(meteorobs) Meteor Activity Outlook for July 23-29, 2004

[Robert Lunsford]

This week will be an excellent time to view meteor activity. The moon
reaches its first quarter phase on Sunday July 25. At this time the moon
lies ninety degrees east of the sun and will set near 0100 local daylight
time. This will be the best time in July to view meteor activity as the moon
will not hamper observations during the most active morning hours. Get out
now though as the moon will become more and more of a factor, especially
next week. It will be interesting to see the many radiants in Aquarius and
Capricornus increase in strength during this period. The estimated total
hourly rates for evening observers this week should be near two for those in
the Northern Hemisphere and two for those observers south of the equator.
Rates are reduced during the evening due to moonlight. For morning observers
the estimated total hourly rates should be near twenty five for those
located in the Northern Hemisphere and thirty 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 July 24/25. 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 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.

These are the showers that may be observed this week:

The Alpha Capricornids (CAP) are active from a diffuse radiant located at
20:12 (303) -11. This position lies in northwestern Capricornus, one degree
northwest of the wide third magnitude double star Alpha Capricornii. The
radiant is best placed near midnight local daylight time, when it lies
highest in the sky. Current rates would be 3-4 shower members per hour at
best. With an entry velocity of 23 km/sec., most activity from this radiant
would be slow.  This radiant is well seen except for far northern latitudes
where it remains twilight all night long and the radiant does not rise as
high into their sky.

The Alpha Cygnids (ACG) are listed as the Psi Cygnids in the list of
radiants published by the Dutch Meteor Society. No matter what you call them
they are often overshadowed by the many other better known radiants active
at this time. This radiant was well represented in the studies coordinated
by Dr. Peter Jenniskens between 1980 and 1991. Their studies indicate a ZHR
near 3 at maximum activity on July 18. The radiant position is currently
located at 20:36 (309) +48. This position is located in northern Cygnus,
three degrees northwest of the first magnitude star Deneb (Alpha Cygni).
With an entry velocity of 37 km/sec. these meteors are of average velocity.
The radiant is best placed near 0200 LDT when it lies nearly overhead for
much of the Northern Hemisphere. Due to its high northern declination this
activity is not well seen from the Southern Hemisphere.

The Anthelion radiant is now centered at 21:08 (317) -15. This area of the
sky is located in central Capricornus, one degree north of the fourth
magnitude star Theta Capricornii Since this radiant is large and diffuse,
any slow to medium speed meteor from Capricornus, southeastern Aquila,
northern Microscopium, northwest Piscis Austrinus or southwestern Aquarius
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 one shower member per hour north of the
equator and three per hour for those situated south of the equator.  At this
time of year the Anthelion radiant overlaps both the Alpha Capricornid and
North Delta Aquarid radiants so distinguishing between these showers will be
difficult but not impossible if you face toward this general area of the
sky.

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. At this time of year the IMO has no designation
for the anthelion radiant as it overlaps several other radiants as it moves
through Capricornus and Aquarius during July and August.

The South Iota Aquarids (SIA) reach ZHR's of one on July 25. This shower
peaks on August 4 so current rates would be quite low, certainly less than
one shower member per hour. The radiant is currently located at 21:28
(322) -17. This area of the sky is located in eastern Capricornus, very
close to the fourth magnitude star Iota Capricornii. This position is also
very close to the anthelion radiant and without plotting and velocity
estimates, it would be impossible to separate meteors from the two sources.
At 34 km/sec. the SIA's have virtually the same velocity as the anthelion
meteors. This radiant is most active near 0100 LDT when it lies highest in
the southern sky.

The North Delta Aquarids (NDA) are currently active from a radiant located
at 21:32 (323) -09. This area of the sky is located in northeastern
Capricornus, three degrees south of the third magnitude star Sadalsuud (Beta
Aquarii). This position is also close to the anthelion radiant and care must
be taken to distinguish meteors from these sources. At 42 km/sec. the NDA's
are a bit faster than the antihelion meteors. This radiant is most active
between 0200 and 0300 LDT when it lies highest in the sky. Like many of the
radiants active this time of year the NDA's are better seen from areas south
of the northern tropics or 30 degrees north latitude. Maximum hourly rates,
seen from the southern tropics, are currently two per hour.

The South Delta Aquarids (SDA) radiant provides the strongest activity in
July. This radiant peaks in strength on July 27 with an average ZHR of
twenty. The current radiant position lies at 22:28 (337) -17. This area of
the sky is located in southern Aquarius, five degrees west of the third
magnitude star Delta Aquarii. The radiant is best positioned near 0300 local
daylight time, when it lies highest in the sky. These meteors encounter the
earth at a speed of 41 km/sec. which is a bit faster than average. Activity
is best seen from the Southern Hemisphere where the radiant passes high
overhead.

The Piscis Austrinids (PAU) are active from a radiant located at 22:32
(338) -31. This area of the sky is located in central Pisces Austrinus, two
degrees north of the fourth magnitude star Beta Piscis Austrini. ZHR's are
currently 4-5 per hour. Don't expect to see this many unless you live near
30 degrees south latitude where the radiant passes high overhead between the
hours of 0200 and 0300 LDT. These meteors are of average velocity, slower
than the Aquarids but faster than the Alpha Caps and the anthelion meteors.

Perseid (PER) activity is slowly increasing as we approach the August 12
maximum. The current radiant position lies at 01:32 (023) +54, which is
located on the Perseus/Cassiopeia border, eight degrees southeast of Shedar
(Alpha Cassiopeiae). The radiant is well placed for those in the Northern
Hemisphere during the last few hours before dawn. Due to the high northern
declination (celestial latitude) of the radiant, rates seen from the
Southern Hemisphere are very low or non-existent. Current Perseid rates for
those observers north of the equator should be near three per hour. At
59km/sec., Perseid meteors are usually swift, often exhibiting persistent
trains.

The Northern Apex radiant is now centered at 02:08 (032) +27. This position
lies in northwestern Aries, three degrees north of the second magnitude star
Hamal (Alpha Arietis). 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 Pisces, southern
Andromeda, northern Aries or Triangulum could be a candidate from this
source. Rates would be now close to two per hour for observers north of the
equator and one per hour for those observers south of the equator.

Like the anthelion area, both apex areas are active all year long and travel
approximately one degree eastward per day. Unlike the anthelion 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 02:08 (032) -03. This position lies in northern Cetus, two
degrees west of the variable star Mira (Omicron Ceti). 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 northern Cetus, southwestern Aries, northwestern
Eridanus or southeastern Pisces could be a candidate from this source. Rates
would now be two per hour regardless of your location.

The Sporadic rates for the Northern Hemisphere are now increasing. One would
expect to see perhaps six 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 equivalent to those seen in the north with perhaps six random meteors
being seen during the late morning hours and two per hour during the
evening. Evening rates are reduced due to moonlight.

Clear Skies!
Robert Lunsford
AMS Operations Manager