 1930s. A theory that arose during the 1950s postulated that the Moon had formed elsewhere in the solar system and was then later captured by the Earth. This idea was also proved to be physically implausible and was dismissed. Today, most investigators favor an explanation known as the giant-impact hypothesis, which postulates that a Mars-sized body struck the proto-Earth early in the history of the solar system. As a result, a cloud of fragments from both bodies was ejected into orbit around the Earth, and this later accreted into the Moon.

Moon Phases, 2008-2009

As the Moon orbits the Earth, more or less of the half of the Moon illuminated by the Sun is visible on Earth. During the lunar month the Moon’s appearance changes from dark (the new moon) to being illuminated more and more on the right side (waxing crescent, first quarter, and waxing gibbous) to the full disc being illuminated (the full moon). The phases of the Moon are completed by the Moon being illuminated less and less on the left side (waning gibbous, last quarter, and waning crescent) and end with another new moon. The cycle of the Moon takes place over a period of around 29 days; the time from new moon to new moon is referred to as a lunation.

The phases of the Moon are caused by the positions of the Sun in relationship to the Moon. Thus, when the Sun and Moon are close in the sky a dark new moon is the result (the Sun is lighting the half of the Moon not visible to Earth). When the Sun and Moon are at opposition (in opposite parts of the sky) the full moon occurs (the Sun illuminates fully the half of the Moon seen on Earth). When the Sun and Moon are at about a 90-degree angle, one sees either a first quarter or last quarter moon.

The dates for the new moon, first quarter, full moon, and last quarter for June 2008-December 2009 are given in the table below.

The distance between the centers of mass of the Earth and the Moon varies rather widely due to the combined gravity of the Earth, the Sun, and the planets. For example, during the period 1969-2000, apogee (when the Moon is at the greatest distance from Earth) varied from 404,063 to 406,711 km (251,073 to 252,719 mi), while perigee (when the Moon is closest to Earth) varied from 356,517 to 370,354 km (221,529 to 230,127 mi). Tidal interactions have braked the Moon’s spin so that presently the same side always faces the Earth. Dates are Universal Time/GMT.

Moon at apogee
DATE	OCCURS
23 January	between last quarter and new moon
19 February	between last quarter and new moon
19 March	between full moon and last quarter
16 April	between full moon and last quarter
14 May	between full moon and last quarter
10 June	at full moon
7 July	between first quarter and full moon
4 August	between first quarter and full moon
31 August	between first quarter and full moon
28 September	at first quarter
25 October	between new moon and first quarter
22 November	between new moon and first quarter
20 December	between last quarter and new moon

Moon Phases, 2008-2009

Moon at perigee
DATE	OCCURS
10 January	between first quarter and full moon
7 February	at first quarter
7 March	between new moon and first quarter
2 April	between new moon and first quarter
28 April	between new moon and first quarter
26 May	at new moon
23 June	between last quarter and new moon
21 July	between last quarter and new moon
19 August	between last quarter and new moon
16 September	between full moon and last quarter
13 October	between full moon and last quarter
7 November	between full moon and last quarter
4 December	at full moon

Mars

Mars is the fourth planet in order of distance from the Sun and the seventh in order of diminishing size and mass. It orbits the Sun once in 687 Earth days and spins on its axis once every 24 hr 37 min.

Owing to its blood-red color, Mars has often been associated with warfare and slaughter. It is named for the Roman god of war; as far back as 3,000 years ago, Babylonian astronomer-astrologers called the planet Nergal for their god of death and pestilence. The Greeks called it Ares for their god of battle; the planet’s two satellites, Phobos (Fear) and Deimos (Terror), were later named for the two sons of Ares and Aphrodite.

Mars moves around the Sun at a mean distance of approximately 1.52 times that of the Earth from the Sun. Because the orbit of Mars is relatively elongated, the distance between Mars and the Sun varies from 206.6 to 249.2 million km (128.4 to 154.8 million mi). Mars completes a single orbit in roughly the time in which the Earth completes two. At its closest approach, Mars is less than 56 million km (34.8 million mi) from the Earth, but it recedes to almost 400 million km (248.5 million mi). Mars is a small planet. Its equatorial radius is about half that of Earth, and its mass is only one-tenth the terrestrial value.

The axis of rotation is inclined to the orbital plane at an angle of 24.9°, and, as for the Earth, the tilt gives rise to the seasons on Mars. The Martian year consists of 668.6 Martian solar days (called sols). The orientation and eccentricity of the orbit (eccentricity denotes how much the orbit deviates from a perfect circle: the more elongated, the more eccentric) leads to seasons that are quite uneven in length.

The Martian atmosphere is composed mainly of carbon dioxide. It is very thin (less than 1% of the Earth’s atmospheric pressure). Evidence suggests that the atmosphere was much denser in the remote past and that water was once much more abundant at the surface. Only small amounts of water are found in the lower atmosphere today, occasionally forming thin ice clouds at high altitudes and, in several localities, morning ice fogs. Mars’s polar caps consist of frozen carbon dioxide and water ice. Intriguing spacecraft observations confirm that water ice also is present under large areas of the Martian surface and hint that liquid water may have flowed in geologically recent times.

