Starts With A Bang!

Why is Venus the Brightest?

Sometimes, when you look up in either the early, pre-dawn morning or early, post-sunset evening sky, there’s one point of light that outshines all the others. It’s usually relatively close to where the Sun was, and unlike most points of light you see in the night sky, this one doesn’t twinkle. I’m talking, of course, about the planet Venus:

Yes, it isn’t nearly as bright as the Moon, but it’s certainly much brighter than everything else you can see. Well, it was only a matter of time before someone wrote in and asked why. Reader Dan asks:

Can you explain why Venus is so bright in the sky right now? I don’t think I have ever seen it so bright in my life.

Venus, as we’ve talked about before, is covered in a very thick layer of greenhouse gases. This makes its surface extremely reflective, so that about 70% of the sunlight that comes in to Venus gets reflected as visible light.

But Venus is also extremely close to Earth. In fact, Venus gets within about 40 million km of Earth, which is less than 1/3 of the distance to the Sun. At its farthest, Venus is about 250 million km from Earth. Why? Because Venus and Earth both orbit the Sun; sometimes they’re on the same side as one another, and sometimes they’re on opposite sides. When they’re both on the same side, Venus has its closest approach to Earth:

But I’m going to force you to think about the geometry of this a little bit. One side of Venus faces the Sun, and gets illuminated; the other one is dark. Imagine that the Earth is far away from Venus. What will the planet Venus then look like from Earth? It will be mostly “full”, and the closer to being in direct opposition to Earth, the more “full” Venus will appear. But the more “full” it appears, the further away it is, and so it should also look smaller. Take a look at these shots of Venus, through the same telescope, when it’s far away from Earth:

When it’s more full, it’s also smaller. But when less of it is illuminated, it’s closer to us. Take a look at what Venus looks like when it’s closest to us:

So when Venus is close to us, it looks like a crescent, but it looks like a very large crescent, and when it’s far from us, it looks like a disc, but a much smaller disc. Which of these two things is more important for the brightness of Venus?

Well, we can find out. You see, Dan is right; Venus is nearly at its brightest right now. Have a pair of binoculars? Well, about 11 days ago, somebody did, and photographed Venus and the Moon together in the sky. Here’s what they saw:

Venus is at its brightest when it’s a crescent! It turns out that being closer to us means everything. Astronomers use magnitudes to measure brightness, and the smaller your number is, the brighter you are. This is useful for stars, where very bright stars are typically 0 or 1 (the very brightest, Sirius, is -1.5), and the dimmest ones visible with the naked eye are 5 or 6.

What about things that aren’t stars, though? Not surprisingly, the Sun is the brightest, at -26.7, and the Full Moon is second, at -12.6. But Venus is third! When Venus is a crescent, it’s -4.6, and when it’s full (but far away), it’s -3.8, which is interesting! Why? Because when it’s full, you can’t see it during the day, even under optimal conditions. But right now, when it’s a crescent, you can! Check out this photograph by John Harper of Venus during the day:

It’s a crescent, just like we said it should be! So that is why Venus is the brightest thing in the sky, and that’s why it’s brighter now than it usually is. Get out your binoculars and have a look! And if you do it tonight, look on the other side of you too, because the Moon is full and it rises at 8:30 PM! Enjoy!