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(meteorobs) Meteor Activity Outlook for March 7-13, 2003
The moon reaches its first quarter phase on Tuesday March 11. At this
time the moon will set near midnight local standard time and will not
interfere with morning observing. The estimated total hourly rates for
evening observers this week should be near one for those in the Northern
Hemisphere and two for those viewing south of the equator. For morning
observers the estimated total hourly rates should be near nine for those
located in the Northern Hemisphere and fifteen 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. Moonlight reduces evening rates this week.
The positions listed below are exact for Saturday night/Sunday morning
March 8/9 The positions do not change greatly day to day so these
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 radiants
below are listed in a west to east manner in order of right ascension
(celestial longitude). The radiants 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 following radiants will be
active this week:
The Delta Leonids peaked on February 25. The current radiant position is
located at 11:56 (179) +12. This area of the sky is located in eastern
Leo, three degrees southeast of the second magnitude star Denebola (Beta
Leonis). This position is close to the antihelion source so care must be
taken to separate the two. It is quite possible that this radiant is a
northern branch of the antihelion source. The hourly rate, even at
maximum, rarely exceeds one shower member per hour so current rates
would certainly be much less than one per hour. The best time to view
this activity is near 0100 local time, when the radiant lies on the
meridian and at its highest point in the sky. With an entry velocity of
23 kilometers per second, these meteors will appear to move slowly.
The Antihelion source is now centered at 12:08 (182) -01. This area of
the sky is located in eastern Virgo, two degrees west of the fourth
magnitude star Eta Virginis. Since this source is large and diffuse, any
slow to medium speed meteor from eastern Leo or western Virgo could be a
candidate for this shower. The area of the sky 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 2 shower members per hour from all
locations.
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. Those who share their reports with the
I.M.O. should call these meteors Virginids or "VIR".
The Theta Centaurids was the first and is also the last of several
radiants that are active in Centaurus from late January through early
March. The date of maximum activity was February 14 with a predicted ZHR
of 4. Current ZHR's would be less than 1. The radiant currently lies at
15:40 (235) -50. This position is in western Norma close to the variable
star R Normae. Due to the southern declination this shower is not well
seen north of the northern tropical regions. This area of the sky is
best placed near 0500 local standard time when it lies highest in the
sky. At 60 km/sec. the Theta Centaurids normally produce meteors of
swift angular velocities.
The Gamma Normids are active from a radiant located at 16:16 (244) -51.
This position lies in eastern Norma, very close to the fourth magnitude
double star Gamma Normae. Since the date of maximum activity is not
until March 14, current ZHR's would still be less than 1. Due to the
southern declination this shower is not well seen north of the northern
equatorial regions. This area of the sky is best placed near 0600 local
standard time when it lies highest in the sky. At 56 km/sec. the Gamma
Normids normally produce meteors of swift angular velocities.
The Northern Apex area is now centered at 17:08 (257) -08. This position
lies in southern Ophiuchus, some eight degrees north of the second
magnitude star Eta Ophiuchi. This area of the sky is best placed for
viewing during the last dark hour before dawn. Since this source is
large and diffuse any meteors from southern Serpens Cauda or southern
Ophiuchus could be a good candidate for this shower. This source should
provide at least 2 meteors per hour no matter your location.
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 debris 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 17:08 (257) -38. This position lies in central Scorpius,
four degrees west of the "stinger of Scorpius", also known as the third
magnitude star Lesath (Upsilon Scorpii). Like the northern apex, these
meteors are best seen toward dawn when the radiant lies highest above
the horizon in a dark sky. Any meteor from central Scorpius could a
candidate from this source. Rates should be near 3 per hour from the
Southern Hemisphere and less than 1 per hour from the Northern Hemisphere.
The Sporadic rates for the Northern Hemisphere are now slowly declining
and will do so until June. One would expect to see perhaps 5-6 random
meteors per hour during the last hours before dawn from rural observing
sites. During the evening hours perhaps 1-2 random meteors can be seen
per hour. Rates seen from the Southern Hemisphere would be approximately
7-8 random meteors being seen per hour during the late morning hours and
2-3 during the evening hours. Moonlight reduces evening rates this week.
Clear Skies!
Robert Lunsford
AMS Operations Manager
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