The characteristic temperature in the lower atmosphere is about -70 °C (-100 °F). Unlike that of Earth, the total mass (and pressure) of the atmosphere experiences large seasonal variations, as carbon dioxide “snows out” at the winter pole.

The surface of Mars shows some of the most dramatic variation in the solar system: the massive extinct volcano Olympus Mons stands some 21 km (13 mi) above the planet’s mean radius and is 540 km (335 mi) across, and Valles Marineris, a system of canyons, is some 4,000 km (2,500 mi) long and from about 2 to 9 km (1 to 5.6 mi) deep.

The two satellites of Mars—Phobos and Deimos—were discovered in 1877 by Asaph Hall of the United States Naval Observatory. Little was known about these bodies until observations were made by NASA’s orbiting Mariner 9 spacecraft nearly a century later. The moons of Mars cannot be seen from all locations on the planet because of their small size, proximity to the planet, and near-equatorial orbits.

Two Mars Exploration Rovers—Spirit and Opportunity— landed on Mars in January 2004. In 2008 they continued to explore features of the planet, notably stratigraphy and volcanic activity, in their third Martian winter. In May 2008 the spacecraft Phoenix successfully landed on the planet and began its mission to be the first spacecraft to retrieve and study water (ice) from another planet. In late July it confirmed the presence of water on Mars.

Small Celestial Bodies

Small bodies are defined as all the natural objects in the solar system other than the Sun and the major planets and their satellites. The solar system is populated by vast numbers of these small bodies, which can be grouped as asteroids, comets, and meteoroids (at times, however, the distinctions between these groupings can be somewhat blurred).

Small bodies in stable orbits are found in several regions of the solar system. Most asteroids reside in a belt between Mars and Jupiter at approximately 300-600 million km (190-380 million mi). Others, called Trojan asteroids, are found at gravitationally stable points near the orbits of Mars and Jupiter.

The trans-Neptunian objects (considered comets) are located outside the orbit of Neptune, from around 4.5-7.5 billion km (2.8-4.7 billion mi) in the area known as the Kuiper belt. A spherical cloud known as the Oort cloud also contains comets at a distance of some 3-15 trillion km (1.8-9 trillion mi).

Other small bodies travel in unstable paths which cross planetary orbits. These include: all observed comets; near-Earth asteroids, whose orbits either cross or closely approach Earth’s orbit; and other planet-crossing objects (a mixture of both asteroids and icy cometlike bodies). All objects on planet-crossing orbits will eventually collide with the Sun or a planet or be permanently ejected from the solar system, although some of these objects do survive for long periods of time due to stabilizing orbital resonances.

Comets originate, and most are still located, in the Kuiper belt and Oort cloud. Even though comets are brief visitors to the inner solar system, their population is constantly replenished through perturbations of the comets in these areas.

There are several characteristics that traditionally have distinguished asteroids, comets, and meteoroids. These are based upon origin, orbital, and physical differences. An object is classified as a comet when it displays a coma or tail (or any evidence of gas or dust coming from it). In addition, the icy objects found in the Kuiper belt (and the Oort cloud, though none of these are observable) are also considered to be comets. They do not display cometary activity because of their great distance from the Sun. Nevertheless, they are believed to be made up of the same volatile material—primarily water and carbon dioxide—as the nuclei of observed comets, and it is the presence of these volatiles on the surface that is responsible for cometary activity. Finally, objects on parabolic or hyperbolic (nonreturning) orbits are generally considered to be comets.

Meteoroids are defined as any small object in space, especially one less than a few tens of meters in size. When a meteoroid enters the Earth’s atmosphere, the heat of friction creates a glowing trail of hot gases called a meteor. Should any part of a meteoroid reach the ground without being completely vaporized, that object is termed a meteorite. The term asteroid is traditionally reserved for the larger rocky bodies in solar orbit, which range up to nearly 1,000 km (600 mi) in size.

Asteroids and the Asteroid Belt

Asteroids are any of a host of small rocky bodies, about 1,000 km (600 mi) or less in diameter, that orbit the Sun. About 95% of the known asteroids move in orbits between those of Mars and Jupiter in an area known as the asteroid belt. The orbits of the asteroids, however, are not uniformly distributed within the asteroid belt, but exhibit “gaps.” Known as Kirkwood gaps, these asteroid-less areas are maintained by the gravitational force exerted by Jupiter upon asteroids in certain orbits.

The vast majority of asteroids have orbital periods between three years and six years—i.e., between one-fourth and one-half of Jupiter’s orbital period. These asteroids are said to be main-belt asteroids. Within the main belt are asteroids that share certain traits. Known as families, about 40% of all known asteroids belong to such groupings. Families are usually assigned the name of the lowest numbered (first discovered) asteroid in the family. The three largest families (Eos, Koronis, and Themis) have been determined to be compositionally homogeneous; each is thought to comprise fragments from a larger parent body that broke apart in a collision.

Besides the few asteroids in highly unusual orbits, there are a number of groups that fall outside the main belt. Those that have orbital periods greater than one-half that of Jupiter are called outer-belt asteroids. There are four such groups: the Cybeles, Hildas, Thule, and Trojan groups.

