"We should do astronomy because it is beautiful and because it is fun. We should do it because people want to know. We want to know our place in the universe and how things happen."
Astronomy is one of the oldest sciences. People have observed the heavens for as long as people have existed. Many cultures developed accurate calendars thousands of years ago by carefully observing the motions of the sun, moon, and stars. However, it wasn’t until the invention of the telescope in the seventeenth century that the modern science of astronomy truly developed. The subject matter that astronomers study is very broad: everything beyond Earth’s atmosphere.
Why do we only see one side of the moon?
Every night the familiar "man in the moon" looks down over Earth. The face of the moon is unchanging. Even though the moon rotates on its axis, we always see the same side of it. The only way to map the rest of the moon is to send satellites around it with cameras. Why can’t we see the rest of the moon?
The simple answer is that the moon rotates on its axis once every month and it revolves around Earth once every month, so we always see the same side of the moon. But that does not answer why this should be the case. There must be an explanation, other than coincidence, for these two cycles being the same.
The reason for the identical timing is gravity. Any two bodies in the universe exert a gravitational pull on one another. If the bodies are very large, the pull on the side that is closer is greater than the pull on the side that is farther away. This difference creates tidal effects. On Earth, these effects can be observed as water moves toward one side of the planet, making a bulge in the oceans. This bulge creates the tides. Gravity also pulls on the solid parts of Earth, causing them to bulge, but because the rock does not flow as readily as water, it is not easily detected.
Because Earth has a mass that is about 81 times the mass of the moon, Earth’s tidal effect on the moon is much stronger than the moon’s effect on Earth—about 20 times as large. The gravitational pull of the planet pulls the moon slightly out of shape with a bulge toward Earth. The side with the bulge is attracted toward Earth a bit more strongly than the side away from it. The result is that the moon is tidally locked to Earth so that the bulging side is always facing the planet and the period of rotation has slowed to become equal to the period of revolution around Earth.
Fast Facts
A small lake does not have tides because tides depend on a difference in gravity, the pull of gravity itself. A lake experiences the gravitational pull of the moon, as does everything else on Earth. However, the distance between the point in the lake that is closest to the moon and the point that is farthest from the moon is very small. The distance between a point in the ocean on Earth’s surface and a point on the opposite side of the planet is about 8,000 miles, so tidal effects are significant in the oceans but not lakes.
Why doesn’t the same thing happen to Earth, so that it always has the same side facing the moon? Actually, the tidal effects of the moon have slowed Earth’s rotation over the past 4 billion years, but the change is slow because of the differences in sizes of the two bodies. Every century, the length of a day on Earth increases by about a thousandth of a second due to the drag of tides. Given enough time, Earth would eventually become tidally locked to the moon, but the solar system is likely to be destroyed by an explosion of the sun before that time.
Do planets show phases as the moon does?
The moon does not always look the same. Sometimes it has the shape of a bright circle; other times it is a thin sliver that appears to be the edge of a circle. There is a cycle of changes in the apparent shape of the moon, called the phases of the moon. Since the planets, like the moon, are seen by reflected sunlight, do they also show phases?
First let’s take a look at what causes the moon’s phases. We see the moon by the sunlight that reflects from its surface. The moon does not produce any light of its own. Sunlight falls on one half of the moon’s surface at all times. The specific part of the moon that is lighted changes as the moon rotates, just as the United States and India are in sunlight at different times as Earth rotates.
When the sun is on one side of Earth and the moon is on the other, the entire surface of the moon that we can see is lighted and the moon is full. If the moon is on the same side of the planet as the sun, then the entire lighted side is facing away from us, making a new moon. When the moon, Earth, and sun make a right angle, half of the part of the moon that we see is lighted and half is not lighted. This phase is called the quarter moon. In between these phases, we see different amounts of the lighted part including the crescent moon, and the gibbous moon (between quarter and full).
Fast Facts
The moon does not generate its own light. Moonlight is light from the sun that is reflected in our direction. Earthlight reflects to the moon in the same way. If you look at a crescent moon on a clear night, you can often see the dark part of the moon illuminated very faintly. This illumination is reflected Earthlight.
