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The moon is Earth's only natural satellite--an object that travels in an orbit, or curved path, around a larger object. The moon's orbit is elliptical, or oval shaped, which means that its distance from the Earth changes as it travels in its orbit. At its closest point to the Earth, known as perigee, the moon is 221,500 miles (356,600 kilometers) away. At its greatest distance, or apogee, it is 252,700 miles (406,800 kilometers) away.

The moon travels in its orbit at an average speed of 2,237 miles (3,600 kilometers) per hour. As it gets closer to the Earth, it speeds up because of the Earth's strong gravitational pull. As it gets farther away, it slows down. The Earth's gravitational pull also affects the moon's shape. Although the moon appears perfectly round, it is not. There is a slight bulge on the side of the moon facing the Earth that scientists think is caused by the Earth's gravitational pull.

The moon is quite large in relation to Earth. With a diameter of 2,160 miles (3,478 kilometers), it is about one fourth the diameter of the Earth. The only planet that has a larger satellite in relation to its size is Pluto--Pluto's moon is more than half the size of Pluto.

Rotation and Libration

Like all planets and natural satellites, the moon rotates, or spins, on its own axis. One rotation of the moon takes 27 days, 7 hours, and 43 minutes--just about the same amount of time it takes the moon to orbit the Earth. The fact that these two periods of time are about the same means that the same side of the moon is always facing the Earth . Because of this, you might think that it is impossible to see the far side of the moon from Earth. To a large extent this is true. However, we do see small sections of the moon's far side at different times as a result of motions called librations.

One type of libration involves the moon's axis. The moon's axis, like the Earth's, is slightly tilted. This tilt makes it possible to see other areas of the moon's surface during its orbit around the Earth.

Another kind of libration occurs because of changes in the moon's speed. As the moon circles the Earth, its speed changes slightly. Its rate of rotation on its axis does not change, however. As a result, it is possible to see even a little more of the far side of the moon.

Because of these librations, we are able to see about 59 percent of the moon's surface at one time or another. The rest of the moon's surface was a mystery, however, until 1959. In that year a Russian space probe, Lunik III, circled the moon and took the first pictures of the surface of the moon's far side. Later, other Russian and American space probes took additional pictures of the far side.

Phases of the Moon

The moon has no light of its own. Moonlight is really sunlight that is reflected off the moon's surface. Here on Earth, the amount of moonlight we see varies. Sometimes we can see the entire lighted side of the moon. At other times, we see only a portion of it lit. Because of this, the moon appears to change its shape from night to night. We call these changes the phases of the moon.

These phases occur as a result of the moon's orbit around the Earth. During its orbit, when the moon is between the sun and the Earth , the side facing the Earth receives no sunlight, and the moon is not visible on Earth. This phase is called the new moon. Two or three days after the new moon, as the moon moves farther in its orbit, a small portion of the side of the moon that is lit becomes visible. This is called the crescent moon. As the moon travels farther in its orbit, the crescent waxes, or grows . When half the side that is lit is visible, this phase, which occurs a week after the new moon, is called the first quarter because the moon has traveled through one quarter of its orbit. Next comes the moon's gibbous phase, in which more than half the side that is lit is visible. This is followed by the full moon. At this point, about two weeks after new moon, the whole side that is lit is visible. A day or two after full moon, the moon begins to wane, or shrink. Another gibbous phase appears next, but with the right side of the moon in darkness instead of the left. About three weeks after new moon, the moon is in its last quarter. At this point, the moon has completed three quarters of one trip around the Earth. At the last quarter, the moon's left half appears lighted and right half appears dark--the opposite of the first quarter moon. The moon appears next as a waning crescent, and a few days after that, there is another new moon.