There is only one known group of inner-belt as-teroids—namely, the Hungarias. The Hungaria asteroids have orbital periods that are less than one-fourth that of Jupiter. Finally, asteroids that pass inside the orbit of Mars are said to be near-Earth asteroids. There are two groups of near-Earth asteroids that deeply cross the Earth’s orbit on an almost continuous basis. The first of these to be discovered were the Apollo asteroids. The other group of Earth-crossing asteroids is named Atens. A third group, the Amors, comprises part-time Earth crossers.

Asteroids are thought to be made of the same rocky (stony, metallic, and carbon-rich) material that formed the planets. Scientists believe that at the time the planets were forming the gravitational influence of what became Jupiter kept the asteroids from aggregating into a single planet. Since that time they have been evolving through ongoing collisions so that most of the present-day asteroids are remnants or fragments of larger bodies. As of 2008 astronomers had detected and numbered more than 90,000 asteroids.

Jupiter

Jupiter is the most massive of the planets and is fifth in distance from the Sun. When ancient astronomers named the planet Jupiter for the ruler of the gods in the Greco-Roman pantheon, they had no idea of the planet’s true dimensions, but the name is appropriate, for Jupiter is larger than all the other planets combined. It has a narrow ring system and at least 63 known satellites, 3 larger than the Earth’s Moon. Jupiter also has an internal heat source— i.e., it emits more energy than it receives from the Sun. This giant has the strongest magnetic field of any planet, with a magnetosphere so large that, if it could be seen from Earth, its apparent diameter would exceed that of the Moon. Jupiter’s system is the source of intense bursts of radio noise, at some frequencies occasionally radiating more energy than the Sun.

Of special interest concerning Jupiter’s physical properties is the low mean density of 1.33 grams per cubic cm—in contrast with Earth’s 5.52 grams/cm3—coupled with the large dimensions and mass and the short rotational period. The low density and large mass indicate that Jupiter’s composition and structure are quite unlike those of the Earth and the other inner planets, a deduction that is supported by detailed investigations of the giant planet’s atmosphere and interior.

Jupiter has no solid surface; the transition from the atmosphere to its highly compressed core occurs gradually at great depths. The close-up views of Jupiter from the Voyager spacecraft revealed a variety of cloud forms, with a predominance of elliptical features reminiscent of cyclonic and anticyclonic storm systems on the Earth. All these systems are in motion, appearing and disappearing on time scales dependent on their sizes and locations. Also observed to vary are the pastel shades of various colors present in the cloud layers—from the tawny yellow that seems to characterize the main layer, through browns and blue-grays, to the well-known salmon-colored Great Red Spot, Jupiter’s largest, most prominent, and longest-lived feature.

Because Jupiter has no solid surface, it has no topographic features, and latitudinal currents dominate the planet’s large-scale circulation. The lack of a solid surface with physical boundaries and regions with different heat capacities makes the persistence of these currents and their associated cloud patterns all the more remarkable. The Great Red Spot, for example, moves in longitude with respect to Jupiter’s rotation, but it does not move in latitude.

The Voyager 1 spacecraft verified the existence of a ring system surrounding Jupiter when it crossed the planet’s equatorial plane. Subsequently, images from the Galileo spacecraft revealed that the ring system consists principally of four concentric components whose boundaries are associated with the orbits of Jupiter’s four innermost moons. The ring system is comprised of large numbers of micrometer-sized particles that produce strong forward scattering of incident sunlight. The presence of such small particles requires a source, and the association of the ring boundaries with the four moons makes the source clear. The particles are generated by impacts on these moons (and on still smaller bodies within the main part of the ring) by microme-teoroids, cometary debris, and possibly volcanically produced material from Jupiter’s moon Io.

With the exception of snakes and bees, scorpions cause more deaths than any other nonparasitic group of animals. It is thought that more than 5,000 people die each year from scorpion stings. A long curved tail with a venomous stinger and grasping, fingerlike first appendages are typical scorpion features.

Did you knows

The satellites orbiting Jupiter are numerous; there are at least 63 Jovian moons and likely additional ones to be discovered.

The first objects in the solar system discovered by means of a telescope (by Galileo in 1610) were the four brightest moons of Jupiter. Now known as the Galilean satellites, they are (in order of increasing distance from Jupiter) Io, Europa, Ganymede, and Callisto. Each is a unique world in its own right. Callisto and Ganymede, for example, are as large or larger than the planet Mercury, but, while Callisto’s icy surface is ancient and heavily cratered from impacts, Ganymede’s appears to have been extensively modified by internal activity. Europa may still be geologically active and may harbor an ocean of liquid water, and possibly even life, beneath its frozen surface. Io is the most volcani-cally active body in the solar system; its surface is a vividly colored landcape of erupting vents, pools and solidified flows of lava, and sulfurous deposits.

Data for the first 16 known Jovian moons (discovered 1610-1979) are summarized below. The orbits of the inner eight satellites have low inclinations (they are not tilted relative to the planet’s equator) and low eccentricities (their orbits are relatively circular). The orbits of the outer eight have much higher inclinations and eccentricities, and four of them are retrograde (they are opposite to Jupiter’s spin and orbital motion around the Sun). The innermost four satellites are thought to be intimately associated with Jupiter’s ring and are the sources of the fine particles within the ring itself.

