Geoscience Reference
In-Depth Information
Our Place in Space
The center of our particular solar system is the star that
we refer to as the Sun. Eight official planets orbit the Sun
(Figure 3.2), with Pluto now considered a dwarf planet. To give
you an idea of the size of the solar system, consider that Pluto
is approximately 5.9 billion km (3.6 billion mi) from the Sun.
A probe launched in January 2006 to study Pluto is currently
zooming through space at about 14,500 km/h (9000 mi/h). Even
at that remarkable speed, the probe will not reach Pluto until
July 2015! Given this great distance, Pluto is much too far away
from the Sun to receive sufficient energy to support life. Of the
remaining planets, most are either too far away from the Sun
(and thus are too cold) or too close (and thus are too hot) for
life as we know it to exist. The lone exception is Earth, the
third planet from the Sun, whose orbit happens to be a perfect
distance from the Sun for supporting life as we know it.
A good way to begin a discussion of Earth-Sun relationships
is with a brief discussion of the Universe. The origin of the
Universe seems to be best explained by the
Big Bang theory
,
which is based on the concept that the Universe emerged from a
singular, enormously dense and hot state about 14 billion years
ago. The primary evidence for this theory comes in two forms,
specifically that (1) the most distant star clusters are moving
away from us at greater speeds than those that are relatively
close, and (2) the amount of cosmic microwave background ra-
diation is remarkably uniform throughout the Universe, which
implies that it represents the leftover energy from an early
period of rapid expansion.
According to present estimates, the Universe is approxi-
mately 20 billion light-years across, with a light-year being the
distance that light travels over the course of a year at the speed
of 1,079,252,848.8 km/h (670,616,629.4 mi/h). With respect to
the ultimate fate of the Universe, it will either continue to ex-
pand for all of eternity or it will one day collapse upon itself,
depending on the average density of matter and energy within
the entire Universe. Given that the speed of distant star clus-
ters seems to be accelerating, it appears that the Universe may
indeed expand forever.
The largest clearly definable unit within the Universe
is a galaxy. According to astronomers, the Universe has ap-
proximately 50 billion galaxies, with each galaxy containing
billions of stars. We happen to live in the Milky Way Galaxy
(Figure 3.1), which includes approximately 400 billion stars.
The Milky Way is a typical spiral galaxy, consisting of a bright
central region with a high density of stars and a flat circular
region containing most of the other stars. Younger, brighter
stars form in long spiral arms that extend out from the galactic
center. Our solar system lies in one of these spiral arms.
The Shape of Earth
Let's now turn our attention to the shape of Earth. Although
Earth appears to be a perfect sphere, it bulges slightly at the
Equator and is flattened somewhat at the poles to form a shape
known as an
oblate spheroid
. The circumference of Earth mea-
sured at the Equator is 40,075 km (24,902 mi), while the cir-
cumference measured through the poles is slightly less: 40,008
km (24,860 mi). The bulge at the Equator, which gives Earth
a slightly thicker middle, is caused mainly by the centrifugal
force of the Earth's rotation and by differences in density of
the Earth's crust and gravitational field. This centrifugal force
is similar to the sideways push you feel when rounding a curve
quickly in your car.
The shape of Earth is an important factor to consider re-
garding the Earth-Sun relationship. Because Earth presents a
curved surface to the arriving rays of the Sun, the intensity of
solar radiation received varies by latitude. This difference is a
reflection of the
Sun angle
, which is the angle at which the
100,000 light-years
Sun
Spiral arms
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Mercury
Galactic center
93,000,000 miles
Sun
3,640,000,000 miles
Figure 3.1 The Milky Way Galaxy.
Our Sun is but one of
approximately 400 billion stars in this galaxy. Note our Sun's
location in one of the spiral arms of the galaxy.
Figure 3.2 Our solar system.
The solar system consists of eight
official planets (Pluto is now considered a dwarf planet) that orbit
the Sun. Earth is the third planet from the Sun. (Not drawn to scale.)
Big Bang theory
The theory that the Universe originated
about 14 billion years ago when all matter and energy erupted
from a singular mass of extremely high density and temperature.
Sun angle
The angle at which the Sun's rays strike the Earth's
surface at any given point and time. This angle is high at low
latitudes and is progressively less at higher latitudes.
