Thursday, December 13, 2012

December 13-14 meteor shower

Tonight, Thursday December 13 into the early hours of Friday, December 14 will be the annual Geminids meteor shower.  Meteor showers are named from the constellation they appear to radiate from, in this case Gemini which rises here in the Hudson Valley around 7 pm EST (look to the left of the more-recognizable constellation Orion).

Meteor showers are the result of the Earth moving through a field of debris typically left by a comet at some point in the past.  The Geminids are an exception, however, since the debris is from an asteroid named 3200 Phaethon.  The fields of debris are scattered around the path of the Earth's orbit around the Sun and therefore certain meteor showers periodically occur each year.

As a general rule, meteor showers are best after midnight.  In the diagram below, position A is just after sunset on the trailing side of the Earth while position B is before sunrise (after midnight) on the leading side of the Earth in its orbit around the Sun. At position B, we're plowing right into the debris field left by a comet and the meteor shower will be at its best. Continuing on in that direction off into space is the constellation of Gemini - the radiant of the meteor shower.

Below is the view you'll see high in the southern sky at midnight showing Gemini, Orion, Taurus, and a bright Jupiter (Betelgeuse and Aldebaran are distinctive too, being reddish stars).

As mentioned above, this meteor shower is caused by debris from asteroid 3200 Phaethon which is unusual.  This asteroid has a highly elliptical orbit, inclined 22 from the plane of the ecliptic, which brings it from out beyond Mars (2.4 A.U.) to inside the orbit of Mercury (0.14 A.U.).  This makes it an Apollo asteroid - one that crosses the Earth's orbit.  It's close approach to the Sun is why it was named PhaĆ«ton after the son of the Sun god Helios in Greek mythology.

While most Apollo asteroids don't leave debris behind to form meteor showers, 3200 Phaethon is apparently composed of ice as well as rock (making it similar to a comet, but more rocky) and as it approaches the Sun, some of the ice is sublimated away and it sheds material.

Tonight's meteor shower should be pretty good for the Hudson Valley because we should be mostly clear (take it from me, having taught Observational Astronomy this fall, that's relatively rare!).  It's also a new moon which is good because moonlight washes out the fainter meteors.  The forecast is for the Geminids to have 100+ meteors per hour at its peak which is a couple every minute.  Given how cold it will be tonight (mid-twenties), that's good because it will be hard to stay out too long waiting!

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