With the arrival of May, observers in the tropical areas of the world are enjoying good views of the Eta Aquarids during the morning hours. The Antihelion radiant moves from eastern Libra, through northern Scorpius and into southern Ophiuchus during May. These areas of the sky are south of the celestial equator, favoring observers watching from south of the equator. Northern observers still suffer with low sporadic rates, especially during the evening hours. This situation will not improve until mid-July, when sporadic rates dramatically rise for northern observers. The overall meteor activity is much like April, possibly slightly better in the south and slightly worse in the north. The most favorable time to view meteor activity this month would be during the first week of the month, when the Eta Aquarids are near maximum activity.
During this period the moon will reach its last quarter phase on Saturday May 20. At this time the moon will rise near 0100 local daylight saving time. Observations the remainder of the morning can be successfully undertaken as long as you keep the moon out of your field of view. As the week progresses the waning crescent moon will rise later in the morning, becoming less of a nuisance. The estimated total hourly rates for evening observers this week is near two for observers in the northern hemisphere and three for those south of the equator. For morning observers the estimated total hourly rates should be near eight for those in the northern hemisphere and eleven for those south of the equator. These rates assume that you are watching from rural areas away from all sources of light pollution. 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 during the morning hours due to lunar interference.
The radiant positions listed below are exact for Saturday night/Sunday morning May 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. 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 Tau Herculids (THE) are produced by comet 73P/Schwassmann-Wachmann , which is currently making headlines as it passes close to the Earth. This shower was once included in many shower lists. As our understanding of the shower dynamics increased with this shower it became apparent that this shower is visible only on rare occasions when the Earth passes through the trails of debris produce by this comet. Therefore the Tau Herculids have been removed from most lists and are only rarely discussed. With the close approach of 73P/Schwassmann-Wachmann to the Earth in 2006, there exists the small possibility that some activity may be seen from this shower during the next two weeks. The Earth passes closest to the largest fragment of this comet near 20:00 Universal Time on May 31. This is not a particularly close approach so very low numbers, if any at all, are expected to appear then, or during the remainder of this encounter. Using the IMO’s shower parameters listed at: http://www.imo.net/files/data/vmdb/vmdbrad.txt, I have calculated that the radiant for this shower will be located near 15:16 (229) +40 this weekend. This area of the sky is located in northeastern Bootes, three degrees east of the fourth magnitude star Beta Bootis. The actual radiant of shower members may differ significantly due to the fragmentation of this comet and the very slow entry velocity. The best time to view possible activity is near midnight when the radiant lies highest in the sky. This shower is well placed for viewing in the northern hemisphere as it passes through the zenith for those located at 40N latitude. At extreme northern latitudes the length of the night is shortened appreciably, limiting the time one can view activity. As one progresses southward the length of night increases but the radiant altitude decreases. South of the equator the radiant altitude becomes a major factor, reducing the possibility of seeing any activity from this source. The radiant does not rise above the horizon south of 50S latitude. With an entry velocity of only 15 kilometers per second, these meteors will appear to move slowly, helping to aid in their identification. A great majority of these meteors will possess an angular velocity less than 5 degrees per second with the highest possible angular velocity certainly being less than 10 degrees per second. Please report any positive sightings to the AMS, the IMO News list, or Meteorobs as soon as possible after your observation.
The Antihelion (ANT) radiant is now centered at 16:52 (253) -23. This area of the sky is located in southern Ophiuchus, seven degrees northeast of the orange first magnitude star Antares (Alpha Scorpii). This radiant is best placed near 0200 local daylight time when it lies on the meridian and is highest in the sky. Rates at this time should be near two per hour for those north of the equator and three per hour south of the equator. With an entry velocity of 30 km/sec., the average Antihelion meteor would be of medium-slow speed.
The Eta Aquarids (ETA) are particles from Halley’s Comet, which last passed through the inner solar system in 1986. Even though this is now twenty years ago, material is still encountered every year in late April and throughout most of May. We passed closest to Halley’s orbit on May 6. At that time Eta Aquarid rates can reach 30 shower members per hour as seen from the northern tropical areas southward. Current rates would only be 1-2 per hour at best. The radiant is located at 23:24 (351) +05. This area of the sky is located in western Pisces, two degrees northeast of the fourth magnitude star Gamma Piscium. This lies within the “Circlet” of Pisces. The best time to view this activity is just before the start of morning twilight, when the radiant lies highest in a dark sky. With an entry velocity of 66 kilometers per second, a majority of these meteors will appear to move swiftly.
The Sporadic rates for the Northern Hemisphere are now nearing their annual minimum activity. One would expect to see perhaps four random meteors per hour during the last hours before dawn from rural observing sites. During the evening hours perhaps two random meteors can be seen per hour. Sporadic rates seen from the southern hemisphere are now increasing toward a secondary maximum in July. One would see approximately six random meteors per hour during the late morning hours and three per hour during the evening. Morning rates are reduced due to moonlight.