The planet Venus, so bright it is often mistaken for an alien spaceship, reaches maximum brilliancy this week.
May 3, 2004: Picture this: You're in a car riding along a country road at night. The sky is clear; the stars are twinkling. The silhouettes of moonlit trees glide by the side window. Flash! A blue-white light beams through a gap in the forest. Flash! It happens again. And again, and again. It's following you.
In the movies, this is when the spaceship lands. A door opens. Eerie-green lights flood the roadside. Something alien steps out ... and you have a Close Encounter. Time to dial 9-1-1!
Relax. It's only Venus, the second planet from the Sun.
Right: Venus and the Moon beam through some trees in Brittany, France. Photo credit: Laurent Laveder.
Venus is the brightest of all planets. It makes Sirius, the brightest star in the night sky, look feeble. At sundown Venus materializes close to the western horizon where it can beam through trees and make you think you're being chased by something from outer space. No wonder so many people call 9-1-1 to report a UFO when they see it.
This week Venus (magnitude -4.5) is at maximum brightness. It is 8 times brighter than the planet Jupiter (magnitude -2.3), 23 times brighter than Sirius (magnitude -1.1), and 275 times brighter than the planet Mars (magnitude +1.6). Venus can actually cast faint shadows; only the Sun and Moon outshine it.
Above: Looking west at sunset in early May 2004.
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.
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.