NAME	MEAN DISTANCE
(DESIGNATION)	FROM JUPITER	DIAMETER
Metis (JXVI)	128,000 km	40 km
	(79,500 mi)	(25 mi)
Adrastea (JXV)	129,000 km	20 km
	(80,000 mi)	(12 mi)
Amalthea (JV)1	181,000 km	189 km
	(112,500 mi)	(117 mi)
Thebe (JXIV)	222,000 km	100 km
	(138,000 mi)	(62 mi)
Io(JI)1	422,000 km	3,630 km
	(262,000 mi)	(2,256 mi)
Europa (JII)1	671,000 km	3,130 km
	(417,000 mi)	(1,945 mi)
Ganymede (JIII)1	1,070,000 km	5,268 km
	(665,000 mi)	(3,273 mi)
Callisto (JIV)1	1,883,000 km	4,806 km
	(1,170,000 mi)	(2,986 mi)
Leda (JXIII)	11,127,000 km	10 km
	(6,914,000 mi)	(6 mi)
Himalia (JVI)	11,480,000 km	170 km
	(7,133,000 mi)	(106 mi)
Lysithea (JX)	11,686,000 km	24 km
	(7,261,300 mi)	(15 mi)
Elara (JVII)	11,737,000 km	80 km
	(7,293,000 mi)	(50 mi)
Ananke (JXII)	21,269,000 km	20 km
	(13,216,000 mi)	(12.5 mi)
Carme (JXI)	23,350,000 km	30 km
	(14,509,000 mi)	(18.6 mi)
Pasiphae (JVIII)	23,500,000 km	36 km
	(14,602,000 mi)	(22.3 mi)
Sinope (JIX)	23,700,000 km	28 km
	(14,726,500 mi)	(17.3 mi)

Beginning in 1999 some 47 tiny moons (including one seen in 1975 and then lost) were discovered photographically in observations from Earth. All have high orbital eccentricities and inclinations and large orbital radii; nearly all of the orbits are retrograde. Rough size estimates based on their brightness place them between 2 and 8 km (1.2 and 5 mi) in diameter. They were assigned provisional numerical designations on discovery; many also have received official names.

In the table, “sync” denotes that the orbital period and rotational period are the same, or synchronous; hence, the moon always keeps the same face toward Jupiter. “R” following the orbital period indicates a retrograde orbit. Unspecified quantities are unknown.

MASS (1020 KG) 0.001	ORBITAL PERIOD (EARTH DAYS) 0.295	ROTATIONAL PERIOD (EARTH DAYS) sync
0.0002	0.298	sync
0.075	0.498	sync
0.008	0.675	sync
893.2	1.769	sync
480.0	3.551	sync
1,482.0	7.155	sync
1,076.0	16.689	sync
0.00006	234
0.095	251	0.4
0.0008	258	0.5
0.008	256	0.5
0.0004	634 R	0.4
0.001	729 R	0.4
0.003	735 R
0.0008	758 R	0.5

Jupiter’s complex ring was discovered and first studied by the twin Voyager spacecraft during their flybys of the giant planet in 1979. It is now known to consist of four main components: an outer gossamer ring, whose outer radius coincides with the orbital radius of the Jovian moon Thebe (222,000 km; 138,000 mi); an inner gossamer ring bounded on its outer edge by the orbit of Amalthea (181,000 km; 112,500 mi); the main ring, extending inward some 6,000 km (3,700 mi) from the orbits of Adrastea (129,000 km; 80,000 mi) and Metis (128,000 km; 79,500 mi); and a halo of particles with a thickness of 25,000 km (15,500 mi) that extends from the main ring inward to a radius of about 95,000 km (59,000 mi). For comparison, Jupiter’s visible surface lies at a radius of about 71,500 km (44,400 mi) from its center. The four moons involved with the ring are believed to supply the fine particles that compose it.

Saturn

Saturn is the sixth planet in order of distance from the Sun and the second largest of the planets in mass and size. Its dimensions are almost equal to those of Jupiter, while its mass is about a third as large; it has the lowest mean density of any object in the solar system.

Both Saturn and Jupiter resemble stellar bodies in that the light gas hydrogen dominates their bulk chemical composition. Saturn’s atmosphere is 91% hydrogen by mass and is thus the most hydrogen-rich atmosphere in the solar system. Saturn’s structure and evolutionary history, however, differ significantly from those of its larger counterpart. Like the other giant planets—Jupiter, Uranus, and Neptune—Saturn has extensive satellite and ring systems, which may provide clues to its origin and evolution. The planet has at least 60 moons, including the second largest in the solar system. Saturn’s dense and extended rings, which lie in its equatorial plane, are the most impressive in the solar system.

Saturn has no single rotation period. Cloud motions in its massive upper atmosphere can be used to trace out a variety of rotation periods, with periods as short as about 10 hours 10 minutes near the equator and increasing with some oscillation to about 30 minutes longer at latitudes higher than 40°. The rotation period of Saturn’s deep interior can be determined from the rotation period of the magnetic field, which is presumed to be rooted in an outer core of hydrogen compressed to a metallic state. The “surface” of Saturn that is seen through telescopes and in spacecraft images is actually a complex layer of clouds.

