What should be the best meteor shower of 2010 will occur in a little over a week beginning on the evening of Monday, Dec. 13.
Like most meteor showers, the Geminids will be at their best after midnight (early on the morning of Dec. 14), when the Earth is heading directly into the meteoroid stream. But some will be visible earlier in the night, on the evening of Dec. 13, because the meteors' radiant – where they appear to originate – is nearly circumpolar, so they will stay in view above the horizon all night.
This sky map shows where to look to see the Geminid meteor shower.
What causes a meteor shower?
Most meteor showers are caused by fragments of old comets scattered along a comet's orbit. When the Earth passes through a comet's orbit, it sweeps up the fragments, which are heated by friction with the Earth's atmosphere to incandescence, and are visible as bright streaks of light. The Geminid shower is unique in being associated not with a comet, but with an asteroid – 3200 Phaethon.
Phaethon is a very odd asteroid indeed. Its orbit brings it closer to the sun than any other known asteroid, well inside Mercury's orbit. Its orbit is more like that of a comet than an asteroid, but it has never exhibited any of the features which characterize comets: no coma, no gas jets, no dust tail.
When we look toward the radiant of this meteor shower, we are looking into the line of Phaethon's orbit. The meteors appear to radiate from this point in the sky, but this is an effect of perspective, much as railroad tracks appear to diverge as they get closer to us.
The Geminids' radiant is, as the name implies, in the direction of the constellation Gemini, just north of the northernmost of Gemini's two brightest stars, Castor. In the early evening of Dec. 13, the radiant is low in the northeast. By 1:00 a.m. ET, after the date has changed to Dec. 14, the radiant is almost directly overhead. By 6.a.m., when the shower is at its peak in the Eastern time zone, the radiant is low in the west.
This circumpolar pattern for the radiant means that meteors may be seen all night long, though they tend to be faster and brighter after midnight. It will also help that the moon sets close to midnight, allowing fainter meteors to be seen.
How do you observe a meteor shower? The first thing is to dress warmly — much more warmly than the weather might normally make you think — because you won't be moving much. The second thing is to make yourself comfortable so that you can watch the sky: A lawn chair with a reclining back is ideal. And a blanket or a sleeping bag will help keep you warm.
You don't need binoculars or a telescope; in fact these will prevent you from seeing the meteors by restricting your field of view. You want the wide field given by the human eyeball.
It doesn't really matter much which part of the sky you watch, as the meteors can appear anywhere. Early in the evening, they often appear low on the horizon; later at night they are most frequent overhead. You will generally see more meteors if you look away from the radiant, as the meteors 90 degrees away from the radiant have the longest trails.
The most important thing is to be patient. It takes a while for your eyes to adapt to darkness. Then, there are often long periods with no meteors at all, interspersed with more active times when four or five may flash by in a minute. The Geminids are a bit slower moving than their summer cousins, the Perseids, and often end with a tiny explosion — you may hear a popping sound.
If you're clouded out on Dec. 13 or 14, try the night before or the night after: There still should be quite a few Geminids about.
Good luck and clear skies!
This article was reprinted with permission from SPACE.com.
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