Thursday, December 13, 2007

Twin Treats from Gemini

On the evening of December 13th into the morning of December 14th, cold-hardy sky watchers in North America will be rewarded with a generous sprinkling of cosmic fairy dust. The top-billed meteor shower of December--the Geminids--returns once more to fill the sky with “shooting stars.” The 13th/14th is the peak of the shower, when observers can expect to see 50-plus meteors per hour streak across the sky. But don’t worry, if you can’t get out under the stars on Thursday night or if clouds thwart you, try again on Friday night into Saturday morning, December 14th/15th, because rates should still be high enough for a good show.

A meteor, aka shooting star, is the streak of light we see in the sky when a bit of dust or space debris hits Earth’s atmosphere at high velocity. Typically, the meteoroid--the bit of debris--vaporizes upon impact with the atmosphere. But occasionally, a large chunk may survive its screaming 75,000-plus mph entry and hit the ground, at which point we refer to it as a meteorite.

The Willamette Meteorite at the Rose Center for Earth and Space, NYC

You can venture out under a dark sky any night of the year and see a handful of meteors. These are usually sporadics, meteors not associated with a particular shower. But when Earth encounters one of the streams of debris left behind by a comet’s close approach to the Sun, we experience a meteor shower.

3200 Phaethon’s orbit - image from NASA/JPL

In the early 1980s, astronomers were stunned to discover that the source body for the Geminid meteor stream was not a comet, but rather what appeared to be an asteroid, 3200 Phaethon (pronounced FAY uh thahn). It is now suspected that 3200 Phaethon may in fact be the burned-out shell of a comet, charred by too many close encounters with the Sun. The rocky skeleton that remains would naturally resemble an asteroid.

This map shows the Geminid shower radiant, that is, the point in the sky from which the meteors appear to emanate, as the Earth slams into 3200 Phaethon‘s debris. In the case of the Geminids, the radiant is a point within the constellation of Gemini, hence the shower’s name.

Bundle up now, and put on the teakettle for a thermos of hot chocolate!

1) Go to as dark of a site as you can, with no line-of-sight outdoor lights. Dress in many layers because you won’t be moving around much and you’ll get cold fast.

2) Take a lounge chair and a pile of blankets. Reclining is really the best way to watch a meteor shower, because it allows you to see the most sky at one time. Although the meteors originate from the radiant, you can see them fizzling anywhere in the sky. I normally lie with the top of my head pointed toward the rising radiant (which will be in the northeast) so that I can be scanning the rest of the sky.

3) The radiant will rise about two hours after sunset. You won’t see any meteors before then. The moon will set about four hours after sunset, but it will be a small crescent hanging low in the west and shouldn’t be bright enough to impact your viewing.

If you can stand the cold and can wait until midnight to go out and begin observing, the radiant will be better placed in the sky--nearly overhead. The hourly rate will be greater and you should see many more meteors. Be brave! The experience of seeing a terrific meteor shower--even with some physical discomfort--is a memory that will stay with you always.

Another alternative is to get up a couple hours before sunrise on Friday to watch the sparks fly before day breaks.

4) As you watch the fireworks, keep in mind that meteors are typically no bigger than a grain of sand. The really bright ones that go off like Roman candles may be no bigger than a grain of rice. It is their high entry velocity as they slam into our atmosphere that causes the wonderful light show. Amazing, isn’t it? Before I knew any better, I always imagined that the shooting stars I saw were each about the size of a basketball!

Meteor storm - 1889 engraving by Adolf Vollmy

Just as the Geminids peter out on December 18, another gift from the Gemini Twins--planet Mars--takes center stage. Mars will be great to look at all week. But on the night of the 18th, Mars will be at its closest to Earth since 2005, a mere 55 million miles away. It will not come closer to Earth until 2016.

Mars is unmistakable in the night sky now, shining like a drop of molten copper, brazen, in your face, the dude with a bad ‘tude. It is not much of a stretch to see why the planet is the namesake of the Roman god of war.

NASA and the Hubble Heritage Team (STScI/AURA)

You will find Mars rising in the east about one hour after sunset, to the left (north) of the easy-to-spot Orion constellation. He will be the brightest object in the sky, after the Moon, which unfortunately will be 65% illuminated on the 18th. But even the placid-faced Moon can’t keep the warmonger at bay. Mars marches her across the sky until she sets, and then he dominates the night, sunlight reflecting off his terrible shield.

Peace on Earth, good skies to all.