The atmosphere of Saturn shows many smaller-scale time-variable features similar to those found in Jupiter, such as red, brown, and white spots, bands, eddies, and vortices. The atmosphere generally has a much blander appearance than Jupiter’s, however, and is less active on a small scale. A spectacular exception occurred during September-November 1990, when a large white spot appeared near the equator, expanded to a size exceeding 20,000 km (12,400 mi), and eventually spread around the equator before fading.

Saturnian Moons

At least 60 natural satellites are known to circle the planet Saturn. Data for the first 18 Saturnian moons (discovered 1655-1990) are summarized below. As with the other giant planets, those satellites closest to Saturn are mostly regular, meaning that their orbits are fairly circular and not greatly inclined (tilted) with respect to the planet’s equator. All of the satellites in the table except distant Phoebe are regular.

Titan is Saturn’s largest moon and the only satellite in the solar system known to have clouds and a dense atmosphere (composed mostly of nitrogen and methane). The moon is also enveloped in a reddish haze, which is thought to be composed of complex organic compounds that are produced by the action of sunlight on its clouds and atmosphere. That organic molecules may have been settling out of the haze onto Titan’s surface for much of its history has encouraged some scientists to speculate on the possibility that life may have evolved there. Observations by the Cassini-Huygens spacecraft showed Titan to have a varied surface sculpted by rains of hydrocarbon compounds, flowing liquids, wind, impacts, and possibly volcanic and tectonic activity. Saturn’s second largest moon is Rhea, followed by lapetus and Dione.

An unusual Saturnian satellite is Hyperion. Owing to its highly irregular shape and eccentric orbit, it does not rotate stably about a fixed axis. Unlike any other known object in the solar system, Hyperion rotates chaotically, alternating unpredictably between periods of tumbling and seemingly regular rotation.

Between 2000 and 2005 about 30 additional tiny moons occupying various (mostly distant) orbits were discovered. Like the numerous outer moons of Jupiter, nearly all of the recent finds around Saturn belong to the irregular class, meaning that their orbits are highly inclined and elliptical. More than half of them, plus Phoebe, are in retrograde orbits (they move opposite to Saturn’s spin and orbital motion around the Sun).

In March 2008 it was announced that the Cassini spacecraft had taken images of Rhea in 2005 that appeared to show the first known ring around a moon.

In the table, “sync” denotes that the orbital period and rotational period are the same, or synchronous. Unspecified quantities are unknown.

Saturn’s rings rank among the most spectacular phenomena in the solar system. They have intrigued astronomers ever since they were discovered telescopically by Galileo in 1610, and their mysteries have only deepened since they were photographed and studied by Voyagers 1 and 2 in the early 1980s. The particles that make up the rings are composed primarily of water ice and range from dust specks to car- and house-sized chunks. The rings exhibit a great amount of structure on many scales, from the broad A, B, and C rings visible from Earth down to myriad narrow component ringlets. Odd structures resembling spokes, braids, and spiral waves are also present. Some of this detail is explained by gravitational interaction with a number of Saturn’s 56 moons (the orbits of well more than a dozen known moons, from Pan to Dione and Helene, lie within the rings), but much of it remains unaccounted for.

Numerous divisions or gaps are seen in the major ring regions. A few of the more prominent ones are named for famous astronomers who were associated with studies of Saturn.

The major rings and gaps, listed outward from Saturn, are given below. For comparison, Saturn’s visible surface lies at a radius of about 60,300 km (37,500 mi).

Uranus is the seventhth planet in order of distance from the Sun and the first found with the aid of a telescope. Its low density and large size place it among the four giant planets, all of which are composed primarily of hydrogen, helium, water, and other volatile compounds and which thus are without solid surfaces. Absorption of red light by methane gas gives the planet a blue-green color. The planet has at least 27 satellites, ranging up to 789 km (490 mi) in radius, and 13 narrow rings.

Uranus spins on its side; its rotation axis is tipped at an angle of 98° relative to its orbit axis. The 98° tilt is thought to have arisen during the final stages of planetary accretion when bodies comparable in size to the present planets collided in a series of violent events that knocked Uranus onto its side.

Although Uranus is nearly featureless, extreme contrast enhancement of images taken by the Voyager spacecraft reveals faint bands oriented parallel to circles of constant latitude. Apparently the rotation of the planet and not the distribution of absorbed sunlight controls the cloud patterns.

Wind is the motion of the atmosphere relative to the rotating planet. At high latitudes on Uranus, as on the Earth, this relative motion is in the direction of the planet’s rotation. At low (that is, equatorial) latitudes, the relative motion is in the opposite direction. On the Earth these directions are called east and west, respectively, but the more general terms are prograde and retrograde. The winds that exist on Uranus are several times stronger than are those of the Earth. The wind is 200 m (656 ft) per second (prograde) at a latitude of 55° S and 110 m (360.8 ft) per second (retrograde) at the equator. Neptune’s equatorial winds are also retrograde, although those of Jupiter and Saturn are prograde. No satisfactory theory exists to explain these differences.

Uranus

Uranus has no large spots like the Great Red Spot of Jupiter or the Great Dark Spot of Neptune. Since the giant planets have no solid surfaces, the spots represent atmospheric storms. For reasons that are not clear, Uranus seems to have the smallest number of storms of any of the giant planets. Most of the mass of Uranus (roughly 80%) is in the form of a liquid core made primarily of icy materials (water, methane, and ammonia).

