July 12, 2011 1 Comment
The following is a slightly edited version of Bob Lunsford’s excellent weekly summary of meteor activity. The original version can be found at the American Meteor Society’s site.
Meteor season finally gets going in July for the northern hemisphere. The first half of the month will be much like June. After the 15th though, both sporadic and shower rates increase significantly. For observers in the southern hemisphere, sporadic rates will be falling but the overall activity will increase with the arrival of the Delta Aquariids.
During this period the moon reaches its full phase on Friday July 15th. At this time the moon will lie opposite the sun and will remain in the sky all night long. This weekend the waxing gibbous moon will set shortly after midnight and will allow most viewers a couple of hours of dark skies before the start of morning twilight. The estimated total hourly rates for evening observers this week is near two as seen from the northern hemisphere and three as seen from the southern hemisphere. For morning observers the estimated total hourly rates should be near nine from the northern hemisphere and twelve as seen from south of the equator. 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. Evening rates are reduced slightly this week due to moonlight.
The radiant (the area of the sky where meteors appear to shoot from) positions and rates listed below are exact for Saturday night/Sunday morning July 9/10. These positions do not change greatly day to day so the listed coordinates may be used during this entire period.
The following showers are expected to be active this week:
The wide Antihelion (ANT) radiant is now centered at 20:00 (300) -19. This area of the sky lies in northeastern Sagittarius, five degrees southwest of the third magnitude star Dabih (Beta Capricorni). This radiant is best placed near 0200 LDT, when it lies on the meridian and is located highest in the sky. Due to the large size of this radiant, any meteor radiating from Sagittarius, Scutum, southern Aquila, or western Capricornus could be a candidate for this shower. Rates at this time should be near one per hour as seen from the northern hemisphere and two per hour as seen from south of the equator. With an entry velocity of 30 km/sec., the average Antihelion meteor would be of medium-slow speed.
Studies by Sirko Molau and Juergen Rendtel of the IMO’s video data has revealed an active radiant located in Andromeda this time of year. The c-Andromedids (CAN) are active from July 4-16, with maximum activity occurring on the 12th. The radiant position is currently located at 02:00 (030) +48. This area of the sky lies in northeastern Andromeda, six degrees north of the famous second magnitude double star Almach (Gamma Andromedae). This radiant is best placed during the last dark hour before dawn, when it lies highest above the horizon in a dark sky. Rates at this time should be less than one no matter your location. With an entry velocity of 59 km/sec., the average c-Andromedid meteor would be of swift speed.
Thanks to studies by Sirko Molau and Juergen Rendtel of the IMO’s video data, the July Pegasids (JPE) have returned to the ranks of an active shower. Recent visual observations of these meteors have been scarce to the point of having the shower removed from the IMO’s list. Video data shows a much longer activity period than previously thought, July 7 through the 29th with maximum occurring on the 10th. The radiant position is also further east than previously thought. It is currently located at 23:08 (347) +11. This area of the sky lies in southern Pegasus, six degrees south of the second magnitude star Markab (Alpha Pegasi). This radiant is best placed during the last dark hour before dawn, when it lies highest above the horizon in a dark sky. Rates at this time should be less than one no matter your location. With an entry velocity of 68 km/sec., the average July Pegasid meteor would be of swift speed.
As seen from the mid-northern hemisphere (45N) one would expect to see approximately seven sporadic meteors per hour during the last hour before dawn as seen from rural observing sites. Evening rates would be near two per hour. As seen from the mid-southern hemisphere (45S), morning rates would be near ten per hour as seen from rural observing sites and three per hour during the evening hours. Locations between these two extremes would see activity between the listed figures. Morning rates are reduced due to moonlight.
The list below presents a condensed version of the expected activity this week. Rates and positions are exact for Saturday night/Sunday morning.
Shower Name RA DEC Vel Rates km/s NH SH ANT Antihelions 20h 00m -19 30 1 2 CAN c-Andromedids 02h 00m +48 59 <1 <1 JPE July Pegasids 23h 08m +11 68 <1 <1 RA - Right Ascension DEC - Declination Vel - Velocity relative to Earth (in km per sec) Rates - Rate of visible meteors per hour from a dark site NH - Northern Hemisphere SH - Southern Hemisphere