April 30, 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.
May is a fairly slow month for meteor activity. The Eta Aquariids are very active the first two weeks of the month then fade as the month progresses. The only other showers active this month are the weak Nu Cygnids, the Eta Lyrids, and the Antihelion radiant. These will add only 1-2 meteors per hour to the total count. Sporadic rates are low but steady as seen from the mid-northern hemisphere (45 N). Sporadic rates seen from the mid-southern hemisphere (45 S) are strong but beginning to decline.
During this period the moon reaches its new phase on Tuesday May 3. At this time the moon will lie near the sun and will not be visible at night. Toward the end of this period a waxing crescent moon will enter the evening sky but will not interfere with observing. The estimated total hourly rates for evening observers this week is near three as seen from the northern hemisphere and four as seen from the southern hemisphere. For morning observers the estimated total hourly rates should be near fourteen from the northern hemisphere and twenty 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.
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 April 30/May 1. 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 15:32 (233) -19. This area of the sky lies in eastern Libra, eight degrees northwest of the second magnitude star Dschubba (Delta Scorpii). This radiant is best placed near 0200 local daylight time (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 extreme eastern Hydra, Libra, northern Lupus, or western Scorpius could be a candidate for this shower. Rates at this time should be near two per hour as seen from the northern hemisphere and three 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.
The team of Sirko Molau and Juergen Rendtel have discovered a weak radiant active at this time in the constellation of Cygnus. Video data shows that the Nu Cygnids (NCY) are active from April 18th through May 7th. Maximum activity occurred on the morning of April 20th. The radiant is currently located at 21:40 (325) +47. This position lies in northeastern Cygnus, just north of the faint star known as Rho Cygni. The radiant is best placed during the last dark hour before dawn, when it lies highest above the horizon in a dark sky. At 42km/sec. the Nu Cygnids would produce meteors of medium velocity. Expected rates are less than one per hour. Due to the northerly declination of the radiant, these meteors are not well seen from south of the equator.
The Eta Aquariids (ETA) are particles from Halley’s Comet, produced in Earth-crossing orbits many centuries ago. We pass closest to these orbits from May 5 through the 9th. During this period the Eta Aquariids are at their best, capable of producing ZHR’s of sixty. The actual visible rates are most often less than half this figure due to the low altitude of the radiant at dawn. Observed hourly rates at maximum normally vary from zero at 60 degrees north latitude to 30 near the equator and back down to near zero again in Antarctica, where the radiant elevation is very low. Hourly rates this weekend are anywhere from zero to five per hour depending on your latitude and observing conditions. Rates will increase significantly as the week progresses as we approach the May 7 maximum. The radiant is currently located at 22:20 (335) -03. This area of the sky is located in northern Aquarius, just south of the fourth magnitude star Gamma Aquarii. The best time to view this activity is during the hour before the start of morning twilight, when the radiant lies highest in a dark sky. No matter your location these meteors will appear from the eastern sky and shoot in all directions. If the radiant has sufficient altitude Eta Aquariid meteors can also be seen shooting down toward the eastern horizon. With an entry velocity of 67 kilometers per second, a majority of these meteors will appear to move swiftly with a high percentage of the bright meteors leaving persistent trains. Surprisingly, this shower produces very few fireballs.
As seen from the mid-northern hemisphere (45N) one would expect to see approximately nine 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 fourteen 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.
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 15h 32m -19 30 2 3 NCY Nu Cygnids 21h 40m +47 42 <1 <1 ETA Eta Aquariids 22h 20m -03 67 2 3 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