This year has flown by and once again it is Geminids time! One of the highlights of the winter months, the shower is active from the 8th until 17th December. The peak is predicted to occur at around 08:00 GMT on the morning of 14th December, so the highest rates will be during the early hours of the morning. However, because it has a broad peak, this is a shower that is definitely worth observing a couple of nights before and after as well.
Geminid meteors are not as fast moving as those observed during other showers. They move at around 35 kilometres per second, which is half the speed of meteors seen during the Perseid and Orionid meteor showers. This makes them a great photography subject. Geminids generally burn up in our atmosphere at a height of about 38 kilometres.
The quoted zenith hourly rates (ZHR) for this shower vary widely, with figures ranging from 100 to 450 meteors per hour because the media like to use rates detected by amateur radio enthusiasts. Radio detectors will pick up meteors that can’t be seen visually as well as the ones that can, so the radio astronomy rates are always much higher than the visual rates. But the media love to use the highest number they can find to give themselves a dramatic headline! A more sensible ZHR is around 120 meteors per hour, but remember that this does not equate to the number of meteors that you may see during the peak of this shower. The zenith hourly rate assumes that the radiant of the shower is directly overhead, includes meteors across the entire sky and assumes that the location is a dark sky site with no Moon interference. It’s impossible for us to visually observe the entire sky at once, so a more realistic number would be between 30 and 60 meteors depending on sky conditions. Even though conditions are favourable in terms of the Moon (the Waxing Crescent Moon will set by around 21:50 GMT), and the fact that the Geminids radiant is high above the horizon all night long, factors such as light pollution or poor seeing can easily halve this rate. Not allowing your eyes to be full dark adapted will also reduce the chance of spotting the fainter meteors. Rates are always higher during the early hours of the morning because Earth is then face-on to the meteor stream.
Meteors associated with a specific shower appear to trace back to the same spot in the sky and this is called the radiant, and the shower bears the name of the constellation that the radiant resides in. In the case of the Geminids, the radiant is just above and to the right of the star Castor in Gemini. It’s a common misconception that the radiant is the best place to look for meteors during a shower; this is not the case! Meteors can occur anywhere in the sky; if you trace back the path they have taken across the sky then they will converge at the radiant. If you are looking directly at the radiant then the chances are that you may miss a lot of meteors. A better way to observe is to look about 45 degrees to the side of the radiant and at around 45 – 50 degrees above the horizon, because this is where you are likely to spot the brightest meteors.
Geminids radiant graphic. Source: stardate.org
If you are planning to observe the Geminids, make sure you are dressed in very warm clothes because it can become extremely cold on clear December nights. I will always have fond memories of observing the 2017 Geminids, when I was sitting outside for 5 hours surrounded by 5” of snow! Try to observe somewhere away from light pollution and allow your eyes to become fully dark adapted. Keep accurate records of what you observe using a Dictaphone or pencil and paper and if you are observing with other people, try to all face in different directions so that you have more sky coverage.
If you want to photograph the Geminids, focus your camera on a bright star then point it about 45 degrees away from the radiant then set the ISO to a suitable setting for your sky. If you put your camera into continuous mode and set a shutter speed of about 15 seconds, you can use a remote shutter cable and just leave the camera constantly taking photos throughout the night. Don’t be tempted to stop the camera if you think you’ve caught a bright meteor – if you keep shooting afterwards you may be able to capture the ionisation cloud slowly dissipating after the event, and this makes a lovely timelapse video subject (see here to view videos of Perseid and Quadrantid meteors with ionisation clouds by Mary McIntyre: https://youtu.be/Jq0DkAQ5_Mc ) To prevent your camera lens from fogging up on a frosty night, make a low cost dew heater by cutting off the end an old, warm sock and placing it over the camera lens. Then tuck in a couple of reusable pocket hand warmers.
The Geminids is one of only 2 showers whose parent body is not a comet (the other being the Quadrantids). The parent of the Geminids was in fact a mystery for many years, then in 1983 the minor planet 3200 Phaethon was discovered by Simon Green and John Davies when studying images from the Infrared Astronomical Satellite (IRAS). It an “Apollo Asteroid” with an orbit that brings it closer to the Sun than any other named asteroid. It is thought that 3200 Phaethon was once a comet which has lost all of its volatile compounds during previous orbits around the Sun. It was the astronomer Fred Whipple who realised that the orbital elements of this 5km diameter minor planet fitted perfectly with the Geminids.
Astronomy Now Magazine (December 2018 issue)
Sky at Night Magazine (December 2018 issue