Shifts of the Moon's Position in the Sky

Because the moon is traveling in an orbit around the Earth as it goes through its phases, we see the moon in a slightly different place in the sky from night to night. When the moon is a waxing crescent, for example, it becomes visible in the western sky just after sunset and it sets a few hours later. Thereafter, from night to night, the moon appears slightly to the left of where it was at the same time the night before. By first quarter, the moon appears in the southern sky as soon as the sky gets dark. When the moon is full, it appears opposite the sun, rising in the east as the sun sets in the west. By the time the moon is waning, it rises after the sun has set. The last quarter moon typically rises around midnight. The waning crescent moon rises even later, and it is usually visible in the eastern sky within a few hours of sunrise.

Depending on its phases, the moon can sometimes be seen both at night and during the day. The first quarter moon, for example, is sometimes visible an hour or two before sunset on a clear day. The waning gibbous moon is sometimes visible for a few hours after sunrise. And the last quarter moon is sometimes visible shortly after sunrise.


Since the moon circles the Earth, you might think that it would pass directly between the Earth and the sun on each orbit. The moon's orbit, however, is slightly tilted, and it passes directly between the Earth and sun only at certain times. When the sun's light is blocked by the moon, and the moon's shadow falls at certain places on Earth, a solar eclipse, or eclipse of the sun, occurs.

During a solar eclipse, the diameter of the moon's shadow on the Earth varies from almost nothing up to about 160 miles (258 kilometers). The narrow path that this shadow makes on the surface of the Earth is called the path of totality. People who are directly in this path see a total eclipse. People who are outside the path see only a partial eclipse, in which only a portion of the sun is blocked by the moon. During a solar eclipse, the moon's shadow crosses the Earth at speeds of more than 1,000 miles (1,600 kilometers) an hour. The period of time during which the moon completely blocks the sun can last from a little more than seven minutes down to a fraction of a second.

A lunar eclipse, or eclipse of the moon, occurs when the Earth passes directly between the sun and the moon and the Earth's shadow blots out the moon. The shadow of the Earth has two sections--a dark inner part called the umbra and a fainter outer part surrounding it. If the moon passes entirely through the umbra, there is a total lunar eclipse. If it passes through only a portion of the umbra, there is a partial eclipse.

At the distance of the moon from the Earth, the umbra is considerably larger than the moon, so in passing through it, the moon can stay totally eclipsed for up to about one hour and forty minutes.

An eclipse of the sun can only take place when the moon is new, and an eclipse of the moon can only take place when the moon is full. But we do not see an eclipse every time the moon is new or full. This is because the moon's orbit is slightly tilted relative to the orbit of the Earth around the sun. So the new moon usually passes a little above or below the sun in our sky and the full moon passes a little above or below the shadow cast by the Earth into space. The moon's orbit slowly wobbles, however, so once in a while--at the new or full moon--the Earth, moon, and sun lie along a perfectlystraight line and an eclipse occurs.

Gravitational Pull of the Moon

If you have ever seen movies or a video of astronauts on the moon, you must have noticed that they did not move in the same way people move on the Earth. They almost seemed to float, despite the cumbersome and heavy space suits and backpacks they wore. This is because the pull of gravity on the surface of the moon is only about one sixth as strong as the pull of gravityon the surface of the Earth, so the weight of an astronaut wearing such equipment is only about 60 pounds (27 kilograms) on the moon while about 350 pounds (158 kilograms) on the Earth.

The gravitational pull between the Earth, moon, and sun causes the Earth's ocean tides. The sun is more massive than the moon but the moon is much closer to the Earth, so the moon has a greater effect on the Earth's tides than the sun does. The tides are caused by the fact that the moon pulls with greater force on the side of the Earth that is closer to the moon than on the side of the Earth that is opposite the moon. This difference in force causes two bulges of water (the high tides) to form on opposite sides of the Earth (the side facing the moon and the side facing away from the moon). As the Earth rotates under these bulges, the oceans are seen to go through two high tides and two low tides each 25 hours or so. The moon's gravitational pull also causes weak tides in the Earth's atmosphere and in the Earth itself.

