The Moon is Earth’s only natural satellite. It is 2,160 miles (3,476 kilometers) across, which is a little more than one quarter of Earth’s diameter, or about the distance from New York City to Tucson, Arizona. The Moon orbits Earth once every 27.3 days.

The Moon has no atmosphere and no liquid water at its surface, so it has no wind or weather at all. On the lunar surface, there is no protection from the Sun’s rays, and no ability to retain heat like the greenhouse effect on Earth. Temperatures on the moon range from about –280 degrees Fahrenheit (–138 degrees Celsius) to –148 degrees Fahrenheit (–100 degrees Celsius). The Moon’s surface is covered with rocks, mountains, craters, and vast low plains called maria (“seas”).