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BAA electronic bulletin
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ENJOY THE GEMINID METEOR SHOWER
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.
This e-bulletin issued by:
Dr John Mason
Director, BAA Meteor Section
email:
docjohn@dircon.co.uk 