The Moon's Surface

If you look up at the moon at night, you can see dark and light areas on its surface even with the naked eye. The pattern formed by these dark and light areas creates the familiar sight commonly called "the man in the moon." The light areas are actually heavily cratered and mountainous terrain known as the lunar highlands. The dark areas are known as maria (singular: mare), from the Latin word meaning "seas." Early astronomers used this term because they originally thought the dark areas might be bodies of water.

Mountains. Looking through a telescope, it is possible to see more clearly the light regions known as lunar highlands. These high, mountainous areas contain some peaks as high as Mount Everest on the Earth. Some mountains in these regions form ranges, but most of them form the rims of craters.

Craters. Craters are depressions in the lunar surface that are roughly circular in shape. Millions of craters cover the moon's surface. Many are surrounded by high walls, or rims. Others are ringed by hills, and the smallest are mere pits in the moon's surface. The walls of craters frequently have different slopes inside and out. The outside crater rim usually slopes gently down toward the surrounding surface. Interior walls, however, usually have steeper slopes. In most craters, the interior floor is lower than the surface outside the crater, sometimes by as much as 10,000 feet (3,048 meters).

Almost all the craters on the moon have been formed by the impact of objects from space. The Earth is bombarded by such objects as well. Many people have seen the light of shooting stars, or meteors, streaking through the night sky. These are actually small pieces of rock or other material moving through the Earth's atmosphere. As they enter the Earth's atmosphere, however, friction causes most of the objects to burn up before reaching the ground. Those that reach the surface are called meteorites. The moon has no atmosphere. As a result, there is nothing to stop meteorites from striking the lunar surface, creating craters and grinding lunar rocks into a thin layer of fine dust.

The moon has hundreds of very large craters. Some, such as Clavius, can be seen without a telescope. Clavius is about 145 miles (233 kilometers) in diameter and is surrounded by crater walls up to 17,000 feet (5,182 meters) high. Yet millions of the smallest craters on the moon are less than 1/2500 inch (1/100 millimeter) in diameter.

Most larger craters are primary craters, which means that they were formed when an object from space struck the moon's surface. Many smaller craters are secondary craters, which means that they were not formed directly by the impact of an object from space. When an object from space strikes the lunar surface, it sends material flying out from the point of impact (the primary crater). As this material crashes onto the lunar surface, it creates secondary craters. Sometimes these impacts send more material flying outward, creating even smaller craters in turn.

By closely examining the moon's craters, it is often possible to tell which ones are older than others. For example, a crater that lies inside another crater or cuts through its walls must be younger than the other one. In this way, astronomers are able to determine relative dating of craters and other lunar features.

Rays and Rilles. Some of the moon's craters have light-colored streaks called rays, which look like the spokes of a wheel stretching away from the crater. The most noticeable rays are those stretching outward from the crater Tycho. Some of these rays are more than 1,500 miles (2,415 kilometers) long. Giant craters, such as Tycho, were gouged out of the lunar surface by gigantic meteorites. The rays are the broken and ground-up rocks that were blasted outward by the impact of the meteorites. Rayed craters are among the youngest craters on the moon. Astronomers know this because the rays tend to darken with time, and after awhile they are no longer visible.

Powerful telescopes on Earth, lunar probes, and the Apollo astronauts who explored the moon have photographed another feature of the moon's surface called rilles. Rilles are cracks and valleys in the lunar surface. They may be straight or they may twist and turn.

Maria and Mascons. The scientist Galileo thought that some areas of the moon were seas, or maria, because they appeared flat, dark, and smooth through his telescope. As telescopes improved, astronomers discovered that the maria are not smooth. They contain many craters--a few large ones and millions of small ones. Astronomers now know that the maria are large plains of solidified lava and that there is no water on the moon.

