While other planets have as many as 61 moons orbiting them, the moon is the Earth's only satellite. Unlike the Earth, the moon is an inhospitable realm with no atmosphere and extreme temperatures. Because the moon lacks an atmosphere, it is perpetually dark. The moon is full of impact craters, rocks, smooth plains, and mountains. One crater, known as Clavius, is more than 100 miles in diameter. There are also rilles on the moon. A rille is a long, narrow valley. The Hadley Rille is 75 miles long and over 1,300 feet deep. The Earth is thought to be about 60 million years older than the moon. The moon was probably formed following a collision between Earth and a planet-sized object.

The moon is located about 238,900 miles from the Earth, though it can be as close as 221,000 miles and as far as 252,000 miles. If you were to walk 238,900 miles, assuming you could walk four miles in an hour, it would you about seven years if you never took a break. The moon orbits the Earth (actually, the moon and Earth orbit the sun together in a system). It takes 27.8 for the moon to make a complete orbit. At various times during its orbit, different amounts of the moon is visible on Earth (known as moon phases), though the same side of the moon always faces the Earth. Scientists believe the moon's orbit time has increased. About a billion years ago, it took the moon only 20 days to orbit the Earth. The moon's gravitational pull, which is much stronger than the sun's gravitational pull causes ocean levels (tides) to rise and fall twice each day.

The moon is a very different place than the Earth. Its gravity is only about 17 percent of Earth's gravity. A 100 pound Earthling would weigh a mere 17 pounds on the Moon. Temperatures on the moon vary from highs of nearly 265 degrees F to lows of about 170 degrees F. The moon is only a little more than one percent of the Earth's mass.

On July 20, 1969, Neil Armstrong became the first human being to walk on the surface of the moon as part of the Apollo 11 mission. His quote, "That's one small step for [a] man, one giant leap for mankind," while on the moon, is one of the most famous quotes in recent history. As of 2005, 12 people have walked on the moon. Today, scientists are searching for evidence of the current or past existence of water on the moon.