NASA - National Aeronautics and Space Administration

The moon is the brightest object in the night sky. On some nights, the moon looks like a shining circle of light. On others, it looks like a thin, silver fingernail. However, the moon does not make its own light. The light we see comes from the sun and bounces off the moon.

Image to left: The moon's surface shows striking contrasts of light and dark. Credit: Lunar and Planetary Institute

If the moon and Earth were side by side, the moon would look like a tennis ball next to a basketball. It looks bigger than the stars and as large as the sun because it is closer to Earth than any other object in space. But the moon is about 400 times smaller than the sun, and it is not all that close to Earth. A rocket journey from Earth to the moon and back takes about six days.

The moon is a huge rock that travels around the Earth. Nothing lives on the moon. The moon has no air and no wind. Its surface is dry. Its sky is always black, even in the daytime, and the stars are always visible. At night, the moon's rocky surface is colder than any place on Earth. During the day, the rocks are hotter than boiling water.

Astronauts who visit the moon carry air with them to breathe. They talk to each other by radio because there is no air to carry sounds. Space suits protect astronauts from the heat and the cold.

Image to right: The diameter of the moon is about 2,160 miles (3,476 kilometers), or about a fourth of Earth's diameter. If the moon were placed on top of the United States, it would reach almost from San Francisco to Cleveland. Credit: World Book diagram

Astronauts can walk easily on the moon, even though they wear heavy equipment. The equipment feels light because the force of gravity on the moon is weaker than gravity on Earth. Gravity on Earth keeps us from floating away in space.

Earth is not the only planet with a moon. Some of the other planets have moons, too. Jupiter has at least 63 moons.

The "Man in the Moon"

The face of the "man in the moon" is made by light and dark patches of gray on the moon's surface. The dark patches are flat areas of rock covered by a thin layer of rocky soil. The rocky soil of the moon has tiny round bits of glass in it. Some of the rocks are made of hardened lava. Most of the light gray parts are filled with mountains and valleys. The moon also has many big, bowl-shaped holes called craters.

There are so many craters on the moon that the moon has craters within craters. Some craters have hills or mountains in them or around them. Some are surrounded by light gray streaks. The largest crater has a floor covered by dark lava. It forms one eye of the "man in the moon."

Image to left: The far side of the moon has a rugged surface. The large crater in the center of the photograph is International Astronomical Union Crater No. 308. It is about 50 miles (80 kilometers) wide. Credit: NASA

Many small craters were formed when meteoroids hit the moon. Meteoroids are rocks or other objects that travel through space. Many large craters probably formed when larger objects, such as comets or asteroids, hit the moon. A few craters look like the tops of volcanoes.

We cannot see all sides of the moon from Earth. The part of the moon we cannot see from Earth is called the far side. Pictures from spaceships have shown us what it looks like. U.S. astronauts also traveled around the far side in 1968.

The Changing Moon

The moon travels around Earth in an oval-shaped path about once every 27 days. Earth's gravity keeps the moon on this path. The moon also spins around once during each trip around Earth. One full day on the moon lasts about 30 Earth days.

We see the moon change from a slim crescent shape to a full circle and back again. But the moon does not really change its size or shape. The amount of sunlight reflected by the moon toward Earth changes. When the moon is between the sun and Earth, its sunlit side is turned away from Earth. So, we don't see the moon. This is called a new moon. Slowly, more and more of the sunlit side is turned toward Earth. About 14 days later, we can see the entire sunlit side of the moon. It looks like a shining globe. This is called a full moon. Slowly, the sunlit side turns away until the moon looks completely darkened about 14 days later.

Image to right: This diagram shows why we can never see the far side of the moon. Even though the moon turns around once as it goes around the Earth, the far side -- shown by the flag -- is always turned away from us. Credit: World Book illustration by James Conahan

How the Moon Was Formed

The moon is about 4.5 billion years old. Scientists have several ideas about how it may have formed. Some think the moon was a lump on Earth that broke off. Some scientists believe that Earth and the moon were formed at the same time. Others think that the moon was traveling in space when Earth's gravity suddenly captured it. Another idea is that a large comet or asteroid smashed into Earth and knocked off a piece that became the moon.

The Moon in History

For thousands of years, people have used the moon to measure time. For example, the American Indians recorded that a hunt took place a certain number of "moons," or days, ago. Muslim people use a calendar based on the moon.

Early people thought the moon was a powerful god or goddess. Some believed that the moon influenced life on Earth. Even today, many people believe that the moon affects the weather and people's behavior.

The famous Italian scientist Galileo made the first scientific study of the moon with a simple telescope in 1609. In 1959, the former Soviet Union launched Luna 2, the first spacecraft to reach the moon. In 1969, the U.S. astronauts of the Apollo 11 mission became the first people to set foot on the moon. The astronauts collected bits of rock and soil, took photographs, set up experiments, and explored the area nearby. In January 1998, the U.S. probe Lunar Prospector found evidence of ice on the moon. Scientists think that the polar regions may contain millions of tons of ice.

How to cite this article: To cite this article, World Book recommends the following format: "Moon." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.

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