2015 December 13

Enjoy the Geminids

John Mason

The Geminid meteor shower will be at its most active over the next 48 hours and observers are encouraged to go out and enjoy what is now the richest of the regular annual showers, with rates outstripping those of the August Perseids for a 24-hour interval centred on their 14 December maximum – a real treat for observers prepared to brave the usual winter winds, cold and damp. The really good news is that Geminid maximum this year occurs just after new Moon, so there will be no interference by moonlight, enabling many fainter meteors to be seen in addition to the brightest members of the shower.This year, Geminid activity is expected to peak at about 13h on Monday, December 14, when the peak ZHR (Zenithal Hourly Rate) is expected to exceed 100 meteors per hour and maybe even approach 120 meteors per hour (two meteors per minute). This is, unfortunately, during daylight hours for observers across Europe, from where the highest observed rates are most likely in the early morning hours of Monday, December 14 as dawn approaches, and during the following evening.

In recent years, from the UK, the Geminids have shown typical peak observed rates of 60-80 meteors per hour (about two to three meteors per minute) in good skies. The maximum is quite broad, however, and respectable Geminid rates may be expected throughout the nights of December 13/14 and 14/15. The time of the peak favours observers in North America and the Far East. On the early morning of Tuesday, December 15, there may be the added bonus of an increased abundance of bright events after maximum; 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.

The Geminid shower radiant (at RA 07h 32m, Dec +32o, just north of the first magnitude star Castor) rises early in the evening and reaches a respectable elevation above the horizon (> 40o) well before midnight, so observers who are unable to stay up late can still contribute very useful watches. However, the early morning hours of both Monday, 14th and Tuesday, 15th are likely to see the greatest Geminid activity, when the radiant is high in the sky.

As with any meteor shower, when observing the Geminids it is best to look at an altitude of 50o (about the same altitude as the Pole Star from southern parts of the UK) and 40-50o 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. December nights can be quite chilly, especially in the early morning hours, 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.

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 be an extinct cometary nucleus. 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.

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. Here we have an excellent opportunity to follow, year on year, the evolution of a meteoroid stream.

The BAA’s visual meteor report forms, 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.