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(meteorobs) Meteor Activity Outlook for August 9-15, 2002



Meteor Activity Outlook for 
 August 9-15, 2002

By Robert Lunsford 
AMS Visual Program Coordinator

The moon reaches its first quarter phase on Thursday August 15. At this time the moon will lie 90 degrees east of the sun and will set near 1am local daylight time for many locations. For the upcoming Perseid maximum, on Monday and Tuesday August 12 and 13, the moon will be a waxing crescent, setting in the late evening hours. This will allow an unhampered view of the morning sky, when the Perseid radiant lies highest above the horizon. The estimated total hourly rates for evening observers this week should be near 7 for those in the Northern Hemisphere and 2 for those south of the equator. For morning observers the estimated total hourly rates should be near 50 for those located in the Northern Hemisphere and 10 for those located in the Southern Hemisphere. These rates assume that you are watching from rural areas away from all sources of light pollution. The actual activity one may see 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 August 10/11 unless otherwise noted. The positions do not change greatly day to day so these positions may be used during this entire period. Most star atlas's (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 which constellations are in 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. 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 Kappa Cygnids are produced from a radiant located at 18:56 (284) +58. The area of the sky is located in southeastern Draco near the faint star 48 Draconis. The radiant is best placed just as soon as it gets dark when the radiant lies near the zenith for many in the Northern Hemisphere. For those in the Southern Hemisphere the radiant lies close to the northern horizon and not much activity is seen at all. The date of maximum activity is not until August 18, so current rates would be near 1 per hour. The velocity of the Kappa Cygnids is 25 km/sec., which is slow. The Kappa Cygnids are known for their brightly colored fireballs.

The Alpha Capricornid radiant is located at 21:16 (319) -06. The area of the sky is located in western Aquarius, 3 degrees west of the 3th magnitude star Sadalsuud (Beta Aquarii). The activity period for the Alpha Caps is nearly over but occasional shower members may still be seen. The velocity of the Alpha Caps is 25 km/sec., which is slower than your average meteor. The radiant is best placed near 0100 local daylight time when it lies on the meridian. Like the Kappa Cygnids, the Alpha Caps are known for their brightly colored fireballs.

The Antihelion radiant is now located at 22:12 (333) -10. This area of the sky is located in central Aquarius near the 4th magnitude star Theta Aquarii. Any slow to medium speed meteor from eastern Capricornus or central Aquarius could be a candidate for this shower. The radiant 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 1 shower member per hour from locations north of the equator and 2 an hour from areas south of the equator. 

Unlike most of the annual showers the antihelion radiant is produced by debris from unknown sources orbiting in a direct motion like the earth. These sources are most likely asteroids, which produce stony and metallic debris whose density is much greater than material produced by comets. This debris 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 radiant is active all year from an area of the sky nearly opposite that of the sun. The radiant will travel approximately one degree eastward per day and travels through many different constellations over the course of a year. It is easiest to simply list these meteors as " 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.  IMO observers may wish to list these meteors as North Iota Aquarids "NIA ", although the radiant position is listed a bit further west at  21:16 (319) -07. Note this position is nearly identical with that of the Alpha Caps.

The North Delta Aquarids reached maximum activity on August 8.  The current radiant location is 22:24 (336) -05. This area of the sky is located in central Aquarius, 4 degrees south of the 4th magnitude star Gamma Aquarii. This position is close to the antihelion radiant and unless you plot the meteors you see, these meteors will be indistinguishable from one another. 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 southern 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, should be close to 3 per hour.

The South Delta Aquarid activity is now waning after maximum activity occurred on July 27. Current ZHR's should be near 3 for favorable areas south of the equator. The current radiant position lies at 23:16 (349) -13. This area of the sky is located in eastern Aquarius near the faint star 94 Aquarii. The radiant lies low in the southeast at dusk and is not well seen until it approaches the meridian near 0300 LDT. These meteors encounter the earth at a speed of 41 km/sec. which is a bit faster than average. These meteors are also better seen from the Southern Hemisphere where the radiant passes high overhead.

