During this period the moon reaches its last quarter phase on Sunday August 21st. At this time the moon will lie ninety degrees west of the sun and will set near midnight local daylight time (LDT) for observers located in the mid-northern latitudes. While the half illuminated moon will be bright, it is much less bright than the full moon, therefore successful meteor observations can be attempted under these conditions. Just make certain that while observing that you keep the moon far from your field of view. This will allow your eyes to keep their dark adaptation and also allow you to see more meteor activity. Next week the waning crescent moon will rise later each morning and will not cause too much interference for meteor observing. The estimated total hourly rates for evening observers this week is near five as seen from the northern hemisphere and two 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 seven 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. Morning rates are slightly reduced due to glare from the moon.
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 August 20/21. These positions do not change greatly day to day so the listed coordinates may be used during this entire period. Most star atlases (available at science stores and planetariums) 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 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 latitude. It must be remembered that meteor activity is rarely seen at the radiant position. Rather they shoot outwards from the radiant so it is best to center your field of view so that the radiant lies at the edge and not the center. Viewing there will allow you to easily trace the path of each meteor back to the radiant (if it is a shower member) or in another direction if it is a sporadic. Meteor activity is not seen from radiants that are located below the horizon. The positions below are listed in a west to east manner in order of right ascension (celestial longitude). The positions listed first are located further west therefore are accessible earlier in the night while those listed further down the list rise later in the night.
The following showers are expected to be active this week:
The August Draconids (AUD) are active from a radiant located at 18:22 (276) +62. This position lies in southeastern Draco, ten degrees north of the second magnitude star Eltanin (Gamma Draconis). Maximum activity occurs on August 21st but rates would be still low, one per hour at best. With an entry velocity of 23 km/sec. most of these meteors will appear to move slowly. The radiant is best placed near 2200 Local Daylight Time (10pm LDT) when it lies highest in the sky. Due to its high northern declination this shower is not well seen from the southern hemisphere.
The wide Antihelion (ANT) radiant is now centered at 22:44 (341) -07. This area of the sky lies in central Aquarius, eight degrees northwest of the third magnitude star Delta Aquarii. 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 eastern Capricornus, Aquarius, western Pisces, or southern Pegasus 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.
The Delta Aquariids (SDA) reached maximum activity on July 30th. The last of its activity for 2011 may be seen this weekend from a radiant located at 23:57 (359) -10. This position lies in central Aquarius, ten degrees northeast of the third magnitude star Delta Aquarii.. Current hourly rates would be less than one per hour no matter your location. The radiant is best placed near 0300 LDT, when it lies highest in the sky. With an entry velocity of 42 km/sec., most activity from this radiant would be of average velocities..
The Perseids (PER) are still active from a radiant located at 03:58 (060) +60. This position lies in southern Camelopardalis, ten degrees northeast of the second magnitudeÂ star Mirfak (Alpha Persei). The radiant is best placed during the last hour before the start of morning twilight when it lies highest in a dark sky. Hourly rates this weekend during the late morning hours would be near two as seen from the northern hemisphere and less than one as seen from south of the equator. With an entry velocity of 61 km/sec., most activity from this radiant would be swift. This week will be the last chance to see Perseid activity for 2011.
As seen from the mid-northern hemisphere (45N) one would expect to see approximately ten sporadic meteors per hour during the last hour before dawn as seen from rural observing sites. Evening rates would be near four per hour. As seen from the mid-southern hemisphere (45S), morning rates would be near four per hour as seen from rural observing sites and one per hour during the evening hours. Locations between these two extremes would see activity between the listed figures. Morning 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.
|SHOWER||DATE OF MAXIMUM ACTIVITY||CELESTIAL POSITION||ENTRY VELOCITY||CULMINATION||HOURLY RATE||CLASS|
|RA (RA in Deg.) DEC||Km/Sec||Local Daylight Time||North-South|
|August Draconids (AUD)||Aug 21||18:22 (276) +62||23||22:00||1 – <1||IV|
|Antihelions (ANT)||–||22:44 (341) -07||30||02:00||1 – 2||II|
|Delta Aquariids (SDA)||Jul 30||23:57 (359) -10||42||03:00||<1 – <1||I|
|Perseids (PER)||Aug 13||03:58 (060) +60||61||06:00||2 – <1||I|