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Comets Go WHOOSH!

Bad Astronomy: Comets whiz across the sky in a few seconds.

Good astronomy: Comets appear to move very slowly, usually taking days to noticeably move.


What's going on:
There has been a commercial getting a lot of air time on American television recently involving chewing gum. In this particular ad, an astronomer has been waiting all his life to see a bright comet. While he's waiting, he realizes his chewing gum has lost its flavor, and he starts searching through his pockets for another piece. As he is looking down, the comet whizzes past the observatory, and he has missed a once-in-a-lifetime event! Poor guy!

Or is he just a victim of Bad Astronomy?

Comets are usually considered to be collections of rock and ice, held together like a loosely packed snow ball, and orbiting the Sun typically in highly elliptical orbits. The orbital periods of comets are measured in years; the most famous comet of all, Comet Halley (usually--- and incorrectly-- called ``Halley's Comet''), takes about 76 years to orbit the Sun just once. That's a long time, and Halley is considered a short-period comet! Most comets have orbits that take thousands of years to complete one circuit.

Now, when a comet gets near the Earth, it tends to be moving pretty quickly. A typical speed might be 50 or more kilometers per second. But remember, a typical comet might be millions of kilometers away! Even at a speed of 50 k/s, it will appear to be moving v-e-r-y s-l-o-w-l-y. You could watch a typical comet for hours and not see it move!

For a comet to be really bright, it has to get very close to the Earth. The recent apparition of the comet Hyakutake is a good example. Here was a comet that was easily visible to the naked eye, and it got only 20 or so million kilometers away at closest approach. With a pair of binoculars, the movement was obvious after less than an hour, and through a telescope you could watch it move in only a few minutes. But this still a far cry from whizzing across the sky.

Perhaps the advertisers were confusing a comet with a meteor. These do indeed whiz across the sky in a few seconds, and can be very bright. Ironically, many meteors come from comets-- they are chunks of rock and ice that have been separated from the parent body and find their way to the Earth. But note that meteors are VERY close-- they burn up in the Earth's atmosphere. Comets hit the Earth sometimes too, but you won't find too many people happy to see that-- a comet hit Jupiter in 1994 and the ensuing explosion was equivalent to 600 times the entire Earth's nuclear arsenal! Years later we are still seeing the effects.

So actually you can now see that our hero, the patient astronomer, had plenty of time -- maybe even years -- to find his gum. He could have driven to South America and grown his own chicle tree in that time!

Want to know more about comets? Here are some links:

  • Gary Kronk's wonderful meteor and comet site! There are a great number of sites linked on this page.

  • You can also try this page at Harvard that has lots of links as well.

  • (This Bad Idea was donated by Mike Regish. Thanks Mike!)
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