(meteorobs) Meteor Activity Outlook for July 16-22, 2004

[Robert Lunsford]

The moon reaches its new phase on Saturday July 17. At this time the moon
lies in the same area of the sky as the sun and rises and sets with the sun.
This will be the best time in July to view meteor activity as the moon will
not hamper observations at any time of night. 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. For
morning observers the estimated total hourly rates should be near fourteen
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.

The radiant positions listed below are exact for Saturday night/Sunday
morning July 17/18. The radiant 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.

The Alpha Capricornids (CAP) are active from a wide radiant located at 19:48
(297) -12. This position lies in northeastern Sagittarius, six degrees west
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 low, only 2-3 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 overlooked 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 20:20 (305) +47. This position is located in northern Cygnus,
four 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 20:40 (310) -17. This area of the
sky is located in central Capricornus, five degrees west of the fourth
magnitude star Theta Capricornii Since this radiant is large and diffuse,
any slow to medium speed meteor from Capricornus, eastern Sagittarius,
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
the Alpha Capricornid radiant so distinguishing between the two showers will
be difficult but not impossible if you include both radiants within your
field of view.

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 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 North Delta Aquarids (NDA) are currently active from a radiant located
at 21:12 (318) -09. This area of the sky is located in western Aquarius, six
degrees southwest 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. Currently rates are only 2-3 per hour but this will quickly rise as
we approach late July. The current radiant position lies at 22:04 (331) -18.
This area of the sky is located in western Aquarius, three degrees south of
the fourth magnitude star Iota 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 22:08
(332) -33. This area of the sky is located in southern Pisces Austrinus near
the faint star Mu Piscis Austrini. ZHR's are currently 1-2 per hour. Don't
expect to see two per hour 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 now starting and will strengthen slightly with
each passing night. The current radiant position lies at 01:08 (017) +52,
which is located in extreme southern Cassiopeia, seven 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 one per hour. These
rates will gradually increase as we approach the August 12 maximum. At 59
km/sec., Perseid meteors are usually swift, often exhibiting persistent
trains.

The Northern Apex radiant is now centered at 01:40 (025) +25. This position
lies in northeastern Pisces, five degrees northwest 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, western 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 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 01:40 (025) -05. This position lies in northern Cetus, six
degrees northwest of the fourth magnitude star Zeta 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 Cetus, southwestern Aries 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 five 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 still slightly better than those seen in the north with perhaps six
random meteors being seen during the late morning hours and two per hour
during the evening.

Clear Skies!
Robert Lunsford
AMS Operations Manager