Uranus was discovered in 1781 by the English astronomer William Herschel, who had undertaken a survey of all stars down to eighth magnitude—i.e., those about five times fainter than stars visible to the naked eye. Herschel suggested naming the new planet the Georgian Planet after his patron, King George III of England, but the planet was eventually named according to the tradition of naming planets for the gods of Greek and Roman mythology; Uranus is the father of Saturn, who is in turn the father of Jupiter.

After the discovery, Herschel continued to observe the planet with larger and better telescopes and eventually discovered its two largest satellites, Tita-nia and Oberon, in 1787. Two more satellites, Ariel and Umbriel, were discovered by the British astronomer William Lassell in 1851. The names of the four satellites come from English literature—they are characters in works by Shakespeare and Pope— and were proposed by Herschel’s son, John Her-schel. A fifth satellite, Miranda, was discovered by Gerard P. Kuiper in 1948. The tradition of naming the satellites after characters in Shakespeare’s and Pope’s works continues to the present.

Uranian Moons and Rings

Uranus has 27 known satellites forming three distinct groups: 13 small moons orbiting quite close to the planet, 5 large moons located somewhat farther out, and finally, another 9 small and much more distant moons. The members of the first two groups are in nearly circular orbits with low inclinations with respect to the planet.

The densities of the four largest satellites, Ariel, Umbriel, Titania, and Oberon, suggest that they are about half (or more) water ice and the rest rock. Oberon and Umbriel are heavily scarred with large impact craters dating back to the very early history of the solar system, evidence that their surfaces probably have been stable since their formation. In contrast, Titania and Ariel have far fewer large craters, indicating relatively young surfaces shaped over time by internal geological activity. Miranda, though small compared with the other major moons, has a unique jumbled patchwork of varied surface terrain revealing surprisingly extensive past activity. Data for the major satellites are summarized below.

The 5 major moons were discovered telescopically from Earth between 1787 and 1948. Eleven of the 13 innermost moons, with diameters of about 40-160 km (25-100 mi), were found in Voyager 2 images. The rest of the moons, with diameters of 10-200 km (6-120 mi), were detected in Earth-based observations between 1997 and 2003; the orbital motion of nearly all of the outermost moons is retrograde (opposite to the direction of Uranus’s spin and revolution around the Sun).

Thirteen narrow rings are known to encircle Uranus, with radii from 39,600 to 97,700 km (24,600 to 60,700 mi), for the most part within the orbits of the innermost moons. For comparison, Uranus’s visible surface lies at a radius of about 25,600 km (15,900 mi). The ring system was first detected in 1977 during Earth-based observations of Uranus. Subsequent observations from Earth and images from Voyager 2 and the Hubble Space Telescope clarified the number and other features of the rings.

Neptune is the eighth planet in average distance from the Sun. It was named for the Roman god of the sea. The sea god’s trident serves as the planet’s astronomical symbol.

Neptune’s distance from the Sun varies between 29.8 and 30.4 astronomical units (AUs). Its diameter is about four times that of the Earth, but because of its great distance Neptune cannot be seen from the Earth without the aid of a telescope. Neptune’s deep blue color is due to the absorption of red light by methane gas in its atmosphere. It receives less than half as much sunlight as Uranus, but heat escaping from its interior makes Neptune slightly warmer than the latter. The heat released may also be responsible for Neptune’s stormier atmosphere, which exhibits the fastest winds seen on any planet in the solar system.

Neptune’s orbital period is 164.8 Earth years. It has not completely circled the Sun since its discovery in 1846, so some refinements in calculations of its orbital size and shape are still expected. The planet’s orbital eccentricity of 0.009 means that its orbit is very nearly circular; among the planets in the solar system, only Venus has a smaller eccentricity. Neptune’s seasons (and the seasons of its moons) are therefore of nearly equal length, each about 41 Earth years in duration. The length of Neptune’s day, as determined by Voyager 2, is 16.11 hours.

As with the other giant planets of the outer solar system, Neptune’s atmosphere is composed predominantly of hydrogen and helium. The temperature of Neptune’s atmosphere varies with altitude. A minimum temperature of about -223 °C (-369 °F) occurs at pressure near 0.1 bar. The temperature increases with altitude to about 477 °C (891 °F) at 2,000 km (1,240 mi, which corresponds to a pressure of 10-11 bar) and remains uniform above that altitude. It also increases with depth to about 6,730 °C (12,140 °F) near the center of the planet.

Uranian Moons and Rings (continued)

NAME	MEAN DISTANCE		MASS	DENSITY	ORBITAL PERIOD/ ROTATIONAL PERIOD
(DESIGNATION)	FROM URANUS	DIAMETER	(1020 KG)	(GRAMS/CM3)	(EARTH DAYS)*
Miranda (V)	129,390 km (80,400 mi)	472 km (293 mi)	0.66	1.2	1.41
Ariel (I)	191,020 km (118,690 mi)	1,158 km (720 mi)	13.5	1.67	2.52
Umbriel (II)	266,300 km (165,470 mi)	1,169 km (726 mi)	11.7	1.4	4.14
Titania (III)	435,910 km (270,860 mi)	1,578 km (981 mi)	35.3	1.71	8.71
Oberon (IV)	583,520 km (362,580 mi)	1,523 km (946 mi)	30.1	1.63	13.46

*The orbital period and rotational period are the same, or synchronous, for the listed moons.