Astronomers have noted a strange characteristic of those lunar maria that have a roughly circular shape. These areas exert a stronger gravitational pull than do other areas of the moon. Since the gravitational pull exerted by an object depends on the amount of its matter, or mass, scientists reason that the stronger pull of the circular maria is caused by a greater concentration of mass in them. Astronomers have shortened the term "mass concentration" to mascons. The mascons were a problem to the Apollo astronauts who landed on the moon. Because of their stronger gravitational force, mascons can pull a spacecraft slightly off course. Therefore, such changes in course had to be predicted and allowed for when the Apollo spacecraft attempted to land on the lunar surface.

Moon Rocks. Between 1969 and 1972, six teams of Apollo astronauts landed on the moon. Each team consisted of two astronauts. While on the moon, these astronauts collected more than 840 pounds (378 kilograms) of moon soil and rocks to bring back to the Earth for study. A small amount of lunar material also was brought back to the Earth by two Russian space probes.

The rocks collected on the moon are types that are familiar to geologists. Almost all are formed from the cooling of lava. The rocks collected from the maria are mainly basalt--a dark, dense rock. Those collected from the lunar highlands are mainly anorthosite--a type of lava rock that cools more slowly than basalt. Some of the rocks collected in the lunar highlands and the maria were breccia--a type of rock that is compacted and welded together by intense heat and pressure.

Moon rocks have helped scientists learn more about the moon. We now know that the Earth and the moon are generally similar in composition, although some elements that are rare on Earth are somewhat more abundant on the moon (at least among the samples brought back). The age of the oldest lunar rock that has been tested is 4.42 billion years.

The Moon's Interior

While the Apollo astronauts were on the moon, they placed a number of seismometers on its surface. These instruments, which detect movement, measured the strength of the 3,000 or so weak moonquakes that occur on the moon every year. Information about these moonquakes, which was transmitted back to Earth, has enabled scientists to get a partial picture of the moon's interior.

The moon's outer layer, or crust, appears to be about 40 miles (64 kilometers) thick on the side of the moon facing the Earth. On the far side of the moon, the crust is nearly twice that thick. Beneath the crust is a thick layer of dense rock, the mantle, which extends down perhaps 435 miles (700 kilometers). It is not known what lies beneath the mantle.

The Moon's Environment

Scientists have found no evidence to suggest that life has ever existed on the moon. That is not surprising, for the moon's environment is very hostile to life as we know it. Living things need an atmosphere of gases to survive, and the moon has virtually none. On the Earth, every living thing contains carbon and water. On the moon there is almost no carbon. And although scientists have found evidence of ice on the moon, there are no signs of liquid water. Life on Earth also depends on the sun's heat and light, which are forms of radiation. Other forms of radiation--some from the sun and some from farther out in space--can be deadly. The Earth's atmosphere helps to filter out most of this deadly radiation. But the moon, without an atmosphere, is continually bombarded by such radiation.

The lack of an atmosphere poses another obstacle to life. The moon's sunlit, or daytime, side is heated to a temperature of about 250°F (120°C) on the surface. The two-week lunar day is followed by two weeks of night, when the temperature falls to ¯255°F (¯160°C). Such extreme temperatures would kill the kinds of living things we know.

Because the moon has no atmosphere, a person on the moon would see a very different sky from the one we see on Earth. On Earth, the daytime sky appears blue because the atmosphere scatters portions of the sun's light. On the moon, there is no air to affect light. As a result, the sky above the moon's surface is always black, even in the daytime. In these black skies, the sun and the stars can be seen at the same time.

Origin of the Moon

Scientists have offered a variety of theories to explain how the moon came into being. One theory, known as the "capture" theory, was based on the idea that the moon once traveled in an orbit around the sun near the Earth. The Earth's strong gravitational force "captured" its neighbor and pulled it into the orbit it now follows around the planet.

A second theory, known as the "fission" theory, suggests that the Earth and the moon were once a single mass of material. As the mass grew, it spun so fast that a part of it formed a bulge. In time, this bulge pulled away and was thrown out into space, forming the moon. The larger part of the mass became the Earth.

