A regular highlight of December is the peak of the annual Geminid meteor shower. The 2020 Geminid meteor shower is active between 4-17 December, with peak activity occurring towards the end of this weekend, on the night of 13/14 December.
This year the Geminid shower is expected to peak at 00:50 UT on 14 December, in the middle of a long, dark night. The radiant position is good too, culminating south at an altitude nearly 70˚ up around 02:00 UT.
The ‘radiant’ is the area of the sky from which a meteor shower appears to originate. The Geminids are so-called because they seem to radiate out from the constellation of Gemini.
For more on meteor showers, read our guides on how to observe and record shooting stars and how to photograph a meteor shower, what causes a meteor shower or, if you’re a complete beginner, our beginner’s guide to meteor showers.
Many people would say the Perseid meteor shower in August is the best of the year, but this year’s Geminid meteor shower will outstrip the Perseids in a number of ways.
December nights are long and dark, making them perfect for meteor watching. This year’s Geminid peak is favourable. A new Moon occurs at 16:17 UT on 14 December meaning it will not interfere with how many meteors you might expect to see.
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To explain why this year’s Geminids will outshine the Perseids, we should also look more closely at the often-misused value that is the zenithal hourly rate, or ZHR.
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.
How many Geminids will I be able to see?
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
With a well-timed peak, coinciding with a near maximum altitude radiant, this should be quite a show as long as the weather stays clear.
How to see a Geminid meteor
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.
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.
As we’ve already said above, the area of the sky from which a meteor shower appears to originate is known as the ‘radiant’, and the Geminids are so-called because they radiate from the constellation of Gemini.
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.
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.
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.
Pete Lawrence is an experienced astronomer and a co-host of The Sky at Night.