Neptune

As with the other giant planets of the outer solar system, the winds on Neptune are constrained to blow generally along lines of constant latitude and are relatively invariable with time. Winds on Neptune vary from about 100 m/sec (328 ft/sec) in an easterly (prograde) direction near latitude 70° S to as high as 700 m/sec (2,300 ft/sec) in a westerly (retrograde) direction near latitude 20° S.

The high winds and relatively large contribution of escaping internal heat may be responsible for the observed turbulence in Neptune’s visible atmosphere. Two large dark ovals are clearly visible in images of Neptune’s southern hemisphere taken by Voyager 2 in 1989, although they are not present in Hubble Space Telescope images made 2 years later. The largest, called the Great Dark Spot because of its similarity in latitude and shape to Jupiter’s Great Red Spot, is comparable to the entire Earth in size. It was near this feature that the highest wind speeds were measured. Atmospheric storms such as the Great Dark Spot may be centers where strong upwelling of gases from the interior takes place.

Neptune’s mean density is about 30% of the Earth’s; nevertheless, it is the densest of the giant planets. Neptune’s greater density implies that a larger percentage of its interior is composed of melted ices and molten rocky materials than is the case for the other gas giants.

Neptunian Moons and Rings

Neptune has at least 13 natural satellites, but Earth-based observations had found only 2 of them, Triton in 1846 and Nereid in 1949, before Voyager 2 flew by the planet. The spacecraft observed 5 small moons orbiting close to Neptune and verified the existence of a 6th that had been detected from Earth in 1981. Data for these 8 moons are summarized in the table below. In 2002-03, 5 additional small moons (diameters roughly 30-60 km [20-40 mi]) were discovered telescopically from Earth; they all occupy highly inclined and elliptical orbits that are comparatively far from Neptune.

Triton is Neptune’s only large moon and the only large satellite in the solar system to orbit its planet in the retrograde direction (opposite the planet’s rotation and orbital motion around the Sun). Thus, as is also suspected of the solar system’s other retrograde moons, Triton likely was captured by its planet rather than formed in orbit with its planet from the solar nebula. Its density (2 grams/cm3) suggests that it is about 25% water ice and the rest rock. Triton has a tenuous atmosphere, mostly of nitrogen. Its varied icy surface, imaged by Voyager 2, contains giant faults and dark markings that have been interpreted as the product of geyserlike “ice volcanoes” in which the eruptive material may be gaseous nitrogen and methane. Nereid has the most elliptical orbit of any planet or moon in the solar system; it also is probably a captured object.

Neptune’s system of six faint rings, with radii from about 42,000 to 63,000 km (26,000-39,000 mi), straddles the orbits of its 4 innermost moons. (Neptune’s visible surface lies at a radius of 24,800 km, or 15,400 mi.) The outermost ring, named Adams, is unusual in that it contains several clumps, or concentrations of material, that before Voyager 2′s visit had been interpreted incorrectly as independent ring arcs. What created and has maintained this structure has not yet been fully explained; it has been suggested that the clumps resulted from the relatively recent breakup of a small moon and are being temporarily held together by the gravitational effects of the nearby moon Galatea.

Neptunian Moons and Rings (continued)

NAME	MEAN DISTANCE		MASS	ORBITAL PERIOD
(DESIGNATION)	FROM NEPTUNE	DIAMETER	(1020 KG)	(EARTH DAYS)
Naiad (III)	48,230 km (29,970 mi)	58 km (36 mi)	0.002	0.294
Thalassa (IV)	50,070 km (31,110 mi)	80 km (50 mi)	0.004	0.311
Despina (V)	52,530 km (32,640 mi)	148 km (92 mi)	0.02	0.335
Galatea (VI)	61,950 km (38,490 mi)	158 km (98 mi)	0.04	0.429
Larissa (VII)	73,550 km (45,700 mi)	192 km (119 mi)	0.05	0.555
Proteus (VIII)	117,640 km (73,100 mi)	416 km (258 mi)	0.5	1.122
Triton (I)*	354,800 km (220,460 mi)	2,700 km (1,678 mi)	214	5.877 (retrograde)
Nereid (II)	5,509,100 km (3,423,200 mi)	340 km (211 mi)	0.2	359.632

Pluto

Pluto is named for the god of the underworld in Roman mythology. It was long considered the planet normally farthest from the Sun, but on 24 Aug 2006, the International Astronomical Union announced that it was downgrading the status of Pluto to a dwarf planet. The key criterion in this classification was that Pluto, which orbits in the cluttered, icy Kuiper belt, had not cleared the neighborhood around its orbit. This was a controversial decision sure to be revisited.

Pluto has three natural satellites, Charon, Hydra, and Nix. Because Charon’s diameter is more than half the size of Pluto’s and they orbit around a common center of gravity, it was common to speak of the Pluto-Charon system as a double planet. Charon, named for the boatman in Greek mythology who carried the souls of the dead across the river Styx, was discovered in 1978, while Hydra and Nix were both first seen in 2005. The New Horizons spacecraft, launched in January 2006 and scheduled to arrive at Pluto in 2015, will search for yet more new satellites.

