Why does the Moon seem to change shape? Let us start with the basics, one fact at a time. Earth has just one Moon, and it goes around Earth in a path called an orbit. The Moon does not make its own light. It shines because it reflects sunlight. The Sun always lights half of the Moon, but how much of that lit half we can see from Earth changes during the month. The Moon's shape does not really change during the month. What changes is how much of its sunlit side we can see.

Look at the two ends of the month, and then at what lies between them. At a full Moon, we can see all of the Moon's sunlit side. At a new Moon, we can hardly see the Moon at all. The Moon goes through eight phases, from new Moon to full Moon and back again. Between them come the half-lit shapes: at first quarter we see half of the Moon's lit side, which is why it is called a quarter Moon.

Waxing means the lit part of the Moon that we see is growing, and waning means it is shrinking. A full trip through the phases takes time. It takes about 29.5 days to go from one new Moon to the next, or from one full Moon to the next. A calendar month is about the length of time from one full Moon to the next. The Moon's phases happen because the Moon, Earth and Sun change positions as the Moon orbits Earth.

We always see the same side of the Moon from Earth. People sometimes call the far side of the Moon the dark side, but that is misleading, because the far side gets sunlight too. Until a Soviet spacecraft flew past in 1959, nobody had seen the far side of the Moon. Let us look at the side we cannot see, and at how people finally saw it.

Sometimes a shadow changes how the Moon looks, and there are two kinds of eclipse to know. Here is how shadows come into it. A lunar eclipse happens when Earth's shadow falls on the Moon. During a total lunar eclipse the Moon can look red or orange. A solar eclipse is different. A solar eclipse happens when the Moon passes between the Sun and Earth, which can only happen at a new Moon.

What does the lit side of the Moon show if we look closely? The Moon has a rocky surface covered with craters, which were made when space rocks crashed into it. Galileo used a telescope to see that the Moon has mountains and craters and is not smooth. The Moon's dark patches are called maria, a Latin word meaning seas.

Now for some numbers about the Moon. The Moon is about 2,160 miles (3,475 kilometers) across, and it is less than a third as wide as Earth. On average the Moon is about 239,000 miles (384,000 kilometers) from Earth. Its days are very hot and its nights are very cold. In full sunlight its surface can pass 250 degrees Fahrenheit, and in darkness it can fall below minus 240 degrees Fahrenheit.

Finally, people have gone there, and the story of their visits is short. In 1969, astronaut Neil Armstrong became the first person to walk on the Moon. People have landed on the Moon on six Apollo missions: Apollo 11, 12, 14, 15, 16 and 17. As of November 2026, no one has walked on the Moon since Apollo 17 left in December 1972, and the Moon is the only place beyond Earth that people have ever visited.

Words to know

• reflects: bounces light
• phases: moon shapes
• orbit: path around Earth
• eclipse: shadow over light
• craters: bowl-shaped holes