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(meteorobs) Meteor Activity Outlook for July 4-10, 2003
The moon reaches its first quarter phase on Monday July 7. At this time the
moon will lie ninety degrees east of the sun and will set near 0100 local
daylight savings time. The productive early morning hours are still free of
interfering moonlight during most of this period. The estimated total hourly
rates for evening observers this week should be near one for northern
viewers and two for those located south of the equator. For morning
observers the estimated total hourly rates should be near twelve 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
will reduce rates seen during the evening hours this week.
The radiant positions listed below are exact for Saturday night/Sunday
morning July 5/6. The positions do not change greatly day to day so they 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 Theta Ophiuchids reached maximum activity on June 28 with very low
rates. The radiant is currently located at 17:12 (258) -10. This area of the
sky is located in southern Ophiuchus, six degrees north of the second
magnitude star Eta Ophiuchi. This area of the sky is best placed near 2300
(11pm local daylight time) when the radiant lies on the meridian and highest
in the sky. At 27 km/sec. the Theta Ophiuchids are slow compared to most
other showers.
The Alpha Capricornids are now starting their long period of activity. They
reach maximum activity on July 30th with an estimated ZHR of 4. Current
rates would be less than one meteor per hour. The radiant is located at
19:04 (286) -16, which places it in northeastern Sagittarius, some five
degrees north of the third magnitude star Pi Sagittarii. These meteors are
best seen near midnight, local daylight time, when the radiant stands
highest above the horizon. At 25km/sec. the majority of Alpha Capricornids
appear to travel quite slowly. Care must be taken when identifying these
meteors as the antihelion area lies close by, only twelve degrees to the
northeast.
The Antihelion source is now centered at 19:52 (298) -20. This area of the
sky is located in extreme eastern Sagittarius, ten degrees southwest of the
third magnitude star Dabih (Beta Capricorni). Since this source is large and
diffuse, any slow to medium speed meteor from eastern Sagittarius or western
Capricornus could be a candidate for this shower. This area of the sky is
best placed near 0200 local daylight time when it lies on the meridian and
is highest in the sky. At this time expect to see up to four shower members
per hour from locations south of the equator and two per hour from locations
further north.
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 Sagittarids or "SAG".
The Pegasids are now starting their short period of activity. They reach
maximum activity on July 10th with an estimated ZHR of 3. The radiant is
located at 22:36 (339) +14, which places it in western Pegasus, three
degrees northwest of the third magnitude star Zeta Pegasi. These meteors are
best seen during the last dark hour before the start of morning twilight,
when the radiant lies highest above the horizon in a completely dark sky. At
70km/sec. the majority of Pegasid meteors will appear to travel swiftly.
The Northern Apex area is now centered at 00:52 (013) +21. This position
lies in northern Pisces, three degrees southwest of the fourth magnitude
star Eta Andromedae. 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 eastern Pegasus, southwestern Andromeda or northern Pisces
could be a good candidate for this shower. This source should provide at
least 2 meteors per hour from the Northern Hemisphere and one per hour from
the Southern Hemisphere.
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 00:52 (013) -09. This position lies in western Cetus, four
degrees northwest of the third magnitude star Eta Ceti. 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 western Cetus or southern
Pisces could a candidate from this source. Rates should be near 2 per hour
from the Southern Hemisphere and 1 per hour from the Northern Hemisphere.
The Sporadic rates for the Northern Hemisphere are now rising slowly toward
their peak, which occurs during the last quarter of the year. 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 estimate for
the Southern Hemisphere does not include meteors from the apex radiants.
During the evening hours perhaps one random meteor can be seen per hour.
Rates seen from the Southern Hemisphere would also be approximately six
random meteors being seen per hour during the late morning hours and two
during the evening hours. Evening rates are lower this week due to
moonlight.
Clear Skies!
Robert Lunsford
AMS Operations Manager
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