The moon is Earth’s only natural satellite. The moon is a cold, dry orb whose surface is studded with craters and strewn with rocks and dust (called regolith). The moon has no atmosphere. Recent lunar missions indicate that there might be some frozen ice at the poles. The same side of the moon always faces the Earth. The far side of the moon was first observed by humans in 1959 when the unmanned Soviet Luna 3 mission orbited the moon and photographed it. Neil Armstrong and Buzz Aldrin (on NASA’s Apollo 11 mission, which also included Michael Collins) were the first people to walk on the moon, on July 20, 1969.If you were standing on the moon, the sky would always appear dark, even during the daytime. Also, from any spot on the moon (except on the far side of the moon where you cannot see the Earth), the Earth would always be in the same place in the sky; the phase of the Earth changes and the Earth rotates, displaying various continents. The moon is about 238,900 miles (384,000 km) from Earth on average. At its closest approach (the lunar perigee) the moon is 221,460 miles (356,410 km) from the Earth. At its farthest approach (its apogee) the moon is 252,700 miles (406,700 km) from the Earth.The moon revolves around the Earth in about one month (27 days 8 hours). It rotates around its own axis in the same amount of time. The same side of the moon always faces the Earth; it is in a synchronous rotation with the Earth. The Moon’s orbit is expanding over time as it slows down (the Earth is also slowing down as it loses energy). For example, a billion years ago, the Moon was much closer to the Earth (roughly 200,000 kilometers) and took only 20 days to orbit the Earth. Also, one Earth ‘day’ was about 18 hours long (instead of our 24 hour day). The tides on Earth were also much stronger since the moon was closer to the Earth. The surface of the moon is scarred by millions of (mostly circular) impact craters, caused by asteroids, comets, and meteorites. There is no atmosphere on the moon to help protect it from bombardment from potential impactors (most objects from space burn up in our atmosphere). Also, there is no erosion (wind or precipitation) and little geologic activity to wear away these craters, so they remain unchanged until another new impact changes it.These craters range in size up to many hundreds of kilometers, but the most enormous craters have been flooded by lava, and only parts of the outline are visible. The low elevation maria (seas) have fewer craters than other areas. This is because these areas formed more recently, and have had less time to be hit. The biggest intact lunar crater is Clavius which is 100 miles (160 km) in diameter.