Another theory, the "condensation" theory, states that the whole solar system was formed at the same time from a great cloud of dust and gas that whirled in space. More than 99 percent of this cloud collected, or condensed, into a large mass that became the sun. The rest of the cloud condensed into a number of smaller masses, which eventually became the planets and their satellites.

Information about the moon's composition obtained from the Apollo missions put all three theories in doubt. Instead, a new hypothesis emerged as the leading theory in the mid-1980's. The "giant impact" theory suggests that the moon was formed when a large object--perhaps as large as Mars--collided with the Earth. The collision blasted large amounts of debris into space, where it formed a ring around the planet. Over time, the orbiting debris collected together to form the moon.

The giant impact theory is not perfect and modifications are still being made. Scientists are not sure at what point in the Earth's formation the collision occurred. Evidence obtained from a computer model suggested it may have occurred when the Earth was almost fully formed.

Early History of the Moon

The moon's early history was unimaginably violent. Millions of rocks, ranging in size from small pebbles to large boulders many miles in diameter, crashed onto the moon's surface. These collisions generated energy that heated up the lunar surface and melted its rock. The moon's surface became a sea of glowing liquid lava hundreds of miles deep.

After millions of years, fewer rocks remained in space, and so fewer crashed into the moon. The lighter or less dense materials floated to the surface and eventually formed the moon's crust. The denser materials sank below the surface and formed the mantle and the core of the moon. Because the chemical composition of these materials was different, the rocks of the moon's crust and mantle are also different.

After the moon's crust cooled, some rocks continued crashing onto the moon, blasting huge craters on its surface. During this time, the upper part of the moon's crust was broken, remelted, and mixed almost completely. Then about 4 billion years ago, after the crust was cool and solid, the mantle began heating up as a result of radioactive decay. Radioactive decay occurs when unstable radioactive elements such as uranium break down and form other elements. The process generates great amounts of heat, and this is what happened with some elements in the moon's mantle.

The process of radioactive decay generated enough heat to melt portions of the moon's interior, forming lava. Over millions of years, more and more rock was melted, and increasing amounts of lava collected within the mantle. In time, some of this lava worked its way up through the crust and spilled out across huge basins on the lunar surface. As it flowed, the fiery lava cooled and solidified, forming great plains, the lunar maria. Gigantic eruptions of lava took place again and again, over a period of nearly 1 billion years. Then, about 3 billion years ago, the flow of lava came to an end, and the molten rock on the surface cooled. The rock that crystallized in the basins was mainly basalt, a dark rock that led early astronomers to believe that the basins were vast seas. Except for the continuing impact of meteorites, the moon's surface has changed little since the last flows of lava cooled.

Changes on the Earth and the Moon

Most scientists now believe that the Earth was formed in the same general way and at the same time as the moon. After their formation, however, they developed differently.

On both the Earth and the moon, millions of tons of nitrogen, carbon dioxide, water vapor, and other gases were released during the period of lava eruption. On Earth, these substances were held in place by the Earth's gravity, and they formed a protective layer of atmosphere around the planet. Over millions of years, as the Earth's crust cooled, the water vapor in the atmosphere condensed into drops of water and fell as rain. Torrents of water poured across the Earth's surface, forming mighty rivers and vast oceans. This rushing water, together with violent winds and changes in the Earth's crust itself, wore away large craters and other evidence of the planet's early history. The planet's atmosphere and its oceans of water also made life possible.

The moon, however, had a weaker gravitational pull because of its smaller size. As a result, it was unable to hold onto the gases produced by lava eruptions, and all the gases escaped into space. Without an atmosphere or water, the moon had no forces of erosion to change its surface. The changes that have occurred in the last 3 billion years have been caused primarily by meteorites crashing onto its surface. For these billions of years, the moon has looked much as we see it today. Its cratered and mountainous surface is like a partial portrait of Earth when it was young.

William A. Gutsch, Jr. Chairman, American Museum-Hayden Planetarium