Chris Anderson
Skywatch

On Saturday, evening sky watchers will be treated to the closest pairing of planets for 2007, when Venus sidles up to Saturn in the western sky.

The two planets will appear so close together that you won't be able to fit the width of your pinkie finger at arm's length between them. But as close as they'll appear in the sky, the two will look quite different, with Venus shining over eighty times brighter than Saturn.

On average, Venus is the closest of the naked-eye planets, while Saturn is the farthest, which goes a long way toward explaining the difference in their brightness. Furthermore, Venus is nearly thirteen times closer to the Sun than Saturn, making the sunlight that illuminates it over 160 times stronger. Add to that the fact that Saturn is darker than Venus, reflecting about half of the sunlight that falls on it (versus Venus, which reflects two-thirds), and it's perhaps a bit surprising that Venus is only eighty times brighter.

Three factors keep Saturn from being totally overwhelmed by Venus.

Saturn is nearly 10 times wider than Venus, making the surface area of its illuminated side is almost 100 times larger. And, because Saturn is a superior planet (farther from the Sun than Earth), we always see nearly all of its illuminated side. But Venus, an inferior planet, only turns its fully illuminated side towards us when it's too close to the Sun to view. Venus was in its third quarter phase two weeks ago and is currently a waning crescent, with slightly less than half of its lit up side now facing us. Finally, Saturn has its famous rings, which are not only large, but also as reflective as fresh snow.

If Venus doesn't look 80 times brighter than its ringed cousin, it's because the human eye's response to stimulus is non-linear. But that's a topic for another column.

Next week: Has a "second Earth" been found?

Sky calendar through Saturday: Planets: One hour before sunrise: Mars: E, mid-sky One hour after sunset: Venus: W, low Saturn: W, low Jupiter: SSE, low Moon: Full moon Saturday, 7:49am.