2012年10月8日星期一

Do a Wish: October Sky Alight With Shooting Stars, Starting With the Draconids




It’s a waning gibbous moon, ideal for spotting what could be some pretty fiery meteors spewing from the mouth of Draco, the dragon.
Best viewed from the northern hemisphere—that’s Turtle Island to most of us—the Draconid meteor shower is not always terribly exciting, given that the point in the sky they appear to originate from (known as the radiant point, which in this case is the constellation Draco’s head) is at its highest as night falls. Most other meteor showers are visible in the early morning hours after midnight; the Draconids do their thing before that.
Either way, October 7 and 8, as night falls and evening deepens, are the best days to watch what could be a real stunner, according to the website EarthSky.org.
“This shower is usually a sleeper, producing only a handful of languid meteors per hour in most years,” the site says. “But watch out if the Dragon awakes! In rare instances, fiery Draco has been known to spew forth many hundreds of meteors in a single hour.”
The source of tonight’s shower is debris from the mundanely named comet 21P/Giacobini-Zinner, according to National Geographic. Each one is about the size of a grain of sand, National Geographic says. Peak viewing time is at about 10 p.m. on the night of October 7.
“And with the moon in waning gibbous phase the best times will have the skies completely moonless and as dark as possible—really the best you can hope for in terms of viewing even the fainter shooting stars,” National Geographic says. “This year observers can expect to see anywhere from 10 to 20 meteors per hour late nights on both October 7 and 8. However, it’s worth keeping your eyes on the heavens for possible surprises. Historically Draconids have been known to peak to hundreds or even thousands of meteors per hour.”
Although rare, it did happen last year over Europe, when Earth hit an especially dense part of the debris field, National Geographic said.
NASA explains that Draconids move slowly, hitting the Earth’s atmosphere at a “relatively leisurely” 20 kilometers per second. This makes them of little danger to satellites and other orbiting craft, and causes them to stand out visually, NASA said in its 2011 preview of the showers.
“A Draconid gliding leisurely across the sky is a beautiful sight,” said Bill Cooke of NASA’s Meteoroid Environment Office, in the NASA statement.
This marks just the first of two meteor showers set to grace October skies. Later in the month come the Orionids, consisting of fragments of the tail of Halley’s Comet. Stay tuned.

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