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The Phases of Venus

Week of December 15, 1997

The Bitesized snacks have seemed to spend a lot of time these past few weeks relatively nearby the Earth. I want to continue that for this week too. After all, in the southwest sky Venus is hard to ignore!

As the sky darkens outside right after sunset Venus is hard to miss. Shining at about magnitude -4.5, it is the brightest thing in the sky (the Moon this week rises late, after Venus sets, and of course the Sun is not up at night!). Venus is currently so bright for two reasons: one is that it reflects nearly all the sunlight it receives because of its dense cloud layer, and the other is that it is so close to the Earth.

Venus travels around the Sun like all planets do. Since its orbit is smaller than the Earth's, Venus can go through dramatic phases just like the Moon does. When Venus is on the far side of the Sun, we see the entire near side lit by the Sun. But then it is as far away as it gets, nearly 300 million kilometers away. When it is on the near side of the Sun, it shows a beautiful thin crescent, because it is between us and the Sun and we see the mostly unlit dark half. Even though less of Venus is lit percentage-wise when it is a crescent, it is over six times closer to the Earth than when it is on the far side of the Sun and so it looks much bigger. At a mere 50 million kilometers away, the amount of lit planet we actually see is much larger then when it is full, and so it appears brighter.

To see this for yourself, draw a circle one centimeter across, then, next to it, draw a circle 6 centimeters across. The small circle represents Venus when it is on the far side of the Sun, and the big circle is when it is nearby. Now draw an arc inside the big circle so that it looks like a crescent Moon, making the thickest part of the crescent one centimeter thick. Compare the area of the crescent with the area of the small circle. You can see why Venus is so bright right now! Much more apparent area of it is lit and so it looks brighter.

Incidentally, when Venus is very low on the horizon, it can still appear very bright. Atmospheric effects from the Earth's air can make it flicker and twinkle, rapidly changing color from green to red to orange to blue to yellow. Not only that, but imagine you are seeing it as you drive a car down a tree lined road. It looks like a bright light is following you, changing colors and brightness as you drive! No wonder so many people report it as a UFO.

©2008 Phil Plait. All Rights Reserved.

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