It’s time for the Geminid meteors to burst out bright

The peak of the Geminid meteor shower is predicted to take place on the night of 13/14 December (Monday night into Tuesday morning). Year on year, the Geminids have become the best and most reliable of the annual meteor showers, eclipsing even the ever-popular Perseid meteor shower in observed rates of meteors. The night of the Geminids is a showpiece of the year and as such is always heavily ringed on observing calendars across the world.

A Geminid meteor flares out over the sky from Northumberland on 14 December 2018. Image: John Rowland.

The Geminids provide a great opportunity to witness a shooting-star spectacular; the sight of a bright meteor suddenly streaking across the sky is one of the most exciting sights in astronomy. The Geminids at their peak are expected to produce around 60–70 meteors per hour from a dark-sky site under clear and transparent skies!

The only caveat for this year’s celestial pyrotechnics is the annoying presence of a gibbous Moon, which is at full phase on the morning of 19 December, to put a dampener on proceedings, its glare drowning out many of the fainter meteors. However, it sets at around 1.30am GMT, just in time for when the Geminids are expected to be at their peak.

A meteor shower is named for the constellation in which the shower’s radiant is located. This is the point in the sky from which the meteors appear to emanate from when tracing back their paths across the sky. The Geminids radiant lies close to the bright star Castor (alpha Geminorum; magnitude +1.6) in Gemini, the fainter and more northerly of Gemini’s twins (the other twin being the star Pollux).

The Geminid meteor shower radiant lies close to the bright star Castor in Gemini. AN Graphic by Greg Smye-Rumsby.

Meteor showers are in the main spawned by periodic comets. Known as the parent comet. Minute debris is shed on the comets repeated visits to the inner Solar System, which spreads out along their orbit. When Earth encounter this stream, a meteor shower occurs when particles enter the atmosphere and are vaporised, producing streaks of light. However, the Geminids debris stream is different from the overwhelming majority of meteor showers in that the debris is not shed by a periodic comet but an object known as (3200) Phaethon, a 5.8-kilometre-wide Apollo asteroid that’s termed a ‘rock comet’. This is essentially an asteroid which comes unusually close to the Sun, with the resultant solar scorching striping off copious amounts of robust grains.

Depending on which source you check, the peak of this year’s Geminids occurs at 02h UT or 07h UT. It’s always worth trying to observe a major meteor shower on the night before or after that predicted for the observed peak. The night of 14/15 December will probably be the most fruitful, especially if the peak occurs closer to UK dawn on 14 December. The Moon, now at a 86 per cent gibbous phase, sets at around 2.50am GMT on 15 December.

On Monday, 13 December, the Geminid radiant is well up (~33 degrees from London) in the eastern sky by 9pm, though a 77-per cent-illuminated gibbous Moon, lying among the stars of Pisces, looms large to the south. The radiant culminates due south at about 2am, by which time the Moon has set. When conducting a meteor watch, an observing site (a rural one if possible) from where the sky at an altitude of 50 degrees from the horizon may be watched is best. Rather than peering directly at the radiant, look in a direction some 30 or 40 degrees away, where meteors will often be longer and easier to see; they will appear shortest near the radiant. Orion and Taurus to the west are fertile areas to the west, with Cancer and Leo good places to watch to the east. In late-evening watches, your best bet is to face east, keeping your back to the Moon in the west. As the radiant climbs, gradually turn your direct view westwards.

However long you intend to look for meteors you should dress appropriately for the cold weather by wearing plenty of layers of warm clothing, including a wooly hat and a pair of gloves. Also have a hot drink and a couple of snacks to hand to keep you going. Get comfortably sitting in a reclining chair and try become fully dark adapted (for at least 15–20 minutes) before starting a watch and use only a red torch for illumination.

A bright 2020 Geminid slices through the star trails of Orion. Image: Glenn Bates

So how many meteors is it possible to see? The Geminids has one of the highest quoted ZHR’s (zenithal hourly rate, the likely rate of meteors seen by a single observer assuming the shower’s radiant lies at the zenith in a dark sky), which can be as high as 150. Higher rates of shooting stars this year should be seen after midnight, especially after the Moon has set and the radiant lies high to the south. Observed rates of 60 meteors per hour is a realistic expectation if observing conditions are fine and you’re dodging major sources of light pollution. Remember, meteors will not occur at a steady one a minute. There may be a five minute or longer hiatus, following by three or four meteors in a short space of time.

The Geminids are slow meteors, entering Earth’s atmosphere at a relatively sedate 35 kilometres per second. By comparison, Perseids and Leonids are turbo-charged at speeds of around 60 and 70 kilometres per second, respectively. Geminids are often bright and multicoloured and often produce persistent trails (or trains) across the sky; Geminids will appear longest and fastest the farther away from the radiant they appear.