Four keys to understanding moon phases

EarthzineEarth Observation

Artist’s concept of the moon, Earth and sun aligned in space. Image via

Why does the moon seem to change its shape every night? Why can I see the moon in the daytime?
The answer to both questions is the same. It’s that the moon is a world in space, just as Earth is. Like Earth, the moon is always half illuminated by the sun; 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. And the moon is in the daytime sky about half the time. It’s just that it’s sometimes it’s so near the sun we don’t notice it. How can you understand moon phases? Here are four things to remember:
1. When you see the moon, think of the whereabouts of the sun