Phases of the Moon

The moon doesn't produce its own light. You can see the moon because it reflects light from the sun. When you look at the moon, it sometimes appears full and bright, and sometimes appears as only a sliver. The different shapes of the moon visible from Earth are called phases.

The moon's phases are caused by changes in the relative positions of the moon, sun, and Earth as the moon revolves around Earth. Sunlight illuminates half of the moon. The phase of the moon depends on how much of the sunlit portion of the moon is facing Earth.

If you were to track the appearance of the moon from day to day, you would notice that the moon's phases change according to a regular cycle. Figure 8 shows the positions of the moon during its phases. A full moon occurs when the moon is on the opposite side of Earth from the sun, and the whole side of the moon facing Earth is lit by the sun. A new moon occurs when the moon is directly between the sun and Earth. A new moon cannot be seen from Earth because the sunlit side of the moon is facing away from us.

It takes about 29.5 days for the moon to complete a full cycle of phases from full moon to new moon and back again. This period of time is known as a lunar month.

Figure 7 The moon is thought to have formed in a spectacular collision between the early Earth and a Mars-sized object. A The collision knocked part of Earth's mantle into space. B Much of the ejected material began to revolve around Earth. It soon began to clump together to form the moon. C Earth and the moon today.