A regular highlight of December is the peak of the annual Geminid meteor shower. It’s one of the favourite observing events of the year.
As with any meteor shower, the shower’s success is dictated by the phase of the Moon and the weather.
There’s not much we can do about the latter, except to move to a location where the clouds are less likely, but the Moon’s phase is very predictable, as is its rise and set time.
Meteor showers are one of the easiest astronomical events to observe as they are best seen with the naked eye, making it the perfect opportunity to get both children and adult beginners excited about astronomy.
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When is the 2021 Geminid meteor shower?
The Geminid meteor shower reaches peak activity around 07:00 UT on 14 December, making 13 and 14 December ideal times to look for its meteors.
The Moon is in an advanced waxing phase around this time. 78% illuminated in a waxing gibbous phase on the evening of 13/14 December, it sets around 03:00 UT on 14 December.
Although this isn’t ideal, long dark December nights mean there are still around three hours of darkness left to enjoy what the shower has to deliver.
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What is zenithal hourly rate?
The zenithal hourly rate, or ZHR, of a meteor shower is the total number of meteors you might expect to see during peak activity with absolutely perfect conditions, i.e. under clear, dark skies away from light pollution.
The ZHR is often quoted during big meteor showers, however given the above caveats, observers should expect the actual number to be somewhat lower.
A shower’s zenithal hourly rate is a normalised value: a figure that compensates for all the issues that degrade a shower’s visual performance.
The ZHR considers how much of the sky you can, or cannot see. It takes account of the radiant’s altitude and how clear your sky is.
Applying the ZHR correction to the number of meteors seen over a set period gives you a figure that more accurately represents a shower’s activity.
The Geminid shower has an excellent zenithal hourly rate (ZHR) of 140–150 meteors per hour. A medium entry speed for the meteoroids also makes Geminid trails easier to photograph. A bright event can look pretty spectacular.
How to see a Geminid meteor
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The best observing advice to spot a Geminid in 2021 will be to get some sleep on the night of 13/14 December, setting your alarm clock for a 02:00 UT wake up.
Prepare yourself with warm clothing and perhaps a hot drink in a flask. A sun lounger or deck chair makes a great viewing platform as it allows you to watch in relative comfort.
Aim to view the sky at an altitude of 60º or so. Any direction is fine, although bear in mind that trails will be shortest closer to the radiant and longest 90° from the radiant.
Convention dictates that a good compromise is to look 40–50° from the ‘radiant’ position, which is located near the star Castor (Alpha (α) Geminorum) in Gemini (see chart above).
To find Gemini, imagine a line between Orion’s right foot (Rigel) and left shoulder (Betelgeuse), then follow that line on for around the distance between your thumb and little finger stretched out at arm’s length.
There should be a pair of bright stars here: Castor and Pollux, the ‘heads’ of Gemini’s twins.
The radiant is just by Castor, but you want to look slightly away from this region rather than directly at it. If you spot a meteor coming from this region, you’ve seen a Geminid.
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Find a place that is dark and away from light pollution, such as a location away from towns and cities, provided it is safe to do so.
If you are lucky enough to live under very dark skies and can observe from the comfort of your own garden, turn off the lights in your house so they don’t spoil the view.
Avoid using lights such as torches and mobile phones as this will spoil your dark adapted vision. If you need to see in the dark, use a red torch. If you need to look at your mobile phone, you may be able to turn your screen red in the settings.
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It will take around 20 minutes for your eyes to dark adapt, but hopefully after a little wait you should start to see meteors shooting across the sky.
Remember: clear nights are cold nights. You will be sitting still for a long time, so wrap up warm, bring something to eat and perhaps a hot drink as well.
What is the Geminid meteor shower?
Meteor showers usually occur when Earth passes through the trail of debris left by a comet. The Geminids are unusual, however, in that the trail of debris was left behind by an asteroid, 3200 Phaethon.
How many Geminids will you be able to see?
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The period of peak Geminid activity is wide and over recent years its ZHR has been rising. Currently, the Geminid peak is around 140–150 meteors per hour.
The height of a shower’s radiant (where the it appears to originate, as viewed from Earth) is very important; for example, if a radiant is on the horizon, half of all trails would occur below the horizon.
A radiant at the zenith is desirable (hence ‘zenithal’ hourly rate), but 70˚ up is pretty good.
The Geminids enter our atmosphere at 35km/s, producing slower trails that make them relatively easy to photograph. The Geminid meteor’s trails typically appear off-white
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Have you managed to see any Geminid meteors this year? Did you manage to photograph any? Let us know by emailing contactus@skyatnightmagazine.com or get in touch via Facebook, Twitter and Instagram.