Phases.

Why does the moon seem to change its shape every night? Remember that the moon is a world in space – with a day side and a night side.

Why does the moon seem to change its shape every night?

The answer is the moon is a world in space, just as Earth is. Like Earth, it’s always half illuminated by the sun. In other words, the round globe of the moon has a day side and a night side. From our earthly vantage point, as the moon orbits around Earth, we see varying fractions of its day and night sides. These are the changing phases of the moon.

As the moon orbits Earth, it changes phase in an orderly way.

Read the rest at EarthSky.