(meteorobs) Viewing the 2004 Geminid Meteor Shower
Robert Lunsford
lunro.imo.usa at cox.net
Thu Dec 9 15:02:47 EST 2004
The Geminid meteor shower will provide a fine view of celestial fireworks
this weekend and beyond. The expected time of maximum activity will be near
22:20 Universal Time (courtesy the I.M.O.) on Monday, December 13. This time
is favorable for Eastern Europe, the Middle East and Western Asia. Actually,
strong rates are possible twelve hours either side of this time, meaning
that everyone north of the equator will still have a chance of seeing an
impressive shower. Those observers located south of the equator can see some
activity near 0200 local standard time, when the radiant rises highest into
their northern sky. Observers south of 30 degrees south latitude are out of
luck as the radiant will not rise into their sky at all. For observers in
North America, the best rates will be seen Monday morning near 02:00 local
standard time. This will be the best time regardless of what state or
province one is located. After maximum activity the Geminid rates rapidly
fall and by Thursday, December 16, the shower will have fallen below the
level of the sporadic background.
This is one of the few showers that produce decent activity during the
evening hours. The radiant, located five degrees northwest of the bright
star Castor (Alpha Geminorum), rises near 18:00 (6:00pm) local standard
time. You must remember though, at this time the Earth (the horizon and
below) will block a great majority of the Geminid activity. Those that do
appear in the sky at this time are just skimming the upper regions of the
Earth's atmosphere. Therefore they avoid the dense lower layers of air. This
allows the Geminid meteoroids to last much longer than they would if they
struck the Earth at a more direct angle. From the ground one would see these
meteors as long, majestic streaks lasting several seconds. These are
commonly known as "Earth-grazers" and occur for all showers. Only the
strongest annual showers produce enough of these meteors to make it
worthwhile to attempt to view at this hour. Luckily, the Geminids are one of
these showers. These "Earth-grazers" most often appear low in the northeast
or southeastern skies. Occasionally, one will appear high in the sky and the
sight is memorable.
As the night progresses, the Geminid radiant rises higher into the eastern
sky. As the radiant gains altitude, the number of meteors lost to hills and
trees lessens. One will also notice the average Geminid meteor becoming
shorter as they strike the Earth at a more direct angle. By 22:00 local
standard time (10:00pm) the radiant lies approximately half-way up in the
eastern sky. It is near this time the action really increases. On the
evening of December 12, at 22:00 one would expect to be seeing 20-25
Geminids per hour from dark sky sites. This rate will increase and peak near
02:00, when the radiant lies highest in the sky. No one knows exactly what
the peak rates will be. Those observers located in areas where the peak
occurs with the radiant high in the sky should experience counts in excess
of one hundred Geminids per hour. Others will experience rates between sixty
and one hundred per hour, depending on their longitude and weather
conditions. After 02:00, Geminid rates will begin to fall and will continue
to fall until the onset of dawn.
The Geminids occurring before maximum activity are normally dim. Therefore
do not expect to see good rates this weekend unless you view from rural
locations far from city lights. The Geminids seen after maximum are usually
brighter but unfortunately rates descend rapidly therefore there will be
less of them to see. To see the most activity it would be wise to view from
midnight to 0300 local standard time on Monday morning December 13. It is
advisable to sit in a comfortable lounge chair and face halfway up into the
sky. At this time of night the Geminid radiant will be high in the sky for
observers in the Northern Hemisphere. Therefore meteors will be appearing in
all directions so it would be wise to face toward the darkest and least
obstructed direction. All Geminid meteors will appear to trace back near the
bright star Castor, in the constellation of Gemini. Another slightly
brighter star (Pollux) lies only five degrees to the southeast. The
brightest object in this area of the sky will be the planet Saturn, which
lies in eastern Gemini on a line from Castor to Pollux, roughly the same
distance that Castor lies from Pollux. There are other weak meteor radiants
active at this time adding to the activity one will see. There are also
sporadic (random) meteors that occur throughout the night. At this time of
year one could expect to see up to twelve sporadic meteors and perhaps one
or two meteors from other active radiants per hour.
There are interesting projects one can undertake while viewing meteor
showers such as the Geminids. The basic experiment is to see how many
meteors you see each hour. One can also collect scientifically useful data
by categorizing each meteor and assigning an estimated magnitude for each
one. You also would need to estimate the faintest star you can see during
each hour so that scientists can calibrate your viewing conditions verses
other observers. Experienced observers also record the time, velocity and
other useful data such as the appearance of a persistent train. For more
information on observing techniques visit the following web pages:
http://skyandtelescope.com/observing/objects/meteors/article_98_1.asp
http://www.amsmeteors.org/visual.html
http://www.namnmeteors.org/
Unlike the Leonid and Perseid meteoroids, the Geminid meteoroids strike the
Earth at a perpendicular angle. The streaks they leave in the sky often last
as long as one second or more. This is good news for photography buffs as
this makes it easier to capture Geminid meteors on film. All you need is a
camera capable of taking long time exposures. You will need exposures up to
fifteen minutes long, depending on the darkness of your night sky. A typical
50mm f1.8 lens is fine and has produced many impressive meteor photos. I
personally like to use a wider field lens such a 24 or 28mm. These lenses
are not as "fast" as the typical 50mm lens but you can make up this deficit
by using a high speed film such as ISO 800 or above. To capture a meteor
just aim you camera approximately half-way up in the sky and then lock the
shutter open. Stars will appear as parallel trails across the frame but a
meteor will bisect these trails and will appear as a streak with sharp ends
while the star trails appear blunt. You can eliminate the star trails by
using a motor-driven mount. This mount may be especially designed for
cameras or one can take advantage of your telescope's mount and attaching
your camera directly to the tube of your scope. Don't use the telescope
itself to attempt to photograph meteors as the field of view is much too
small and the chances of capturing a meteor will be extremely remote. There
is also no advantage of using color films other than seeing the different
star colors. Those viewing from rural areas can take exposures up to fifteen
minutes long while those viewing closer to town should limit their exposures
to five minutes. It all a matter of luck if any meteor passes through the
cameras field of view. Using normal techniques, meteors must be at least as
bright as first magnitude to be photographed. From previous experience I
have found that I average 2-3 meteors per 24 exposure roll of ISO 400 during
the Geminids.
This may be your last opportunity to view a major annual meteor shower in
all its glory for quite some time. Many of the major showers are spoiled by
moonlight in 2005. Next year a full moon will spoil the Geminid show
therefore don't pass up this opportunity to view and photograph this shower
under optimum conditions.
Clear Skies!
Robert Lunsford
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