December 25, 2009
The Meteor Activity Outlook is a weekly summary of expected meteor activity written by Robert Lunsford, Operations Manager of the American Meteor Society and contributor to this blog. The original unedited version of this week’s Meteor Activity Outlook can be found at the American Meteor Society’s site.
No matter where you live, the first half of December provides some of the best meteor activity of the year. In the northern hemisphere the sporadic rates are still strong plus you can also count on strong activity from the Geminids, which peak on December 13. There are also several minor radiants that add a few meteors each hour. All of these centers of activity are located high in the sky during the early morning hours this time of year. Unfortunately this year the bright moon spoils the show during the first week of the month. During the second week of December the moon will pass its last quarter phase and will not be such a nuisance .
As seen from the southern hemisphere the sporadic rates are increasing toward a January maximum. Shower rates are also good but the Geminids suffer a bit from the lower elevation seen from southern locations. Still with the warmer weather now occurring south of the equator, December is a great time to view celestial fireworks.
During this period the moon reaches its full phase on Thursday December 31st. On that date the moon lies opposite the sun and is in the sky the entire night. This weekend the waxing gibbous moon sets during the early morning hours allowing a couple of hours of viewing in dark conditions before the onset of morning twilight. The estimated total hourly rates for evening observers this week is ~3 no matter your location. For morning observers the estimated total hourly rates should be ~20 from the northern hemisphere and ~15 as seen from the southern hemisphere. 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. Rates are reduced this week due to moonlight.
The radiant positions and rates listed below are exact for Saturday night/Sunday morning December 26/27. 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.
Now that the activity from particles produced by comet 2P/Encke has ceased encountering the Earth, the Taurid showers for 2009 are over and we resume reporting activity from the Antihelion radiant. This is not a true radiant but rather activity caused by the Earth’s motion through space. As the Earth revolves around the sun it encounters particles orbiting in a pro-grade motion that are approaching their perihelion point. They all appear to be radiating from an area near the opposition point of the sun, hence the name Antihelion. These were once recorded as separate showers throughout the year but it is now suggested to bin them into their category separate from true showers and sporadics. This radiant is a very large oval some thirty degrees wide by fifteen degrees high. Activity from this radiant can appear from more than one constellation. The position listed here is for the center of the radiant which is currently located at 07:12 (108) +22. This position lies in central Gemini, 2 degrees west of the fourth magnitude star Delta Geminorum. Since the radiant is so large, Antihelion activity may also appear from eastern Taurus, northeastern Orion, western Cancer, or southern Auriga. This radiant is best placed near 0100 local standard time (LST) when it lies on the meridian and is highest in the sky. Rates at this time should be ~3 per hour as seen from the northern hemisphere and ~2 per hour from south of the equator. With an entry velocity of 30 km/sec., the average Antihelion meteor would be of slow speed.
Alpha Hydrids (AHY)
Activity from the Alpha Hydrids (AHY) begin to appear on the morning of December 30th. This shower reaches maximum activity on the 31st from a radiant located at 08:24 (126) -08. This position lies in extreme western Hydra, 15 degrees west of the second magnitude star Alphard (Alpha Hydrae). These meteors are best seen near 0200 LST when the radiant lies highest above the horizon. Due to the intense moonlight present at maximum, hourly rates from this shower will be < 1 per hour no matter your location. At 45 km/sec. the Alpha Hydrids produce meteors of medium velocity.
December Leonis Minorids (DLM)
The December Leonis Minorids (DLM) are active from a radiant located at 11:10 (168) +27. This position lies in a blank area of the sky where the borders of Leo Minor, Leo, and Ursa Major meet. The nearest bright stars are Nu and Xi Ursae Majoris which lie 5 degrees to the northeast. These meteors are best seen near 0500 LST when the radiant lies highest above the horizon. This shower peaked on December 20th so current rates would be ~2 per hour as seen from the northern hemisphere and < 1 per hour as seen from south of the equator. At 64 km/sec. the December Leonis Minorids produce mostly swift meteors.
As seen from the mid-northern hemisphere (45N) one would expect to see ~15 Sporadic meteors per hour during the last hour before dawn as seen from rural observing sites. Evening rates would be ~2 per hour. As seen from the mid-southern hemisphere (45S), morning rates would be ~12 per hour as seen from rural observing sites and ~2 per hour during the evening hours. Locations between these two extremes would see activity between the listed figures. Evening rates are reduced this week due to moonlight.
The table below presents a condensed version of the expected activity this week. Rates and positions are exact for Saturday night/Sunday morning but may be used all week long.
Shower Name RA DEC Vel Rates km/s NH SH ANT Antihelions 07h 12m +22 30 3 2 AHY Alpha Hydrids 08h 24m -08 45 <1 <1 DLE Dec Leonis Minorids 11h 10m +27 64 2 <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