From Earth we see the Moon change shape bit by bit each night. We call these changing shapes phases. The phases begin with a new moon. The Moon is not visible during this phase. Gradually, more and more of the Moon becomes visible. When we can see half of the lit portion of the Moon, it is in the first quarter phase. A gibbous moon is more than half visible. The Moon appears to continue expanding until it becomes a full moon and appears at its largest and roundest. After the full moon, the lit portion of the Moon we see begins to diminish until the third quarter phase shows the Moon as a half circle. Our view of the lit part of the Moon continues to become a smaller and smaller crescent until the new moon phase comes once again.

The Sun's reflection makes a portion of the Moon (during its various phases) visible from Earth. The Moon does not make its own light. Half the Moon is always lit by sunlight except during an eclipse. We see different Moon shapes because our viewpoint changes relative to the half of the Moon that is lit by sunlight.

The orbiting Moon changes position in relation to the Earth and Sun. When the Moon is opposite the Earth from the Sun, sunlight reflects off the entire side facing us, showing us a full moon. During the new moon phase, the Moon is between the Sun and Earth. The sunlight reflects off the side of the Moon that is turned away from us. We see different amounts of reflected sunlight in phases between the full moon and new moon.

The same side of the Moon always faces the Earth because it takes the Moon the same amount of time to rotate on its axis (27 days, 8 hours) as it does to orbit the Earth. Its spinning keeps up with its motion around the Earth so that we never see the Moon's "dark side." Each full moon gives us the same view of the Moon.

Because it takes almost a month for the Moon to orbit Earth, each side of the Moon is in sunlight for about two weeks and dark for the other two weeks.

When NASA was planning the first Apollo Moon Landing missions, they had to decide the best time to land on the sunny side of the Moon. The sunlight had to be just right so the pilot would have a good view of the landing site. When the Sun is too high in the sky, the Moon's craters appear washed out, making it hard for the Apollo pilot to see the lunar surface. When the Sun's angle is too low, the shadow of the crater rim fills the floor of the crater and makes it hard for the pilot to see.

Pilots need to see the craters and other landforms clearly and ensure that sunlight doesn't shine directly into their eyes.