(meteorobs) Meteor Activity Outlook for April 9-15, 2004

 

The moon reaches its last quarter phase on Monday April 12. At this time
the moon lies ninety degrees west of the sun and will rise near local
midnight. Observers this weekend will have to contend with bright moonlight
after midnight. As the week progresses the moon will rise later in the
morning providing the observer with more dark skies. 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 six for
those located in the Northern Hemisphere and ten for those in the
Southern Hemisphere. Morning 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 positions listed below are exact for Saturday night/Sunday morning April
10/11. 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 Antihelion radiant is now centered at 14:20 (215) -13. This area of the
sky is located on the Virgo/Libra border, eight degrees northwest of the
third magnitude star Zubenelgenubi (Alpha Librae). Since this radiant is
large and diffuse, any slow to medium speed meteor from Western Libra or
southwestern Virgo 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 regardless of your location.

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 Virginids (VIR).

The Northern Apex radiant is now centered at 19:20 (290) -07. This position
lies in southern Aquila, three degrees southeast of the third magnitude star
Lambda Aquilae. 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 southern Aquila, northeastern
Sagittarius or Scutum 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 19:20 (290) -37. This position lies in the extreme
southeastern Sagittarius, three degrees west of the fourth magnitude star
Gamma Corona Australis. 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 northwestern
Corona Australis or southeastern Sagittarius could be a candidate from this
source. Rates would now be less than one per hour in the Northern Hemisphere
and two per hour in the Southern Hemisphere.

The Delta Pavonids (DPA) are listed among the radiants of the Dutch Meteor
Society. They were predicted to peak on March 28. The radiant currently lies
at 22:00 (330) -66. This area of the sky is located in central Indus, four
degrees east of the fourth magnitude star Gamma Pavonis. This area of the
sky is too far south to be seen north of the northern equatorial areas. The
ZHR at maximum is five shower members per hour. These rates are rarely seen
except from far southern locations such as Australia, South Africa and
southern South America.  Current rates would be less than one shower member
per hour. 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 60 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 three 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 five random meteors being seen per hour during the early morning
hours and three per hour during the evening. Morning rates are reduced by
moonlight.

Clear Skies!
Robert Lunsford
AMS Operations Manager