Elusive Mercury

This is a perfect time to look for elusive Mercury, the planet closest to the Sun. Because it's so much closer to the Sun than we are, it usually appears less than a handwidth (when your arm is fully extended) away from the Sun. But for the next few weeks it will be as far east of the Sun as it ever gets, and we should be able to see it. Mercury is that bright "star" right next to the crescent Moon tonight. By tomorrow the Moon will have moved on; Mercury will then be about halfway between the Moon and the setting Sun.

Three times closer to the Sun than we are, Mercury is baked by its heat. Daytime temperatures rise to over eight hundred degrees Fahrenheit. But since Mercury has almost no insulating atmosphere, at night the temperature can drop to two hundred degrees below zero. Nowhere else in the solar system is there this fantastic range of temperature.

Almost as small as our moon, Mercury's gravity is so weak that a 120 pound woman would weigh only 40 pounds there. But before you dream of a Mercurian vacation, think about those extremes of heat and cold. To make matters worse, Mercury rotates so slowly on its axis that its "day" is almost five months long. That's a lot of time to be very hot -- or very cold. And of course, you would weigh just as much when you returned home.

Hard as it is to believe from those pictures of the solar system we saw in school, Mercury is really our closest neighbor. It's true that the orbits of Venus and Mars are closer to the orbit of the Earth than is Mercury's, but the planet Mercury is more often closer to the Earth than either Venus or Mars. That's because Mercury rotates around the Sun so fast (in about three months) that it's often very close to us. On the other hand, placid Venus and Mars, moving much slower than Mercury, are frequently on the other side of the Sun, and very far away.

(05/06/08)