venus

Why is Venus so bright? It's a cloudy world, only slightly smaller than Earth, and those clouds reflect almost all the sunlight that hits them. The reflection seems especially intense this week because Venus is getting close to Earth: it's only 72 million km away--just a hop, skip and a jump on the vast scale of the solar system.


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Venus' clouds hide the planet's surface. Even the biggest telescopes on Earth can't see what lies below. But if you have a small telescope or binoculars, take a look at Venus anyway. There is something to see: Venus looks like a fat gray banana.

Just like the Moon, Venus has phases. It can be full, gibbous, half or a crescent. These phases occur for the same reason that Moon phases do: geometry. One side of Venus is sunlit (the "dayside"). The other side is dark (the "nightside"). As Venus orbits the Sun it turns one side, then the other, toward Earth. At the moment, Venus is turning its nightside toward us. We can see only a sliver of the dayside--hence the crescent.

In one way Moon-phases and Venus-phases differ: The Moon is bright when it's full, and dim when it's a crescent. Venus is just the opposite. It reaches greatest brilliancy at crescent phase. A full Venus, on the other hand, is dim. Strange but true.

The diagrams below show why. Venus is full when it's on the other side of the Sun. It's dim then because it's far away. Venus is a crescent when it's nearby--big and bright.

Here's something to think about while you're looking at Venus this week: that delicate, beautiful crescent is a hellish world. The planet's bone-dry surface is hot enough to melt lead. Venus' atmosphere, 90 times heavier than Earth's, is almost pure carbon dioxide, a greenhouse gas that traps solar heat. The thick blanketing clouds don't help; they trap heat, too, and they're made of sulfuric acid. Robot-spaceships sent to Venus have landed, but they never last long. Russia's Venera 13 lander operated for 127 minutes--the all-time record--before being overwhelmed by the acid, the heat, and the crushing pressure of Venus' atmosphere.

And you thought Venus was scary when it was just a UFO.