With the Holidays fast approaching like Santa Claus’s sleigh, a yearly meteor shower spectacle presents a chance to make a last Christmas wish upon a shooting star.
The Geminids seem to shoot from a point located near the constellation Gemini the twins. The point where meteors seem to come from is called their radiant; the Geminids’ radiant is located near the bright star Castor in said constellation, but, nevertheless, they shoot outward and can appear anywhere above the horizon. So, instead of focusing on that point, meteor hunters should lay back and take in as much of that starry expanse as possible.
Although they appear to emanate from the radiant near bright star Castor, some 52 light years away, meteors actually occur about 60 miles above Earth’s surface. Meanwhile, a trick of perspective makes it seem like they converge. Like a pair of train tracks that appear to converge on the horizon but in fact run parallel, meteors apparently meet at their radiant, but actually travel in the same direction together along an orbit around the sun—they never meet.
The radiant will be highest in the sky around 2 a.m., and that goes for every time zone. As a general rule, the higher the constellation Gemini is, the more Geminids one will usually spot. Typically, the Geminids favor the northern hemisphere, but they can still be seen from southern regions, albeit in fewer numbers. However, due to interference from moonlight, 2 a.m. might not be the best time for meteor viewing.
Would-be meteor watchers must account for the moon. Moonlight will wash out the faint glint of shooting stars, so viewing conditions are best on moonless, dark nights or before the moon rises. As the last quarter moon falls on December 16, a waning gibbous moon will rise in late evening on December 13, rising slightly later on December 14. That means you probably won’t glimpse many Geminids around 2 a.m. Try earlier in the evening instead—there are opportunities to sight spectacular “earthgrazers” at that time! Earthgrazers are slow-moving, long-lasting meteors that travel horizontally across the sky. The bottom line: under ideal conditions one might spot as many as 120 Geminids per hour, but this year one will see perhaps 50 per hour.
Now that we know when and how we can spy the Geminids, what are shooting stars and where do they come from? The Geminids, like other meteor showers, are a yearly event recurring around the same time every year. Meteors begin as bits of ice and cosmic debris, left from comets or asteroids orbiting the sun, which the Earth periodically passes through. When that matter hits our atmosphere, it lights up like matches and becomes a meteor shower shooting to Earth. In mid-December, Earth transits the orbit of asteroid 3200 Phaethon, believed to be the parent asteroid of the Geminids.
So, what distinguishes a comet from an asteroid and how do they spawn meteors? Comets are amorphous “dirty snowballs” hurtling through space, comprised of frozen gas and rock. They have a solid nucleus, but when they near the sun a coma—a “cloud” surrounding a comet’s head—forms around them. They may also exhibit a tail. The solid nucleus transforms into gas in a process called sublimation, and leaves behind an expansive complex of debris, persisting after the comet is long gone. This stream of debris is what collides with Earth’s atmosphere to create meteors.
An asteroid, meanwhile, has neither a coma nor tail. It is just a chunk of rock. Some are believed to be dormant or extinct comets that have ceased to sublimate and jettison material; or that material somehow became trapped within the comet’s surface. Comets can become asteroids, as 3200 Phaethon is speculated to have done. Meanwhile, some asteroids have become active comets on nearing the sun.
As for 3200 Phaethon, parent asteroid of the Geminids, it was discovered in 1983 by Simon Green and John Davies using the Infrared Astronomical Satellite. After its orbit was calculated, Fred Whipple declared the asteroid had the same orbit as the Geminids. It became the first asteroid purported to be the parent of a meteor shower, but it is not exactly known how material from its surface or its interior was released into the meteoroid stream.
The asteroid gets very close to the sun, as close as half the distance from the sun as the planet Mercury. Then, 3200 Phaethon slingshots around our solar center, whipping as far out as past Mars’s orbit. It’s along its inbound leg that Earth intersects.
So, hopeful meteor watchers, watch for Geminids on the evenings of December 13 and 14, before the waning gibbous moon rises and drowns out their luster. You can take your chances at 2 a.m. when Gemini reaches its height, as the Geminids are bold and white. The odds are good you will catch a few bright flashes despite the moonlight.
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