Planetary phase

Planetary phase is a way of describing how much we can see of a planet. Just like when you look at a moon, sometimes it's a full moon and sometimes it's only a sliver of the moon. With planets, sometimes we can see most of the planet, sometimes just a tiny part. To describe this, astronomers compare the size of the lit area of the planet to a circle. A planet might be in a waxing gibbous phase, which means that it looks like a round circle with a lot of the side lit up. On the other hand, a planet might be in a waning crescent phase, which means it looks like a circle with only a tiny bit of the bright side visible.