As Earth and the other planets revolve around the sun, the same thing happens. The planets also reflect light from the sun and the amount of the surface that is lighted, from our point of view, depends on the relative positions of the sun, the planet, and the place where we are standing. Because the planets are so far from Earth, they appear as a point of light to the naked eye. With a telescope, however, the phases become apparent. The first person to observe the phases of Venus was Galileo Galilei in 1610. His observation supported the Copernican idea that the planets revolve around the sun.
While we can see the full range of phases of the moon, we can only observe some of the phases of the planets. For example, we cannot see the completely full phase of Mercury or Venus, the planets whose orbits are closer to the sun than Earth’s orbit.
Earth can never be between the sun and the planet, the position that creates a full moon. The phases of Venus and Mercury range from gibbous to crescent. When the planet is closest to Earth, we cannot see it because all its light is reflected away from Earth, back toward the sun.
The planets that are farther from the sun than Earth also exhibit phases, but they are less noticeable. Because of the angles at which these planets can be observed, they have phases ranging from gibbous to full. The phases are less distinct than those of Venus and Mercury. The phases are difficult to observe because, as the planets move from full phase, they also become more distant and dimmer. It is not possible for a planet outside the orbit of Earth to exhibit a crescent or new phase.
What are the chances of an asteroid hitting Earth?
It’s a favorite disaster movie theme: a giant asteroid bears down on Earth as people panic. The rock, as large as a small city, strikes the planet as a massive fireball, tsunamis overrun skyscrapers, and the climate is completely disrupted. Is this purely fiction or should we be worried?
Unfortunately, although the movies are fiction, the possibility of a collision between Earth and an asteroid is real. In fact, it has happened countless times in the planet’s history. One theory holds that such a collision was responsible for the extinction of the dinosaurs. It would be difficult, if not impossible, for Hollywood to overstate the devastation that a really big asteroid could wreak on Earth.
Fortunately, we have a plan. NASA has begun a program to find all near Earth objects (NEOs) greater than 1 kilometer in diameter and track their courses. An NEO is a comet or asteroid that has been pushed by the gravitational attraction of a nearby planet into an orbit that brings it close to Earth. NEO discovery teams examine photographs of objects in space, looking for the things that change position from day to day.
When a meteor enters the atmosphere, it falls at thousands of miles an hour and heats to a bright glow. The heat does not come from friction with the air of the atmosphere. Most of the heat is actually generated by compression of the air ahead of it, in the same way that a bicycle pump is heated by compression of the air inside it.
NASA estimates that there are about 1,000 NEOs that are larger than 1 kilometer. A tracking program monitors photos of space with the goal of discovering at least 90 percent of these large NEOs by 2018. Once they are discovered, the objects are tracked to find out which of them could come close enough to Earth that there is a risk of collision. As of early 2008, the project had discovered more than 5,000 NEOs, 733 of which were larger than 1 kilometer.
It is not yet completely clear what will happen if we detect an asteroid or comet that is on a collision course with Earth. There have been a number of proposals for ways to deflect or destroy the objects, but each has its limitations. The movie solution of blasting it apart with a nuclear warhead is not practical because it would break the object into many NEOs still heading toward Earth. More practical suggestions include changing the orbit by striking the NEO with a heavy spacecraft, attaching large thrust engines, or even deflecting it with light from a powerful laser. It would take a long time to implement any of these solutions. The sooner we have notice, the better our chances of dealing with the problem.
Fast Facts
Scientists know that asteroids and comets have hit Earth in the past. These objects are also responsible for the craters on the moon. One theory explains that the extinction of the dinosaurs 65 million years ago was caused by the impact of an asteroid several kilometers across. More recently, in what is known as the Tunguska event, an asteroid, estimated to have a diameter of several hundred meters, exploded above Siberia in 1908. The force of the explosion leveled about 80 million trees over an area of 800 square miles.
Why do we see different stars in summer than in winter?
