December brings the return of the Geminid meteor shower, or the Geminids, for short. This is routinely the strongest and most reliable shower of “shooting stars” of the year.
Unfortunately, conditions will be far from ideal in 2019 for observing the Geminids as the Moon will be very bright, having reached full phase on December 12th.
The Geminids are active from December 4th to the 17th, according to the International Meteor Organization, with a peak on the 14th, and good rates for a couple of nights around that time.
At its peak, meteor rates can reach 120 or more an hour, although this is what is called the zenithal hourly rate (ZHR) and calculated for a single observer with perfect skies and a radiant overhead. In practice you will not see so many, with the Moon drowning out the fainter ones.
The Geminids would probably be a lot better known if they appeared during the summer months, like the Perseids, rather than during mid-winter in the northern hemisphere!
The Geminids are a rich enough shower that you should see some meteors despite the challenge of the moonlight. Wrap up warm and find a comfortable spot to sit away from street lights, and face a direction away from the Moon if you can. Any Geminid meteors you see which will seem to fly at medium speed across the sky. They enter the atmosphere at around 35 km (22 miles) per second.
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The meteors are called the Geminids because their radiant point lies in the constellation of Gemini. They can appear in any part of the sky, but if you trace their paths back, they will all intersect at this point in the sky. You don’t need to look towards Gemini to see them – it is better to direct your gaze well away from the radiant itself.
Wait until the radiant has lifted itself above the horizon before starting to observe as its altitude affects the number of meteors you will see. For northern observers, activity should pick up from around 10pm local time and last the rest of the night. The observing window is shorter and later in the night for observers in the southern hemisphere from where Gemini spends less time above the horizon.
You can follow how active the Geminids are in 2019 by viewing the International Meteor Organization’s live updates, based on their members’ observations.
It is generally known that meteor showers are produced by streams of dust shed by comets. These streams cross the orbit of the Earth so that we fly through them at the same time every year.
What is unusual about the Geminid meteor shower is that the parent body has been identified as an asteroid rather than a comet. The asteroid, named 3200 Phaethon, was only identified in 1983 when it was found in images taken with an infrared satellite called IRAS.
Phaethon has an orbit that brings it to a distance of only 21 million km (13 million miles) from the Sun, and it has been closer in the past. It seems to be still producing dust due to thermal decomposition and fracturing of the asteroid.
One of the world’s leading authorities in meteor science is Professor Galina Ryabova, of the Institute of Applied Mathematics and Mechanics at Tomsk State University, Russia. She told Skymania News: The evidence is overwhelming that Phaethon is the parent body for the Geminids.
“And while we don’t know much about Phaethon, the Geminids is one of the most studied meteoroid streams, so we may use observations of the shower it produces every December to make some conclusions about its origin.
“The Geminid meteoroid stream structure seems to show its cometary origin. I believe that Phaethon was a comet, captured on its present orbit. It lost volatiles at a catastrophic rate, over just one to three orbital revolutions, and is now an asteroid.”
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