时间:Oct 8 – 9 2010


Meteor Activity from Britain

August 2010

August’s popularity for UK astronomers stems from it bringing the end of the midsummer overnight twilight season. It is also “Perseid month”, as this normally strong major meteor shower peaks in mid August, well-timed to take advantage of the school and college summer holidays. This year, the best Perseid rates should fall nicely overnight for British observers if the predictions prove correct, and better still have no Moon! Nocturnal observing in mid August can be very pleasant, with sometimes quite warm temperatures, and plenty of meteors to see in clear, darker skies. August’s weather can be fickle however, on the cusp between late summer and early autumn, thus it can be cold and occasionally frosty by Perseid-time, notably further north in the UK. As always, be prepared before you set out to observe!

Shower of the Month: The Perseids

The Perseid maximum is due between 18:30 UT on August 12 to 07:00 UT on August 13 this year, most likely at some stage from about 23:30 to 02:00 UT. It should produce typical healthy Perseid numbers, with Zenithal Hourly Rates (ZHRs) probably around 100. Unlike in several recent years, no unusual activity is anticipated (though this need not prevent something unexpected happening!). The most plausible peak interval is very favourable for Britain, as the Perseid radiant (illustrated on the accompanying chart) can be usefully-observed from roughly 22h UT onwards here, culminating around 06h, well after dawn. With new Moon on August 10, conditions are ideal to watch whatever takes place – weather permitting. Perseids are swift, often bright, and commonly trained meteors, and those hoping to image some may find their capture rates enhanced by aiming towards And-Cas-Peg-Cyg, aside from recording them passing through some of the more easily-recognisable August constellations that way.

PER Radiant Chart
Lower Perseid activity can be followed away from the peak night for most of the month, ending around August 24 (the shower began back in mid July). Moonless skies will be available for at least part of the night once the Perseid radiant is at a suitable elevation from the UK after August 4 or 5, through to about August 21. Post-midnight observing is preferable, when possible, as the radiant is higher in the sky then, thus observed Perseid rates will be also somewhat higher. Activity builds only slowly however, and if it follows its typical pattern, ZHRs will rise above 10 only late in the first week of the month. Observed rates are usually good to very good for a couple of days before the maximum plus a day or so afterwards, with ZHRs tending to drop steeply once the peak is past. This period of better activity is important to remember if it seems the actual maximum may be clouded-out.

Other Active Showers

The large, loose radiant for the cluster of variable, very weak, near-ecliptic showers we currently call the Antihelion Source (ANT), is a roughly oval area, about 30° in RA (two hours) by 15° in Dec, which lies roughly opposite the Sun in the sky, hence its name. The actual ANT’s radiant centre is about 12° east of this point. Video results suggest it is effectively active virtually throughout the year, producing generally low rates. Radiant centre positions for it are provided in the Section’s Meteor Showers List, and are shown on the accompanying diagram covering July to September. All the radiant drift charts used in these notes are designed to be used in the field with a dim red light once printed out and suitably protected from the cold and damp. Only selected brighter stars and constellation link-lines are given, to help with orientation in the sky. The charts are drawn to a gnomonic projection, so some distortion may be apparent towards the map edges especially, and at times in the radiant tracks too, where the separation between the date markers may not be uniform. The ANT’s radiant motion is not quite constant throughout the year either. During August, the ANT radiant centre moves steadily northwards through Aquarius and just into Pisces, an area that is on-view for most of the night, culminating around 01h UT. In general, one or two meteors per hour is a good level of ANT activity according to the latest International Meteor Organization (IMO) analyses, but ZHRs may be a little better, around 2-3, till mid August. Observations to check on ANT meteors are recommended whenever there is no Moon after mid-evening. Such meteors are of medium speed.

ANT, SDA & CAP Radiant Chart
July-August also brings two of the stronger near-ecliptic meteor showers during the year, which may remain observable as distinct from the ANT. The stronger of these is the δ Aquarids (previously called the “Southern δ Aquarids”, and thus still abbreviated as “SDA” on the accompanying radiant drift chart), which peaked in late July. Declining activity from them should continue till circa August 19, but observed rates from Britain usually peter out some days before this official end-date, thanks to weak activity from a southerly radiant. Even so, Perseid maximum watchers have sometimes spotted the occasional meteor from this source, a few of which can be bright.

The other leading, if much less active, July-August near-ecliptic source, is the α Capricornids (CAP), which also peaked, albeit at a much lower level than the SDA, in late July. As the radiant chart here suggests, the CAP radiant actually overlaps that of the large ANT area’s, and we are not sure if it can be sensibly separated from the ANT by visual observers as a result. Many observers struggled to do so during the moonless 2009 return, certainly, and the difficulties magnify by early August, as in the past, it was unusual to spot any CAP meteors from Britain after the first week in August or so. The shower’s official end date is not until August 15, however. Occasional slow, bright to fireball-class, shower members may still occur until then.

The minor κ Cygnid maximum should happen on August 18, with a waxing gibbous Moon that sets between 22:30-23:30 UT then for British sites, leaving much of the later night with dark skies for watchers. The shower produces wonderfully slow, typically bright, meteors from roughly August 3-25, but its peak ZHRs are just ~3. Occasional fireballs have been seen from it, and these may recur in periodic bursts every 6 or 7 years. In addition, because the shower’s radiant, between Cygnus’ western wing-tip and Draco’s neck (at RA 19h04m, Dec +59° on August 18), is almost overhead for much of the night, observed rates can equal the ZHR in dark, clear skies at the peak. As the radiant lies close to the ecliptic pole in Draco, its daily drift is thought to be almost negligible. The shift in position from August 3 to 25 is from RA 18h50m, Dec +58° to RA 19h16m, Dec +60°.

As August draws to a close, some α Aurigids (AUR) may be spotted, as they probably begin around August 25, lasting into early September. Their activity is from one of a number of known or suspected radiants around Auriga, Cassiopeia, Perseus, Aries and Triangulum during the early autumn, and is probably the annually strongest. The whole series of showers is currently undergoing detailed investigation, after recent IMO video data revealed a number of significant anomalies to the previously-detected sources here, including the September Perseids (SPE) and δ Aurigids (DAU), neither of which is believed active before early September (all three radiants are shown on the accompanying sky map). At least five different near-Auriga showers have been suggested in late August to mid October by the video results, and it is possible none of the radiant positions currently assumed are correct. The α Aurigid maximum seems to fall around August 31 to September 1, producing ZHRs of ~6. The peak is most likely on the latter date this year, with a last quarter Moon in Taurus visible throughout the time the radiant can be properly-observed (after 23h UT). Unexpected α Aurigid outbursts occurred in 1935, 1986 and 1994, which produced ZHRs of ~30-40, though none of these was widely-seen, while the first predicted outburst was seen in moonlit skies in 2007, yielding estimated ZHRs of ~130 briefly. No repeat is expected this year. α Aurigid meteors are swift, like the Perseids, but not usually so bright, though the 2007 event was rich in fireballs.

AUR, SPE & DAU Radiant Chart
Good luck for all your late summer observing, and clear skies!

Compiled by Alastair McBeath, SPA Meteor Section Director
Email: [email protected]
Address in Popular Astronomy

Meteor Showers List, 2010-2011