People began looking at the stars and noticing their patterns before the beginning of recorded history. We recognize constellations in the sky as patterns of stars that stay in constant positions relative to one another. Some of the constellations can be seen year-round, but others are visible only during part of the year. Why do the constellations that we see change during the year?
Although stars do move, they are so far away that we cannot detect their motion. It is not the constellations that change during the year. Instead, it is our observation point that changes.
Fast Facts
Almost every culture in history has seen patterns, or constellations, in t he stars. Not everyone sees the same pattern, even though they look at the same stars. For example, most people in the United States have no trouble finding the Big Dipper. For the ancient Greeks, however, those stars were part of a picture of Ursa Major, the Great Bear. In Britain, it is the Plough and in France, a Saucepan. To Hindus, it is the Seven Sages. Escaped American slaves followed the Drinking Gourd to the north.
Think about how Earth revolves around the sun. At any given place in its revolution, you cannot see the stars that are in the same direction as the sun, because sunlight in the atmosphere is too bright during the day. You only see the stars at night when the planet is between you and the sun.
So in January, you look out into space and see the group of stars that we identify as Orion, the hunter (actually Orion is visible from October to March). In July, though, Earth has moved to the other side of its orbit, so Orion is behind the sun and not visible from Earth. Now, you look up and find Libra, the scales, which was nowhere to be seen in January.
What about Ursa Major, the large bear (or the Big Dipper, if you prefer). It points toward Polaris, the North Star, all year round. Again think about Earth in its orbit. The sun never enters the sky above the North Pole or below the South Pole. It does not matter what the season, the constellations in those directions are always visible from Earth. However, the year-round stars are different in the Northern and Southern Hemispheres.
What causes the seasons?
The seasons have a huge effect on our lives. They determine what we wear, what activities we pursue in work and leisure, what we grow for food, and when we plant it. The ability to predict seasonal changes was so important to early human cultures that they built sophisticated observatories for that specific purpose. What causes the seasons to change in a regular, predictable pattern?
We experience seasons because Earth’s axis is tilted by about 23.5° in relation to the plane of its orbit around the sun. As a result, the intensity of sunlight that reaches a particular place on the surface changes in an annual cycle. To see how this works, consider the most extreme cases—the North and South Poles. During the summer, the tilt of the axis means that the pole is in a direct line to the sun all day, every day. Solar energy provides heat and light round the clock. In the winter, though, the pole is always shaded from sunlight by Earth itself, so the night lasts for about six months, during which the pole receives no energy directly from the sun.
At the equator, seasonal changes are almost nonexistent. The apparent position of the sun changes from a little bit in the north to a little bit in the south. In Singapore, which is very close to the equator, the average high and low temperatures in January are 85°F and 72°F; in July, it’s 86°F and 75°F.
Now, what if you live in Portland (Maine or Oregon, take your pick), about halfway between the North Pole and the equator? The tilt of the axis has a major effect although much less than at the pole. In the early summer, time between sunrise and sunset is about 15′/2 hours, and in early winter, about 9 hours. In addition, the sunlight strikes at a more direct angle during the summer, so more energy is received during an hour of sunlight.
Seasonal changes are not caused by differences in the distance of Earth from the sun; however, these changes do have a small effect on temperatures. Earth is slightly closer to the sun in December than in June. As a result the Southern Hemisphere has slightly milder changes between summer and winter than the Northern Hemisphere.
Why does the moon have more craters than Earth?
If you look at the moon through a small telescope or a pair of binoculars, you can see that it is covered with craters. A detailed photograph, taken through a large telescope, shows that there are many thousands of craters on the moon, giving it a patchwork appearance. Small craters cover the floors of large craters. There are a few craters on Earth, but they are rare by comparison. Why does the moon have so many craters and Earth so few?
There are two ways that craters can form—the collapse of rock around a volcano and the impact of an object from space. Most of Earth’s craters were formed by volcanoes, although there are a few known craters caused by meteors. The moon shows the opposite. There are some craters that show evidence of volcanic activity and there are some very large plains that were formed by lava flow.
