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December 2003 Feature: Mercury At Evening

Mercury, a planet that's often lost in the glare of the Sun, tantalizes sky watchers this December. That's because Mercury swings to the eastern edge of its orbit as seen from Earth, staying out in the evening sky for an elongated amount of time. It's first seen in the southwest sky about thirty to forty-five minutes after sunset, and shines for some thirty to forty-five minutes after it is visible. It sets by the time twilight gives way to night.

Southern latitudes have the advantage, because Mercury reaches a bit higher in the sky, and stays out later after sunset. No matter where you live, however, an unobstructed view to the southwest is a must. Even though Mercury shines respectably brightly, the twilight subdues its lustre. Binoculars help out in the quest.

First of all, look for Venus to blaze rather low in the southwest sky, your signpost to the elusive planet. Excepting the Moon and the Sun, Venus outshines any planet or star and should be easy pickings. Search to the lower right of Venus with binoculars or your eyes alone. Chances are good that you'll spot Mercury, the fleeting phantom of twilight.

Mercury, the closest planet to the Sun, has no protective blanket of atmosphere. For this reason, Mercury has an enormous variation in temperature -- probably the greatest of any planet in our solar system. According to the World Almanac, it can get up to 845 degrees F. during the day and -300 degrees F. at night.

Mercury can boast of other superlatives too. It has the shortest year of any planet in our solar system, circling the Sun in 88 Earth days. More amazing, perhaps, Mercury's day is twice as long as its year. Noon to noon on Mercury is the equivalent of 176 Earth days, the longest of any planet.

The strangest thing about Mercury is that it actually brightens as it gets further away from the Earth. Rather mind-boggling, but apparently this is due to a phenomenon called "opposition effect" -- a possible topic for future article.

copyright December 2003 by Bruce McClure

Another December Feature: Saturn and Sirus