(meteorobs) Meteor Activity Outlook for May 21-27, 2004

[Robert Lunsford]

 The moon reaches its new phase on Thursday May 27. At this time the moon
lies ninety degrees east of the sun and will set near 0100 local daylight
time. This weekend the moon will be a waxing crescent and will set shortly
after the end of evening twilight. It will set long before any post-midnight
meteor observing sessions. The estimated total hourly rates for evening
observers this week should be near two for those in the Northern Hemisphere
and three for those south of the equator. For morning observers the
estimated total hourly rates should be near eight for those located in the
Northern Hemisphere and twelve 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 May 22/23. 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 17:04 (256) -23. This area of the
sky is located in southern Ophiuchus, five degrees northwest of the third
magnitude star Theta Ophiuchi. Since this radiant is large and diffuse, any
slow to medium speed meteor from southern Ophiuchus, northwestern
Sagittarius, southwestern Serpens Cauda or northern Scorpius 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 two shower members per hour north of the equator
and three per hour for those situated 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. Those who send their data to the International Meteor Organization
should call these meteors Sagittarids (SAG).  The antihelion radiant was
once known as the Alpha Scorpiids this time of year.

The Beta Corona Australids (CAU) are listed among the radiants of the Dutch
Meteor Society. The current radiant position is located at 19:36 (294) -39.
This area of the sky is located in southeastern Sagittarius, five degrees
southwest of the fourth magnitude star Theta Sagittarii. The best time to
view this activity is near 0400 local daylight time, when the radiant lies
highest in a dark sky. ZHR's at maximum activity (May 15) were predicted to
be only three shower members per hour so current rates would be less than
one per hour. Observers in the Southern Hemisphere have an advantage with
the radiant rising much higher in their sky. With an entry velocity of 45
kilometers per second, a majority of these meteors will appear to move at
moderate speeds.

The Northern Apex radiant is now centered at 22:04 (331) +04. This position
lies in southwestern Pegasus near the faint star Nu Pegasi. 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 northern Aquarius, southern Delphinus, Equuleus or southwestern Pegasus
could be a candidate from this source. Rates would be now close to two per
hour regardless of 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 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 22:04 (331) -26. This position lies in northern Pisces
Austrinus, ten degrees northwest of the first magnitude star Fomalhaut
(Alpha Piscis Austrini. 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 southern
Aquarius, southeastern Capricornus, or Pisces Austrinus could be a candidate
from this source. Rates would now be one per hour in the Northern Hemisphere
and three per hour in the Southern Hemisphere.

The Eta Aquarids (ETA) are produced by particles from Halley's Comet, which
last passed through the inner solar system in 1986. Even though this is now
eighteen years ago, material is still encountered every year in late April
and throughout most of May. We passed closest to Halley's orbit on May 5.
Current rates would be less than one per hour from a radiant located at
23:28 (352) +06. This area of the sky is located in western Pisces, one
degree south of the faint magnitude star Theta Piscium. The best time to
view this activity is just before the start of morning twilight, when the
radiant lies highest in a dark sky. With an entry velocity of 66 kilometers
per second, a majority of these meteors will appear to move swiftly.

The Sporadic rates for the Northern Hemisphere are declining. One would
currently expect to see perhaps four random meteors per hour during the last
hours before dawn from rural observing sites. This estimate and the morning
estimate for the Southern Hemisphere do not include the apex meteors listed
above. 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 stronger than those seen in the northern skies with perhaps six
random meteors being seen per hour during the early morning hours and three
per hour during the evening.

Clear Skies!
Robert Lunsford
AMS Operations Manager