Most of the moon’s craters, however, were formed when its surface was struck by fast-moving objects. The energy released by these impacts cracked and melted the rock and blew it aside. The walls of the craters are made of this ejected rock. Some of the craters are surrounded by streaks of material scattered tens of miles in every direction. There are a few similar craters on Earth. Barringer Crater in Arizona looks just like a moon crater. It is a bit less than a mile wide and 650 feet deep.
Fast Facts
NASA astronomers photographed the birth of a new crater on the moon on May 2, 2006. As a meteoroid struck the surface and an explosion equaled about 4 tons of explosives, a brilliant flash was recorded. Based on the brightness of the fireball and how long it lasted, astronomers calculated that the rock from space was about 10 inches in diameter and traveling at 85,000 mph. The impact left a new crater that is about 40 feet wide and 10 feet deep.
Earth is much larger than the moon, so you would expect that it would be struck by meteors more often than the moon. In fact, it is. Craters are rare on Earth, though, for several reasons. We have a protective blanket around us—the atmosphere. Most of the objects that strike the planet never make it to the surface. Intense heat, caused by the compression of air ahead of the meteor, usually causes it to disintegrate far above the surface, leaving only a momentary bright streak across the sky. Without an atmosphere to protect it, the moon’s surface makes a much better target.
Even when a crater does form on Earth, it is generally a temporary geological feature. In most places, erosion by wind and water begin to erase the crater immediately. The Barringer Crater is located in the desert of Arizona, where the climate keeps these forces of erosion to a minimum. Even so, there is evidence of erosion on its walls and floor. This crater is about 50,000 years old. Some of the moon’s craters have remained unchanged (except for new impacts within the crater) for about 4 billion years, nearly 100,000 times as long.
Finally, the moon has more craters because its surface is older. The surface of Earth is constantly changing due to plate tectonics, while the moon does not experience this change. As the continents are buried beneath one another and new crust forms, the materials of the surface are continually recycled, erasing any feature on them. Much of the surface of our planet is less than 200 million years old, about one eightieth of the age of the moon’s surface.
How do we know the mass of Earth?
If you want to know your mass, you can step on a bathroom scale. By measuring the force of Earth’s gravity on your body, the scale can tell you that your mass is 50 kilograms (110 lb.), 60 kilograms, 70 kilograms, or whatever your particular reading is. But how can you measure the mass of Earth itself?
The mass of Earth is about 6 x 1024 kilograms. That is about 6 million billion billion kilograms. How can we possibly know that value? Isaac Newton determined that the force of gravity between two objects increases as the mass of the objects increases and decreases as the square of the distance between them.
The period of rotation around a more massive object does not depend on density but only on total mass and the distance from the center of the object. If the sun were to shrink into a black hole the size of an orange, Earth would not spiral into it. Earth would continue orbiting at exactly the same distance. However, an object that approached very close to the tiny sun would experience an extremely high gravitational pull because its distance from the center of mass would be so much smaller.
An object stays in orbit around another object because the acceleration of gravity is exactly equal to the inertia of the orbiting object. If an object is orbiting Earth at a known distance from Earth’s center, you can calculate the amount of acceleration due to gravity. If the mass of the object is much smaller than the mass of Earth, the object’s mass does not matter. At a particular distance from Earth, a marble, a school bus, and a moon will all revolve around the planet at the same rate. Knowing that rate and the distance, you can calculate the mass of the planet.
We know the distance to the moon and we know its acceleration due to gravity from the period of its orbit. With those values we can measure the planet’s mass.
Why don’t we see an eclipse every month?
A solar eclipse occurs when the new moon passes directly between the sun and Earth, so the moon’s shadow falls on the surface of the planet. A lunar eclipse occurs when Earth passes between the sun and a full moon and Earth’s shadow blocks sunlight so that is does not reach the moon. Why don’t we see a solar eclipse and a lunar eclipse every month?
