It was a beautiful summer evening in Western Colorado. The Sun had just set and our bright sister planet was just starting to pop out in the darkening sky. I quickly pointed the telescope at Venus, which was in a crescent phase, for an eagerly awaiting crowd. The first person squinted into the eyepiece and said, “I see the Moon, but I don’t see Venus.” I was taken back a bit, as the Moon was nowhere to be seen in the sky. Assuring the crowd that I was not deceiving them, I responded, “That IS Venus! Venus goes through phases, just like the Moon!”
We all see the Moon go through phases gradually over the course of a month. After the new moon phase we start to see a sliver crescent, with that side becoming more and more illuminated through first quarter and gibbous phases all the way up to full phase. After this the side that was lit gradually becomes dark.
The Moon goes through phases due to two main reasons: 1) It does not produce its own visible light. We see it illuminated because light from the Sun is reflecting off its surface towards us, and 2) The Moon orbits the Earth. When it is in new phase it is directly between the Earth and the Sun, so all of the light hitting the Moon reflects away from Earth. In full phase, the Moon is directly opposite the Sun, therefore, all of the light hitting the surface reflects back at us.
Galileo was the first to observe that Venus also goes through phases. They can’t be seen with the naked eye, but a small telescope or even binoculars can easily make out the different phases. Why does Venus go through phases? Similar to the Moon, Venus does not produce its own light, however, Venus orbits the Sun not the Earth. Galileo used his observations that Venus goes through all phases, new to full, to conclude that Venus must orbit the Sun and not the Earth, which was a very controversial claim at the time. It was a commonly held belief since ancient times that everything orbits the Earth. In order of closest objects, it was the Moon, Mercury, Venus, and then the Sun orbited the Earth between Venus and Mars. Venus is never observed to be opposite the Sun in the sky, meaning you will never see Venus up at midnight, so it was thought that Venus always stays near the Sun. How could Venus ever be observed in a gibbous or near full phase if it never appears opposite the Sun at midnight, and never can get behind the Sun? This is what convinced Galileo that Venus must orbit the Sun and not the Earth. Now we understand why we never see Venus at midnight. It is an interior planet that orbits the Sun closer than Earth, so we will only see interior planets around dawn or dusk, hence Venus’s nicknames of Morning or Evening Star.
Another feature of Venus’s orbit around the Sun that can easily be observed is that it appears much bigger when it is in crescent phase than when it is in gibbous or full phase. This is because the crescent phase occurs when Venus is closer to the Earth and full phase when it is on the opposite side of the Sun and thus farther away.
Venus is currently in a crescent phase as a Morning Star, gradually turning to first quarter phase by June and full in early January 2026.