Have you ever looked at the Moon and seen a face looking back at you? So have lots of other people—many cultures have myths about a face or other shapes on the surface of the Moon. Of course, the face on the Moon isn’t really a face. It’s an arrangement of large flat areas called maria (that’s Latin for seas, because early astronomers thought these areas were oceans) that were formed by volcanic eruptions. Whether the maria look like a face or something else to you, they are always facing Earth, even when they aren’t illuminated by light from the sun. The side of the Moon that faces Earth is always the same.
However, that doesn’t mean that the Moon always looks the same when we see it in the sky. When we look up at the Moon, what we see depends on where the Moon is in its orbit, the nearly circular path that it travels around Earth. You may know that the sun always illuminates half of the Moon, but because the Moon is constantly moving and changing position along its orbit, the half of the Moon that faces the sun doesn’t always face toward Earth. As the Moon moves around Earth, different parts of the Moon are illuminated by the sun. This makes the Moon look different from night to night. These changes in the Moon’s appearance are called the phases of the Moon, and you’ve probably seen them before. In the sections below, you’ll read about five phases of the Moon: the new moon, the crescent moon, the quarter moon, the gibbous moon, and the full moon.