If you illustrate the orbits of Earth and the moon on a sheet of paper, it certainly looks like there should be an eclipse every month. That is not the complete picture of what is happening, though. The lunar orbit falls in a slightly different plane than Earth’s orbit around the sun. In the paper model, that means the moon is above the paper part of the time and below the paper part of the time. We only see an eclipse when the orbit crosses the plane at exactly the same time as the three bodies come into a straight line.
The picture gets even a bit more complicated. When a solar eclipse does occur, the shadow of the moon covers only a small part of Earth’s surface and the eclipse lasts only a few minutes. Even when there is an eclipse, you have to be at the right place in order to see it. Total eclipses occur between two and five times per year, but any particular spot on the surface will only experience a total eclipse about every 360 years.
Fast Facts
It is only an accident of size that we can see a total solar eclipse at all. The distances of the moon and sun from Earth are such that both bodies appear to have the same diameter. Due to tidal effects, the moon is slowing in its orbit and as a result is moving farther away from Earth by a few centimeters per year. In another billion years, the moon’s apparent diameter will be too small to completely cover the sun and there will be no more total solar eclipses.
Lunar eclipses actually occur less often than solar eclipses. Because Earth’s shadow is much larger than the moon, however, the eclipse lasts longer and it is visible from the entire night side of the planet. As a result, any given location can observe as many as three lunar eclipses in a year, although in some years there may be none at all. The maximum possible number of eclipses in a year is four solar and three lunar.
Why did Pluto get demoted from its status as a planet?
Most people learned in elementary school that our solar system has nine planets. The most recently discovered, Pluto, was first detected in 1930. Suddenly, in 2006, the number of planets changed. No, we didn’t lose Pluto to some other star. It is still there, moving around the sun in the same orbit that it was following in 1930. Astronomers have reclassified Pluto, though, and no longer call it a planet. Why did Pluto lose its status?
Before Pluto was discovered in 1930, eight planets were known. They fell into two groups: terrestrial planets and gas giants. Terrestrial planets, including Earth, are made primarily of rocks and metals and they are small, hard, and dense. Gas giants are big balls of hydrogen, helium, and other gases. Even though the gas giants are not nearly as dense as the terrestrial planets, they are so big that their mass is much greater than Earth’s mass. The giants are big and soft and they are located much farther from the sun than their smaller neighbors. Pluto was a bit of a misfit—apparently a terrestrial planet orbiting beyond the gas giants.
Over time, as more was learned about Pluto, the differences between Pluto and the rest of the gang seemed to grow. While the rest of the planets revolve pretty much in the same plane, Pluto’s orbit is off that plane by about 17°. In 1978, astronomers determined that Pluto is not larger than Mercury, as had been previously believed. Its mass is actually only one twenty-fifth that of Mercury, and Pluto is only nine times as massive as the asteroid Ceres. Pluto, and its moon Charon, consist primarily of ice—frozen water—so they are not made of the same material as any other planet.
That still wasn’t too much of a problem until the 1990s, when astronomers began to discover other objects in orbit around the sun in the Kuiper Belt, a region beyond Pluto. We now know of several objects in the Kuiper Belt whose size is similar to that of Pluto. The obvious question came up. Is each of these objects also a planet, even though they are so different from what has traditionally been called a planet? If so, there may be dozens or hundreds of planets and the definition might be so expanded as to be useless. If not, what about Pluto?
Pluto is not the solar system object that is most distant from the sun. A disc of icy objects, known as the Kuiper Belt, extends about 10 times as far from the sun as the orbit of Pluto. The Oort Cloud extends even farther, one third of the way to the nearest star. It is estimated that the Oort Cloud contains as many as a trillion comets. Occasionally one of these comets is deflected by the gravity of a planet, distant star, or other object in the cloud. It may then approach the sun, creating a visual treat for observers on Earth for a few months before disappearing from sight on its way back to the Oort Cloud.
