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(meteorobs) Meteor Activity Outlook for Dec. 19-25, 2003



The moon reaches its new phase on Tuesday December 23. At this time the moon
lies close to the sun as seen from the Earth therefore moonlight will be of
little consequence this week. The estimated total hourly rates for evening
observers this week should be near two for those in the Northern Hemisphere
and one in the Southern Hemisphere. For morning observers the estimated
total hourly rates should be near twelve for those located in the Northern
Hemisphere and ten for those in the Southern Hemisphere. 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 20/21. The radiants do not change greatly day to day so the
given positions may be used during this entire period. Most star atlases
(available at science stores and libraries) 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 location. 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 last rise later in the night. This list also
provides the order of ascending velocity for each radiant with those listed
first usually being much slower than those last on the list. Velocity should
not be the prime factor for shower association as all showers can produce
slow meteors. Slow meteors can be produced from normally swift showers, such
as the Leonids, when meteors appear near the radiant or close to the
horizon. The true velocity is only revealed in shower members seen far from
the radiant and high in the sky.

The Antihelion radiant is now centered at 06:56 (104) +23. This area of the
sky is located in central Gemini, very close to the position now occupied by
the zero magnitude planet Saturn. Since this radiant is large and diffuse,
any slow to medium speed meteor from Gemini, northeastern Orion, or
southeastern Auriga 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 3 shower
members per hour from areas north of the equator and 1 per hour for
locations 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. 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 antihelions 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 Northern Apex radiant is now centered at 11:56 (179) +15. This position
lies in eastern Leo, very close to the second magnitude star Denebola (Beta
Leonis). This area of the sky is best placed for viewing during the last
dark hour before dawn when it lies highest in the sky. Since this radiant is
large and diffuse, any meteor from eastern Leo or western Coma Berenices
could be a candidate from this source. Rates would be now close to two per
hour regardless of your location. Those who send their data to the
International Meteor Organization should call these meteors Coma Berenicids
(COM).

Like the antihelion area, both apex areas are active all year long and
travel approximately one degree eastward per day. Unlike the antihelion
debris, these particles orbit the sun in a retrograde motion opposite that
of the earth and are most likely produced by unknown comets. They strike the
earth after their closest approach to the sun. Since they are moving in
opposite directions these particles strike the earth at tremendous
velocities often creating bright meteors with persistent trains. These
particles strike the earth on the morning side of earth and are best seen
just before morning twilight while the sky is still perfectly dark. This is
not really a "shower" per se, but an artificial radiant created by the
Earth's motion through space. Meteors from both branches are normally
included in the sporadic count. I feel it is a worthy project to see if it
is possible to distinguish these meteors from the normal sporadic
background. On rare occasions there are meteors with a zero inclination that
radiate precisely from the apex point on the ecliptic, exactly 90 degrees
west of the sun. In simplistic terms, these meteors are seldom seen since
the Earth "sweeps clean" much of the material that shares the same orbit as
our planet. Much more material is located just north and south of the
earth's orbit with slightly higher or lower inclinations. This creates the
northern and southern branches of the apex activity.

The Southern Apex source lies exactly 30 degrees south of its northern
counterpart at 11:56 (179) -15. This position lies on the Crater/Corvus
border very close to the faint star Eta Crateris. Like the northern apex,
these meteors are best seen toward dawn when the radiant lies highest above
the horizon in a dark sky. Since this radiant is also large and diffuse, any
meteor from the Crater or Corvus could be a candidate from this source.
Rates would be now close to two per hour regardless of your location.

The Ursids (URS) reach maximum activity on December 23 with an average
estimated ZHR (zenith hourly rate) of twelve. The radiant is currently
located at 14:20 (215) +75. This position lies in southern Ursa Minor, two
degrees northwest of the second magnitude star Kochab (Beta Ursae Minoris).
The faint star 5 Ursae Minoris marks the center of the radiant. The radiant
is circumpolar from all locations north of 15 degrees north latitude. Thus
it never rises for locations south of 15 degrees south latitude. It would be
very difficult to see any Ursid activity south of the equator. The further
north one is located, the higher the radiant lies in the sky. While Ursid
activity may be monitored all night long from northern locations, it lies
closest to the northern horizon near 2000 (8pm) local time. The lowest rates
may be expected near this time of night. After 2000, the radiant slowly
gains altitude and reaches maximum altitude near 0800 (8am). Most locations
are in daylight at this time so the best hour to view Ursid activity would
be the last hour before the start of morning twilight. The Ursids are active
from December 17 through 26 with a majority of the activity occurring on the
22nd, 23rd and the 24th. At 33 km/sec. the Ursids produce meteors of average
velocity. The Ursids are also a faint shower not well seen from urban
locations. To see the Ursids well it is advisable to view from rural
locations away from all sources of light pollution. With the moon totally
out of the way this year, this is a fine opportunity to try to increase our
knowledge of this little known shower.

The Sporadic rates for the Northern Hemisphere are now just past their annua
l peak. One would expect to see perhaps seven random meteors per hour during
the last hours before dawn from rural observing sites. This estimate and the
estimate for the Southern Hemisphere do not include meteors from the apex
radiants. During the evening hours perhaps three random meteors can be seen
per hour. Sporadic rates seen from the Southern Hemisphere are increasing
toward a peak in January. One would see approximately six random meteors per
hour during the late morning hours and two per hour during the evening.

Clear Skies!
Robert Lunsford
AMS Operations Manager

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