Two grand sky-watching spectacles are about to battle it out this week, as the peak of the Geminid meteor shower meets the final supermoon of 2016.
In a cosmic coincidence, the superlarge full moon will be flooding the night sky just as the annual shooting star show ramps up to its best performance on the night of December 13. While both sky events on their own would offer stupendous views, the timing means that meteor-watchers will have a challenge on their hands.
The term “supermoon” is popularly defined as a full moon that coincides with the lunar orb’s closest approach to Earth, or perigee.
Because the moon’s orbit around Earth is egg-shaped, there are times during its cycle when it is closer or farther from us. And because the size of the moon's orbit varies slightly over time, each month's perigee is not always the same distance from Earth.
Last month, November 13 saw the biggest and closest supermoon to Earth since 1948. This month’s close approach will be beautiful, but the moon will technically be a tad farther from us than November’s record-setting event.
This month, the moon officially reaches perigee at 6 p.m. ET (23:00 UT) on December 12, when it will be just 222,737 miles from our planet, as measured from the centers of both Earth and the moon. The moon reaches its full phase just 25 hours later, at 7:05 p.m. ET on December 13 (00:05 UT on December 14).
The absolute closest full moon to Earth this century will occur on December 6, 2052, when our celestial neighbor will be just 221,472 miles away.
For anyone around the world, the best time to catch this double sky event will be late night local time on December 13, as the Geminid meteors reach their maximum rates of 60 to 120 shooting stars an hour. However, with the superbright silvery orb dominating the high southern sky for most of the night, visible meteors will probably be cut down to around a dozen an hour, as seen from a dark location away from city light pollution.
The best bet will be to turn your back on the bright moon, to keep your eyes adapted to the dark. That way you should be able to catch at least the brightest meteors zipping overhead.
Meteors, popularly known as shooting stars, can be seen with nothing more than your unaided eyes on any night, given a sufficiently clear, dark sky. Annual showers are produced when Earth passes through clouds of dusty debris in space. When these tiny grains enter our atmosphere at high speed, they burn up, creating the brilliant streaks of light.
The Geminid meteors can be seen when their namesake, the constellation Gemini, is well above the eastern horizon. The shower runs roughly from December 6 to 19, and it peaks on December 13 and 14.
Gemini rises at about 8 p.m. local time, but the best time to look, depending on sky conditions, is the early morning hours between about 2 a.m. and dawn. That's when your local sky is pointing directly into the Geminid meteor stream. Look for the meteors radiating from near Castor, one of the constellation’s two brightest stars.
To find the constellation at 2 a.m., go outside and face south. Castor and its stellar partner Pollux will appear approximately 45 degrees above the horizon (10 degrees in the sky is equal to the width of your three middle fingers held at arm’s length). Earlier in the evening, from 10 p.m. until local midnight, Gemini will be about 30 degrees above the horizon in the southeast.
Fireballs in Store?
While the glare of the supermoon will drown out all but the brightest shooting stars, it’s still worth going out and looking. That’s because the Geminids are known to produce bolides, the unusually bright and long-lasting meteors more commonly called fireballs.
While most of the dust grains that spark meteors are about the size of sand grains, sometimes this shower also includes grapefruit- to basketball-size space rocks that can outshine the full moon and leave behind smoke trails that can linger for minutes.
So while some meteor-watchers may find the December moonlight a bit challenging, they can enjoy the beautiful supermoon rising just after sunset and still get to make plenty of wishes on shooting stars.