British Astronomical Association
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British Astronomical Association
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Meteor Section |
In roughly one year out of three (a sixth of a Saros cycle), lunar phasing is such that the Moon's glare seriously interferes with the most active parts of the major annual meteor showers, including the Perseids. Such circumstances unfortunately obtain in 2003; indeed, things could hardly be worse, with Full Moon on August 12, the very date of Perseid maximum! There are, however, some other worthwhile observing opportunities during late July and into early August. Observers are encouraged to make best-possible use of clear skies in the interval up to about August 8-9 during which there will still be reasonable amounts of darkness after moonset. The main meteor showers active at this time will be the Delta Aquarids, Alpha Capricornids, Iota Aquarids, Capricornids and Alpha Cygnids. Early Perseids will also, of course, be in evidence. The combined activity of these showers should make watches reasonably rewarding: observers might like to take advantage of any good skies over the opening weekend of August (Friday 1-2, Saturday 2-3 and Sunday 3-4) as opportunities to clock up two- to three-hour stints of useful dark-sky time. Given that Perseid maximum is a moonlit washout, these may prove the most productive nights of the summer for many!
Delta Aquarids
Active July 15 - August 20.
Radiants N RA 23h 04m Dec +02o S RA 22h 36m Dec -17o
This is the most productive of the several showers with radiants to the
southeast of the Square of Pegasus in late July and early August. Activity comes
from two radiants - a Southern branch near the star Delta Aqr, and a Northern
one near the 'Water Jar' asterism. These have separate maxima: the Southern
Delta Aquarids peak on July 28-29, the Northern Delta Aquarids on August 5-6.
Respective peak corrected Zenithal Hourly Rates (ZHRs) are roughly 20 and 10.
Given the low altitudes of the radiants (especially the Southern) from the
latitudes of the British Isles, observed rates are ever likely to be
spectacular, but close to the peaks the Delta Aquarids can yield perhaps half a
dozen or more medium-paced meteors per hour as seen from a dark observing site.
Absence of moonlight favours the Southern Delta Aquarid peak on Monday-Tuesday
July 28-29, just a day after New Moon. The Northern Delta Aquarids contend in
late evening with a waxing gibbous Moon, setting around midnight UT at their
peak The Delta Aquarid radiants are highest in the southeastern sky in the early
morning hours, and watches after midnight will generally be most productive.
Alpha Capricornids
Active July 15 - August 20.
Radiant RA 20h 36m Dec -10o
With a peak ZHR of only 5, the Alpha Capricornids are one of several summertime
showers which produce little more than a steady trickle of activity. What makes
this shower stand out are its occasional long, slow and sometime very bright
meteors. As a result of the low altitude of the radiant (near Alpha Cap) from
the British Isles, meteors from the shower are sometimes seen on near-grazing
atmospheric trajectories, traversing long paths up from the southern horizon
towards the zenith. Normally, peak - such as it is! - occurs around August 2, at
which time the Moon will not interfere. The radiant is highest in the southern
sky just after midnight UT.
Iota Aquarids
Active July and August.
Radiants N 22h 04m Dec -06o S 22h 10m Dec -15o
Another shower with a double radiant structure, the Iota Aquarids are somewhat
less active than the Delta Aquarids, with corrected peak ZHR less than 10. The
shower has a peak around August 6-7, and produces swift, generally faint
meteors. The radiants are about 10 degrees to the southwest of those for the
Delta Aquarids.
Capricornids
Active July and August.
Radiant RA 21h 00m Dec -15o
A poorly-defined shower with a radiant just southeast of that for the Alpha
Capricornids. Peak ZHR is around 5, and the slow meteors sometimes bright. Most
active in late July.
Piscis Australids
Active July 15 - August 20.
Radiant RA 22h 40m Dec -30o
A weak shower, with a radiant near the first-magnitude star Fomalhaut, low in UK
skies. At best - in late July - perhaps a single Piscis Australid might be
recorded in an hour's observing.
Alpha Cygnids
Active July and August.
Radiant RA 21h 00m Dec +48o
There are some doubts as to the reality of this shower, with an apparently
stationary radiant near Deneb (most shower radiants drift eastwards relative to
the background stars by about a degree from night to night, as a results of
Earth's orbital motion). Rates of one or two per hour are recorded on most
nights through July and August.
Perseids
Active July 23 - August 20.
Radiant RA 03h 04m Dec +58o
In most years, the Perseids are the outstanding astronomical event of the
summer, producing high observed rates close to maximum on August 12-13. The 2002
return was well covered, with peak ZHR about 80, corresponding to a meteor per
minute for observers at dark sites late on maximum night when the radiant (near
the northern end of Perseus at the border with Cassiopeia) was high in the
northeastern sky. The Perseid radiant is actually circumpolar, but relatively
low in the early evening. While the maximum will be essentially lost to
moonlight - only the brightest meteors will cut through the glare - the Perseids'
early, rising activity phase up to August 8-9 should still be rewarding for
observation. By this date, observers using the Moon-free last hour or so of the
night might hope to log up to 15-20 Perseids per hour. The Perseids are justly
renowned for their abundance of bright, fast meteors. Bright Perseids often
leave behind persistent ionisation trains, which may take several seconds to
fade from view after the meteor itself has gone. Thanks to their abundance of
bright meteors, the Perseids are a favourite target for photographers close to
maximum. It may still be worth attempting photography despite the lower rates in
the nights when the Moon will be out of the way. Simple meteor photography can
be carried out using undriven time exposures (10-15 minutes) with a 50 mm or
wide-angle 28 mm lens at f/2 to f/2.8, aimed in the direction of Cygnus or the
Square of Pegasus. Fast films (ISO 400-800) are preferable.
While moonlight will rob observers of the most active parts of the Perseids, the activity of the several radiant below the Square of Pegasus, coupled with early Perseids and healthy random background sporadic rates should add up to a reasonable 'consolation prize' in early August. Regular meteor observer will need no reminding that showers other than the 'big four' (Perseids, Leonids, Geminids and Quadrantids) are every bit as worthy of attention. Newcomers may be pleasantly surprised at how active the nights of late July and early August - even before the Perseids have fully 'kicked in' - can prove! The BAA Meteor Section will. of course, welcome reports of watches made by the standard methods (outlined under VISUAL OBSERVING NOTES on these pages) during late July and early August. With so many showers active simultaneously, observers should take care with meteor identification; it is well worth spending some time becoming familiar with radiant positions with the help of a good naked-eye atlas such as Norton's (19th Edition). Observers will be relived to learn that lunar phasing in 2004 is a good deal more favourable for the most productive major shower maxima.
Page last updated: 27/08/2003