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Sunday, 13 December 2009


BAA electronic bulletin No. 00455  


The Geminid meteor shower is now underway, and it is hoped that observers
will pull out all the stops this year to cover the shower as comprehensively
as possible given that conditions are virtually ideal for observers this
December.  Unfortunately, weather forecasts indicate variable observing
conditions across the British Isles and Northern Europe, so it is important
to have a good geographical spread of observers to ensure adequate coverage.

Active from December 7-16, but with a slow rise to maximum, the Geminids are
currently the most active of the regular annual showers, with rates
outstripping those of the Perseids for a 24-hour interval centred on their
13-14 December maximum - a real treat for observers prepared to brave the
winter cold and damp.  Some forecasts have indicated peak Geminid Zenithal
Hourly Rates of up to 140 m/h this year.

This year, Geminid activity is expected to peak during the early morning
hours of Monday, December 14th. The maximum is broad, however, and it is
important to have a spread of observers making observations throughout the
night on the evening of Sunday 13th/early morning of Monday 14th December to
cover the shower maximum well.  In addition, observations made on nights
both before and after the main peak will be welcomed by the BAA Meteor

As the table below shows, the Geminid radiant (at RA 07h 32m  Dec +33o, just
north of Castor) rises early on and reaches a respectable altitude 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.

Local Time    Radiant Altitude (53oN)
20h                              25.4o
21h                              34.1o
22h                              43.1o
23h                              52.1o
00h                              60.5o
01h                              67.1o
02h                              70.0o
03h                              67.4o

There is the added bonus of an increased proportional abundance of bright
events after maximum; past observations show that bright Geminids become
more numerous some hours after the rates have peaked, a consequence of
particle-sorting in the meteor stream.

Geminid meteors enter the atmosphere at a relatively slow 35 km/sec, and
thanks to their robust (presumably rocky/asteroidal as opposed to
dusty/cometary) 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 photographic target.

This year, conditions are ideal as far as the Moon is concerned; New Moon is
on December 16, so there will no interference from moonlight this year.

Unusual in being associated with an asteroid - (3200) Phaethon - rather than
a comet, the shower has grown in intensity since the 1980s as a result of
the meteor 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 meteor 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 meteor stream. In recent years,
from the UK, the Geminids have shown typical peak observed rates of 60-70
meteors/hr in good skies. Wrap up warmly and enjoy what should be a great

For further information, or copies of report forms, observing notes, and
details of how to carry out group meteor watches, please visit the BAA
Meteor Section website at

John W Mason
Acting Director
BAA Meteor Section

BAA electronic bulletins service.      E-mail:
Bulletin transmitted on  Sun Dec 13 11:49:14 GMT 2009
(c) 2009 British Astronomical Association
Good Clear Skies
Colin James Watling
Real Astronomer and head of the Comet section for LYRA (Lowestoft and Great Yarmouth Regional Astronomers) also head of K.A.G (Kessingland Astronomy Group) and Navigator (Astrogator) of the Stars (Fieldwork)

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