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SHARON SHANKS | The Cosmos Want to see Mercury? Just look to the west



Published: Sun, May 13, 2001 @ 12:00 a.m.



I've sometimes wondered which came first: the planet or the god. We teach that the five planets we can see traversing our nighttime skies were named for ancient gods. But couldn't it be the other way around -- couldn't it be that the movements and appearance of the planets influenced the ideas and names of the gods themselves?

Take Mercury, for example. It speeds across our skies, seeming to dart in and out of sight from the protective glow of the sun. We never see it clearly for long. Passed along to us through the murky corridors of time is the idea that Mercury is the Roman name for Hermes, the son of Jupiter and Maia, who served as the quick messenger of the gods. Some believe that the idea of Mercury originated with the Greeks, however, who brought Hermes with them to Rome when they arrived with trade goods.

Mercury was the god of science and commerce. But he was also the god of thieves, rogues and vagabonds.

I tell students that Mercury was named for the messenger of the gods because of its speedy nature (an attribute that works just as well for a thief), but couldn't it be that the idea of the god came from the planet? The seeming quick movements of the planet itself could have led to the idea of a god with similar qualities.

Getting philosophical: Oops. Wait a minute. This is leading into one of those discussions more appropriate for a group of philosophy or linguistics students (and perhaps physics majors, but they'd insist on a spherical god) instead of an astronomy column.

Whatever the origins of its name, Mercury was around long before there were people to look at it, and it is undeniably an elusive target for today's modern stargazers. If you want a good look at Mercury, now's the time to do it, since it's in the best viewing location of the year.

If you have a clear western horizon, you're in luck. If your view to the west is blocked by trees and houses, try to find a spot that clear of today's urban clutter. Watch tonight and the next several nights for Mercury to draw close to Jupiter just above the western horizon at dark. By Wednesday, the two will be only 2 degrees apart as Mercury makes its mad dash into the night sky and passes Jupiter while the giant planet moves in its slow, majestic pace to become lost in the glare of the sun.

When to watch: Start watching at 9 p.m. that night, just after the sun sets. Above the horizon in the west-northwest will be three fairly bright objects. The brightest and farthest to the right will be Jupiter. Just above it and to the right will be Mercury. The other bright object to the left and about the same height above the horizon will be Betelgeuse, the mighty red giant planet in the constellation of Orion. Depending upon how flat your western horizon is, you might be able to see the three until just before 10 p.m., and then they'll be gone.

For a real challenge, try to spot Mercury in the company of the waxing crescent moon the night of May 24.

Jupiter will be lost in the glare of the sun by the end of the month. Saturn has preceded it, already virtually lost in the nighttime skies. Both will reappear around the end of July in the morning skies before the sun rises.

As far as planets go, Mars is taking over as the lord of the nighttime dance. The planet named for Ares, the god of war (or did the idea of a god of war come from the red appearance of the planet?) is rising just before midnight in the east. It will herald the arrival of the bright constellations of summer to our southern skies: the stars of Scorpius and Sagittarius.

Mercury, of course, is the planet closest to the sun. It puzzled ancient astronomers. In those brief periods when it was visible through their telescopes, Mercury seemed to show the same & quot;face & quot; toward early planet watchers all the time. This led to the belief that Mercury was tidally locked in orbit around the sun, in the same way that the moon is tidally locked in its orbit around the Earth.

Similar rotations: In other words, their observations seemed to show that the length of Mercury's & quot;day & quot; (or one rotation) was 88 days, the same time it takes the planet to orbit the sun. (The moon rotates about once every 29.5 days, the same time it takes it to orbit the Earth.)

This & quot;tidal locking & quot; is caused by gravity. Over a long period of time, Mercury has & quot;bulged & quot; toward the sun so that a denser area has migrated toward the control of the sun's great gravitational pull. The same effect has happened to our moon. For our moon, this has created a 1-to-1 resonance with Earth: It shows the same face to us all the time. It's not the same with Mercury.

It wasn't until 1965 that technology allowed us to determine that Mercury isn't showing the same side toward the sun all the time, but that its motion was much more bizarre. Mercury actually rotates about once every 58 days and fraction, exactly two thirds of the time it takes to complete one orbit. Its resonance is 3-to-2: It rotates three times during every two orbits around the sun.




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