Pluto is so distant (its average distance from the Sun is 39.6 astronomical units, or AU) that sunlight traveling at 299,792 km/sec (186,282.1 mi/sec) takes more than five hours to reach it. An observer standing on the dwarf planet’s surface would see the Sun as an extremely bright star in the dark sky, providing Pluto with only 1/1600 the amount of sunlight reaching the Earth.

Pluto has a diameter less than half that of Mercury; it is about two-thirds the size of the Moon. Pluto’s physical characteristics are unlike those of any of the planets. Pluto resembles most closely Neptune’s icy satellite Triton, which implies a similar origin for these two bodies. Most scientists now believe that Pluto and Charon are large icy planetesimals left over from the formation of the giant outer planets of the solar system. Accordingly, Pluto can be interpreted to be the largest known member of the Kuiper belt (which, as discussed, includes the outer part of Pluto’s orbit). Observations of Pluto show that it appears slightly red, though not as red as Mars or lo. Thus, the surface of Pluto cannot be composed simply of pure ices. Its overall reflectivity, or albedo, ranges from 0.3 to 0.5, as compared with 0.1 for the Moon and 0.8 for Triton.

The surface temperature of Pluto has proved very difficult to measure. Observations made from the Infrared Astronomical Satellite suggest values in the range of -228 to -215 °C (-379 to -355 °F), whereas measurements at radio wavelengths imply a range of -238 to -223 °C (-397 to -370 °F). The temperature certainly must vary over the surface, depending on the local reflectivity and solar zenith angle. There is also expected to be a seasonal decrease in incident solar energy by a factor of roughly three as Pluto moves from perihelion to aphelion.

The detection of methane ice on Pluto’s surface made scientists confident that it had an atmosphere before one was actually discovered. The atmosphere was finally detected in 1988 when Pluto passed in front of a star as observed from the Earth. The light of the star was dimmed before disappearing entirely behind Pluto during the occultation. This proved that a thin, greatly distended atmosphere was present. Because that atmosphere must consist of vapors in equilibrium with their ices, small changes in temperature will have a large effect on the amount of gas in the atmosphere.

Comets are a class of small bodies orbiting the Sun and developing diffuse gaseous envelopes. They also often form long luminous tails when near the Sun. The comet makes a transient appearance in the sky and is often said to have a “hairy” tail. In fact, the word comes from the Greek kometes, meaning “hairy one,” a description that fits the bright comets noticed by the ancients.

Despite their name, many comets do not develop tails. Moreover, a comet is not surrounded by nebulosity during most of its lifetime. The only permanent feature of a comet is its nucleus, which is a small body that may be seen as a starlike object in large telescopes when tail and nebulosity do not exist, particularly when the comet is still far away from the Sun. Two characteristics differentiate the cometary nucleus from a rocky body such as an asteroid or meteoroid—its orbit and its chemical nature. A comet’s orbit is more eccentric (less circular); therefore, its distance to the Sun varies considerably. Its material contains more volatile components, with water ice the predominant compound. They have been described as “dirtysnowballs” or “icy mudballs.” When far from the Sun, however, a comet remains in its pristine state for eons without losing any volatile components because of the deep cold of space. For this reason, astronomers believe that pristine cometary nuclei may represent the oldest and best-preserved material in the solar system.

During a close passage near the Sun, the nucleus of a comet loses water vapor and other more volatile compounds, as well as dust dragged away by the sublimating gases. It is then surrounded by a transient dusty “atmosphere” that is steadily lost to space. This feature is the coma, which gives a comet its nebulous appearance.

The astronomer Edmond Halley, a friend of Isaac Newton, endeavored to compute the orbits of 24 comets for which he had found fairly accurate historical documents. Applying a method Newton had developed, Halley predicted that the comet that now bears his name would return to Earth in 1758, and that proved correct. Since its prediction by astronomers and its appearance in 1758/59, Comet Halley has reappeared three times—in 1835,1910, and 1986.

Each century, a score of comets brighter than Comet Halley have been discovered. Many are periodic (returning) comets like Comet Halley, but their periods are extremely long (millennia or even scores or hundreds of millennia), and they have not left any identifiable trace in prehistory. Bright Comet Bennett (1970) will return in 17 centuries, whereas the spectacular Comet West (1976) will reappear in about 500,000 years. Among the comets that can easily be seen with the unaided eye, Comet Halley is the only one that returns in a single lifetime. About 200 comets whose periods are between 3 and 200 years are known, however. Unfortunately, they are or have become too faint to be readily seen without the aid of telescopes.

For faraway objects that contain volatile ices, the distinction between asteroids and comets becomes a matter of semantics because many orbits are unstable; an asteroid that comes closer to the Sun than usual may become a comet by producing a transient atmosphere that gives it a fuzzy appearance and that may develop into a tail. Some objects have been reclassified as a result of such occurrences. For example, asteroid 1990 UL3, which crosses the orbit of Jupiter, was reclassified as Comet P/Shoemaker-Levy 2 late in 1990. Conversely, it is suspected that some of the Earth-approaching asteroids (Amors, Apollos, and Atens) could be the extinct nuclei of comets that have now lost most of their volatile ices.