1 / 9

Wax On, Wane Off: A Guide to All the Lunar Phases

It's the brightest object in the night sky. Our moon is our closest planetary neighbor and has inspired countless astronauts to venture into space, been the muse of many writers and sparked the imaginations of science fiction writers around the world.

We can see the moon because light from the sun is reflected off of its surface. But it doesn't always look the same. When Earth is between the moon and the sun, we see a fully lit moon. A waxing and waning moon is growing or shrinking in illumination, respectively. When the moon slips between Earth and the sun, we see a new moon, shrouded in darkness. Because the moon is a sphere like Earth, half of its surface is always illuminated by the sun. The demarcation line between the light and dark side of the moon is called the terminator.

It takes the moon about one month—29.5 days on average—to complete a full lunar cycle, called lunation, and it should come as no surprise that the word "moon" and "month" have the same root.

Here's a quick guide to the moon's phases.