December 23, 2011
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.
No matter where you live, the first half of December provides some of the best meteor activity of the year. Unfortunately in 2011, the moon will spoil much of this activity as I reaches its full phase on the 10th. 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 14. 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. Much of the activity mentioned above can also be seen from the southern hemisphere. While the sporadic rates are not as strong as those seen from the north, they are stronger than the previous months and heading for a maximum in February. The warm, but short summer nights south of the equator make for some great viewing as long as the moon does not interfere.
During this period the moon reaches it new phase on Saturday December 24th. At this time the moon will be located near the sun and will be invisible at night. As the week progresses the waxing crescent moon will enter the evening sky but will not interfere with meteor observing as it is thin and will set long before the active morning hours arrive. The estimated total hourly rates for evening observers this week is near four 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 fifteen as seen from mid-northern latitudes and eight from mid-southern latitudes. 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 December 24/25. 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 particles produced by comet 2P/Encke are no longer encountering the Earth, the Taurid showers for 2011 are over and we resume reporting activity from the Antihelion (ANT) 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 prograde 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 a 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 07:04 (106) +22. This position lies in central Gemini, four degrees west of the fourth magnitude star Wasat (Delta Geminorum). Antihelion activity may also appear from Monoceros, northeastern Orion, Canis Minor, or Cancer. This radiant is best placed near midnight local standard time (LST), when it lies on the meridian and is located highest in the sky. Rates at this time should be near two per hour as seen from the northern hemisphere and one per hour from south of the equator. With an entry velocity of 30 km/sec., the average Antihelion meteor would be of slow velocity.
The December Leonis Minorids (DLM) are active from a radiant located at 11:04 (166) +28. This position lies in eastern Leo Minor, six degrees northwest of the third magnitude star Zosma (Delta Leonis). These meteors are best seen near 0500 local standard time (LST) when the radiant lies highest above the horizon. This shower peaked on December 20th so current rates would be near two per hour as seen from the northern hemisphere and less than one per hour as seen from south of the equator. At 64 km/sec. the December Leonis Minorids produce mostly swift meteors.
The last of the Ursids (URS) may be seen this weekend from a radiant located at 14:40 (220) +75. This position lies in eastern Ursa Minor, two degrees north of the second magnitude star Kochab (Beta Ursa Minoris). These meteors are best seen during the last dark hour before dawn, when the radiant lies highest above the horizon in a dark sky. This shower is not well seen from the southern hemisphere. Maximum activity is on Friday December 23th, so hourly rates this weekend would probably be less than one. On the morning of maximum, hourly rates of between 5-10 Ursids may be seen. At 33 km/sec. the Ursids produce mostly medium-slow meteors.
As seen from the mid-northern hemisphere (45N) one would expect to see approximately eleven sporadic meteors per hour during the last hour before dawn as seen from rural observing sites. Evening rates would be near three per hour. As seen from the mid-southern hemisphere (45S), morning rates would be near seven per hour as seen from rural observing sites and two 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 07h 04m +22 30 2 1 DLM Dec Leonis Minorids 11h 04m +28 64 2 1 URS Ursids 14h 40m +75 33 <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