Even in ancient times, astronomers knew that Venus changed in brightness in the sky. Sometimes it’s like a dim star, and other times it becomes the brightest object in the sky (after the Moon); bright enough to cast shadows. But it wasn’t until Galileo first turned his rudimentary telescope on Venus in 1610 that astronomers first realized that Venus goes through phases, just like the Moon.
Think about the orbit of Venus for a moment. As you know, Venus orbits closer in to the Sun than Earth. One half of the planet is always in sunlight, and the other half of the planet is in shadow. It’s our view of Venus that changes. Sometimes we see Venus on one side of the Sun, and other times we see it on the other side. We can never see when Venus is completely illuminated because that’s when it’s on the opposite side of the Sun. We also can’t see when it’s completely in shadow because then it’s in between the Earth and the Sun, and the Sun obscures Venus from our view.
Just like the Moon, Venus goes through a full range of phases. When Venus has just passed out from behind the Sun, it’s almost a full circle, but it’s dim because it’s nearly at its most distant point from Earth. Then it “catches up” to Earth’s orbit as it travels around the Sun. Venus becomes brighter and brighter but also does into a half phase and eventually a slim crescent. You might be surprised to know that Venus is at its brightest when it’s a slender crescent.
The only way to see the phases of Venus is through a telescope. So find a friend with a telescope, ask them when Venus is going to be bright in the sky, and ask them for a chance to take a look.
We have written many articles on Universe Today about observing Venus. Here’s one article about a time when Venus, the Moon and Jupiter were all visible in the sky at the same time, and here’s one about Venus and Jupiter.
We have also recorded a whole episode of Astronomy Cast that’s just about planet Venus. Listen to it here, Episode 50: Venus.