2021 December 7

Get Ready for the 2021 Geminid Meteor Shower

Dr John Mason

Active from December 4-17, with a slow rise to maximum activity and a steeper decline, the Geminids are the richest of the regular annual meteor showers at the present time.  Under good observing conditions the shower produces plenty of bright meteors, with rates easily exceeding those of the August Perseids for a 24-hour interval centred on their maximum on the night of December 13/14.  The shower is certainly rewarding for any observers prepared to brave the winter wind, cold and damp. The Geminid maximum is quite broad and respectable Geminid rates may be expected throughout the nights of December 12/13, 13/14 and 14/15.

Geminid maximum this year coincides with a waxing gibbous Moon in Pisces, but observers should not be deterred by this. There will certainly be some interference by moonlight, but observers should remember that on maximum night the Moon will set before 0200 hrs, giving four hours of nice dark skies before dawn. In addition, even when the Moon is above the horizon, observers may minimize the effects of moonlight by positioning themselves so the Moon is behind them and hidden behind a wall, tree or other suitable obstruction.

This year, the time of Geminid maximum is very favourable for observers in Britain and Ireland with peak activity expected at about 02h on Tuesday, December 14, when the Zenithal Hourly Rate (ZHR) may again reach 100 to 120 meteors per hour.  The radiant of the Geminid shower (at RA 07h 34m,  Dec +32.3^o, just north of the first magnitude star Castor) rises early in the evening and reaches a respectable elevation above the horizon (> 40^o) well before midnight, so observers who are unable to stay up late can still contribute very useful watches although, as stated earlier, they will have to be mindful of the effects of moonlight at this time. However, the early morning hours of Tuesday, 14th December are likely to yield the greatest Geminid activity, when the radiant is high in the sky and rates are near their peak – especially after moonset early on the 14th. 

Past observations have shown that bright Geminids become more numerous some hours after the rates have peaked, a consequence of particle-sorting in the meteoroid stream.  This means that the four hours after moonset and before dawn on 14th December are likely to produce the greatest Geminid activity for imaging and visual observers.  In recent years from Britain and Ireland, the Geminids have shown typical peak observed rates of 70-80 meteors per hour in good skies, but such rates may only be expected after the Moon has set this year. 

As with any meteor shower, when observing the Geminids it is best to look at an elevation of 50^o (about the same altitude as the Pole Star from southern parts of the UK) and 40-50^o to either side of shower radiant, rather than looking directly at the radiant itself, although Geminid meteors may appear in any part of the sky.  Given the position of the waxing gibbous Moon this year, observers may find it preferable to look to the eastern side of the radiant until the Moon has set.  December nights can be quite chilly, especially in the early morning hours when activity is likely to be highest, so wrap up well with plenty of layers of warm, dry clothing and make sure that you wear a hat, gloves, thick socks and sensible waterproof footwear.

Geminid meteors enter the atmosphere at a relatively slow 35 kilometres per second, and thanks to their robust (presumably more rocky than dusty) nature tend to last longer than most in luminous flight. Unlike swift Perseid or Orionid meteors, which last only a couple of tenths of a second, Geminids may be visible for a second or longer, sometimes appearing to fragment into a train of ‘blobs’. Their low speed and abundance of bright events makes the Geminids a prime target for imaging.

Increasing Activity

The majority of the annual meteor showers are associated with known periodic comets, yet there is no very short period comet that matches the orbit of the Geminid meteoroid stream. Instead, the orbit of the Geminids is occupied by an object called 3200 Phaethon, which looks remarkably like a rocky asteroid, but may actually be a “rock comet” which is, essentially, an asteroid that approaches so close to the Sun that solar heating scorches dusty debris right off its rocky surface forming a tail of rocky grains.

The Geminid shower has grown in intensity over the past 50 years as a result of the stream orbit being dragged gradually outwards across that of the Earth. A consequence is that we currently encounter the most densely-populated parts of the stream. This happy situation is unfortunately only temporary – in a few more decades, Geminid displays can be expected to diminish in intensity. Mathematical modelling of the Geminid meteoroid stream by  G.O. Ryabova and J. Rentdel (Mon. Not. Roy. Astron. Soc. 475, L77-L80 (2018)) has shown that Geminid activity should indeed rise with time and this has been confirmed by analysis of visual observations between 1985 and 2016. They also show that the node of Phaethon’s orbit should intersect Earth’s orbit in about 2200 CE and the modelled Geminid stream core some time later.  After that Geminid shower activity is likely to decrease.  In 2200 CE peak Geminid activity could reach 1.4 times the current rate, i.e. a ZHR of ~170m/h.  So, in the Geminids we have an excellent opportunity to follow, year on year, the evolution of a major annual meteoroid stream. 

Get Involved

The BAA’s visual meteor report forms, found on the Visual Observing page of the Meteor Section, are available as downloads in both pdf and Excel formats, enable observers to record the details of each meteor seen. These include: time of appearance (UT); apparent magnitude (brightness); type (shower member, or random, ‘background’ sporadic); constellation in which seen; presence and duration of any persistent train. Other notes may mention flaring or fragmentation in flight, or marked colour. Watches should ideally be of an hour’s duration or longer (in multiples of 30 minutes). Observers are reminded to carefully record the observing conditions and the stellar limiting magnitude. Wrap up warmly and enjoy what should be a great show!

By whatever means you observe the Geminids this year, please submit your results to the BAA Meteor Section via meteor@britastro.org.

Dr John Mason
Director, BAA Meteor Section
email: docjohn@dircon.co.uk
2021 December 06

Teaser image by Ian Sharp: Composite of 6 images of  the 2017 Geminids taken near Tarifa, Spain using a Canon 6D DSLR (astro-modified), Samyang 14mm f/2.4 lens at f/2.8 and Astrotrac mount.