Like the Alpha Caps, the Pisces Austrinids activity is also near its end. The radiant position lies at 23:32 (353) -26, which places it in western Sculptor, some 8 degrees northeast of the bright 1st magnitude star Fomalhaut (Alpha Piscis Austrini). Current rates would be less than one per hour. These meteors are of average velocity, slower than the Aquarids but faster than the Alpha Caps and the antihelion meteors.

The Northern Apex radiant is now located at 03:12 (048) +32. This position lies in southwestern Perseus, some 9 degrees south of the famous variable star Algol (Beta Persei). This area of the sky is best placed for viewing during the last dark hour before dawn. Since this radiant is diffuse any meteors from northern Aries, southwestern Perseus, northwestern Taurus or eastern Triangulum could be a good candidate for this shower. This source should provide at least 3 meteors per hour for those in the Northern Hemisphere and less than 1 per hour for those in the Southern Hemisphere. These meteors could the source of many of the Perseid subradiants reported at this time of year. Since these meteors are swift like the Perseids, any observer could easily misidentify these meteors as Perseids, especially if they are facing well away from the Perseid radiant.

Like the antihelion radiant both apex radiants 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. There are meteors with a zero inclination that radiate precisely from the apex point on the ecliptic, exactly 90 degrees west of the sun. These meteors are rare though as the earth orbits the sun it "sweeps clean" much of the material that shares the same orbit. Much more debris 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. Meteors from both branches are normally included in the sporadic count but should also be noted in some manner as to which branch of the apex complex they appear to radiate.

The Southern Apex source lies exactly 30 degrees south of its northern counterpart at  03:12 (048) +02. This position lies in northeastern Cetus, 3 degrees northeast of the 3rd magnitude star Menkar (Alpha 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 eastern Cetus, southwestern Taurus or northwestern Eridanus could be a candidate from this source. Rates would be now close to 2 per hour regardless of your location.

The Perseids  will reach maximum activity on Monday and Tuesday next week. The exact day will depend on your location. I would suggest viewing on both mornings as activity will be impressive. Depending on your viewing conditions, rates could range from 25 to 75 Perseid meteors per hour during the early morning hours of Monday and Tuesday. Since not all Perseid activity is bright, those viewing from dark rural locations will witness the most activity. The radiant position for Monday morning lies at 03:04 (046) +58, which is actually located in extreme southwestern Cassiopeia, 5 degrees north of the 3rd magnitude star Gamma Persei. The radiant is best placed, regardless of location, during the last few hours before dawn, when it lies highest in the sky. The radiant is also circumpolar (never sets) for a majority of locations in the Northern Hemisphere. This means that Perseid activity can be seen all night long. Rates seen during the evening hours will be low due to the lower radiant elevation. At the time the northern horizon will block a majority of the Perseid activity. Any Perseid meteors seen during this time though often appear as "earthgrazers", skimming just the very limits of the earth's upper atmosphere. These meteors are quite spectacular as they are extremely long as last for several seconds. I have also noticed a majority of these early Perseids are orange colored. While Perseid activity can be seen in all areas of the sky, I would suggest facing the north to east quadrant with the radiant somewhere in your field of view. Facing this direction will allow you to see the maximum number of Perseid meteors. Due to the high northern declination (celestial latitude) of the radiant, rates seen from the Southern Hemisphere are very low or non-existent. At 59 km/sec., Perseid meteors are swift, often exhibiting persistent trains and bright colors.

The Sporadic rates for the Northern Hemisphere are now increasing. One would expect to see perhaps 7 random meteors per hour during the last hours before dawn from rural observing sites. This estimate and the morning estimate for the Southern Hemisphere does not include the apex meteors listed above. During the evening hours perhaps 3 random meteors can be seen per hour from the Northern Hemisphere. Rates seen from the Southern Hemisphere are now decreasing with perhaps 5 random meteors being seen per hour during the early morning hours and 2 per hour during the evening. 
 
Clear Skies!
Robert Lunsford