If you have looked into the night sky, you may have noticed the Moon appears to change shape each night. Some nights, the Moon might look like a narrow crescent. Other nights, the Moon might look like a bright circle. And on other nights, you might not be able to see the Moon at all. The different shapes of the Moon that we see at different times of the month are called the Moon’s phases.
Why does this happen? The shape of the Moon isn’t changing throughout the month. However, our view of the Moon does change.
The Moon does not produce its own light. There is only one source of light in our solar system, and that is the Sun. Without the Sun, our Moon would be completely dark. What you may have heard referred to as “moonlight” is actually just sunlight reflecting off of the Moon’s surface.
The Sun’s light comes from one direction, and it always illuminates, or lights up, one half of the Moon – the side of the Moon that is facing the Sun. The other side of the Moon is dark.
The position of the Moon and the Sun during Each of the Moon’s phases and the Moon as it appears from Earth during each phase. Credit: NASA/JPL-Caltech
On Earth, our view of the illuminated part of the Moon changes each night, depending on where the Moon is in its orbit, or path, around Earth. When we have a full view of the completely illuminated side of the Moon, that phase is known as a full moon.
But following the night of each full moon, as the Moon orbits around Earth, we start to see less of the Moon lit by the Sun. Eventually, the Moon reaches a point in its orbit when we don’t see any of the Moon illuminated. At that point, the far side of the Moon is facing the Sun. This phase is called a new moon. During the new moon, the side facing Earth is dark.