(meteorobs) Meteor Activity Outlook for December 17-23, 2004

[Robert Lunsford]

This upcoming period will see the moon reaching its first quarter phase on
Saturday December 18. At this time the moon will be setting near local
midnight, allowing the active morning hours to remain free of bothersome
moonlight. The estimated total hourly rates for evening observers this week
should be near two for those in the Northern Hemisphere and one for those
observers south of the equator. For morning observers the estimated total
hourly rates should be near twenty for those located in the Northern
Hemisphere and ten for those in the Southern Hemisphere. Evening rates are
reduced due to moonlight. These rates assume that you are watching from
rural areas away from all sources of light pollution. The actual rates will
also depend on factors such as personal light and motion perception, local
weather conditions, alertness and experience in watching meteor activity.

The radiant positions listed below are exact for Saturday night/Sunday
morning December 18/19. These positions do not change greatly day to day so
the listed coordinates may be used during this entire period. Most star
atlases (available at science stores and planetariums) will provide maps
with grid lines of the celestial coordinates so that you may find out
exactly where these positions are located in the sky. A planisphere or
computer planetarium program is also useful in showing the sky at any time
of night on any date of the year. Activity from each radiant is best seen
when it is positioned highest in the sky, either due north or south along
the meridian, depending on your latitude. Meteor activity is not seen from
radiants that are located below the horizon. The positions below are listed
in a west to east manner in order of right ascension (celestial longitude).
The positions listed first are located further west therefore are accessible
earlier in the night while those listed further down the list rise later in
the night.

These are the showers that may be observed this week:

The Anthelion radiant is now centered at 06:48 (102) +23. This area of the
sky is located in central Gemini, two degrees south of the third magnitude
star Epsilon Geminorum. Since this radiant is large and diffuse, any slow to
medium speed meteor from northern Orion, southeastern Auriga, Gemini,
northwestern Monoceros or eastern Taurus could be a candidate for this
shower. The center of this area is best placed near 0100 local standard time
when it lies on the meridian and is highest in the sky. At this time expect
to see three shower members per hour from the Northern Hemisphere and one
per hour south of the equator.

Unlike most of the annual showers the antihelion source is produced by
debris from unknown objects orbiting in a direct motion like the earth.
These objects are most likely asteroids, which produce stony and metallic
debris whose density is much greater than material produced by comets. There
is also the possibility that some of this activity may be caused by the
"Jupiter family of comets", comets which have been altered by Jupiter's
gravity into much shorter orbits. This material collides with the earth on
the inbound portion of its orbit, before its closest approach to the sun.
Therefore we best see them just after midnight when we are facing the
direction from which this activity appears. The antihelion source is active
all year from an area of the sky nearly opposite that of the sun. The center
of this source will move approximately one degree eastward per day and
travels through many different constellations over the course of a year. It
may make sense to list these meteors as anthelions or "ANT" but a majority
of meteor organizations prefer that you list them from the constellation in
which the radiant is currently located or the constellation where the shower
reaches maximum activity.

The Coma Berenicids (COM) are active from a radiant located at 11:44 (176)
+24. This area of the sky lies in northeastern Leo, four degrees northeast
of the third magnitude star Zosma (Delta Leonis). This radiant is best
placed for viewing just before dawn, when it lies highest above the horizon.
The predicted date of maximum activity is December 19, when the ZHR reaches
five. At 65 km/sec. the Coma Berenicids produce mostly swift meteors with
the brighter shower members leaving persistent trains.

The Ursids (URS) reach a ZHR of one on December 17. Maximum activity is
predicted to occur on Wednesday morning December 22, near 0700 Universal
Time. For North American observers this corresponds to 2am EST and 11pm PST.
The ZHR reaches an average of ten on this morning. The radiant is located at
14:28 (217) +75. This area of the sky lies in southern Ursae Minor, very
close to the fourth magnitude star 5 Ursae Minoris. The bright orange second
magnitude star Kochab (Beta Ursae Minoris) also lies only two degrees to the
southeast. This radiant is best placed for viewing just before dawn, when it
lies highest above the horizon. For areas north of latitude 20 north, the
radiant lies above the horizon the entire night. Only in higher northern
latitudes is the radiant altitude sufficient for unimpeded observing during
the evening hours. At this time of night though, the waxing gibbous moon
will still be in the sky. On the morning of maximum activity (December 22)
the moon will set near 0315 local standard time at locations at 30 degrees
north latitude. Further north, at 50 north latitude for instance, the moon
will not set until just after 0400 LST. Therefore there will be a small
window of opportunity to view the shower under dark skies. You need not wait
until after the moon has set as it becomes much less of a factor at
altitudes of less than ten degrees.  This shower is not visible from the
Southern Hemisphere due to the low radiant altitude. This is one of the
least viewed major showers and data is desperately needed. If your skies are
clear this week please make an effort to view this shower. Even negative (no
activity) results are important as they help refine the global activity
profile. At 33 km/sec. the Ursids produce meteors of average velocities.

The Sporadic rates for the Northern Hemisphere are now near their annual
peak. One would expect to see perhaps twelve random meteors per hour during
the last hours before dawn from rural observing sites. During the evening
hours perhaps two random meteors can be seen per hour from the Northern
Hemisphere. Rates seen from the Southern Hemisphere are now lower than those
seen in the north with perhaps eight random meteors being seen during the
late morning hours and one per hour during the evening. Evening rates are
reduced due to moonlight..

Anthelion        Radiant Position  = 06:48  (102)  +23
Hourly Rate = 3  N. Hemisphere, 1  S. Hemisphere

Coma Berenicid (COM)   Radiant Position  = 11:44 (176)  +24
Hourly Rate = 3  N. Hemisphere, 1  S. Hemisphere

Ursid (URS)   Radiant Position  = 14:28 (217)  +75
Hourly Rate = 2  N. Hemisphere, 0  S. Hemisphere

Clear Skies!
Robert Lunsford
American Meteor Society