In 2006, the International Astronomy Union took up the question of what exactly makes an object a planet. One proposal was to define a planet as a body that orbits the sun and has enough mass such that gravity pulls it into a spherical shape. The problem with that was that it immediately added three planets: the asteroid Ceres, Pluto’s moon Charon, and a Kuiper Belt object, Eris. Many more objects might follow, leaving the same muddled definition.
Ultimately, the astronomers classified the objects orbiting the sun in three categories:
♦ Planets are round objects that have cleared their neighborhood of smaller bodies.
♦ Dwarf planets are round objects that have not cleared their orbits and are not satellites of a larger object.
♦ Smaller solar system bodies are all the other objects that orbit the sun.
Under this definition Pluto, Charon (which is about the same size as Pluto), Ceres, and Eris are classified as dwarf planets.
Why aren’t all stars the same color?
If you look at the sky on a clear night, you can see hundreds of bright stars. They don’t all look the same. Brightness varies significantly from one star to another. Although they look like white points, there is some variation in color. Some, such as Betelgeuse, appear to have a red or orange tint. Others, such as Rigel, appear to be a bit more blue than most stars. If you look at a color photo taken through a telescope, you will see that stars come in a wide variety of colors. Why do stars have different colors?
Just as people are different from one another, each star has its own characteristics. They vary widely in size, with the largest stars having about 300 times as much mass as the smallest stars. They vary in brightness, too. The brightest stars emit about 100 million times as much light as the dimmest stars. However, this is not the main reason that stars appear to have different levels of brightness to an observer. That is generally due to variations in distance among the visible stars.
And the stars vary in color. The color difference is due to temperature at the surface of the star. The light from a star is caused by the same process as the light from an incandescent light bulb. Atoms near the surface of the star absorb energy and then emit that energy as light when energetic electrons return to their ground state. The color of the light that the atoms emit is related to the amount of energy that they have absorbed. A star’s color is related to the temperature of the atoms near its surface.
Red stars, such as Betelgeuse, are relatively cool at about 3,000 kelvins (K). The yellow light of our sun is produced at a temperature of about 6,000 K. At 10,000 K, stars emit white light. The hottest stars, such as Rigel, have a surface temperature between 20,000 K and 30,000 K. Their light is blue. This does not mean that the stars emit light of only one color. Like sunlight, the light from stars of every color include the entire spectrum. Blue stars emit more light in the higher-energy blue wavelengths than in the lower-energy red. Red stars do emit some blue light but not in the same intensity as red.
When you look at the sky, though, almost all of the stars look white. Does this mean that most stars have a temperature around 10,000 K? No, the white appearance of stars is due to the way our eyes work. At night, when the total amount of light is low, we tend to use receptors in the eye that do not respond differently to different colors. It is as if you were to take a black and white photograph of the sky. The colors are there, but we don’t have enough information to detect them.
The temperature that determines the color of a star is its surface temperature. While the temperatures range from 3,000 K to 30,000 K, the interior temperatures of stars, where nuclear fusion takes place, are much higher. The temperature at the core of the sun is about 15 million K. In a star that is about to explode, the temperature can reach as high as 100 billion K.
How do we measure the distance to stars?
It is important for astronomers to know the distance to stars because we can’t know other properties such as brightness and mass without knowing the distance. But the stars are very far away, so far that we can’t measure gravitational effects. How do we measure distance from our solar system to various stars?
The keys to measuring the distance to stars are knowing that Earth revolves around the sun and knowing the diameter of its orbit. To find the distance, we can take a look at a star and note its position compared to other stars around it. Six months later we take another look. The position of some stars will appear to have changed.
To see why this occurs, hold a pencil up about a foot in front of your face. Close one eye and look at a distant object behind the pencil. Now look at the pencil with the other eye. The pencil seems to move compared to the background because your eyes are several inches apart. This difference based on the point of view is called parallax.
Now go back to stars. When we look at a star in spring and then again in fall, our baseline is not a few inches, but rather about 180 million miles. Nearby stars move slightly compared to a background of very distant stars. By measuring this change we can use parallax to find the distance to the closer stars. If we use distant galaxies as our background, we can measure the distance to stars that are far from Earth.
