British Astronomical Association

Perseids 2004
Observing Prospects

For most observers, the main attraction in late summer 2004 will be the Perseid meteor shower, active from July 23-August 20. The radiant - maximum night position RA 03h 04m Dec +58^o, near the ‘Sword Handle’ at the Perseus/Cassiopeia border - is well-placed high in the sky for UK-based observers, especially later in the night (see table).

One of the most prolific of the dependable, annual showers, the Perseids are always a popular target even for those who aren’t regular meteor observers. Part of the shower’s appeal - apart from the usually-reliable high rates and abundance of bright meteors - is its occurrence during the holiday season at a time when night-time observing conditions are reasonably clement and there is a fair chance of clear skies!

The Perseids, sometimes called ‘The Tears of St Lawrence’ (whose Feast Day falls on August 10), are produced by debris shed by Comet 109P/Swift-Tuttle. In a typical year, activity starts off in late July with a steady trickle of a couple of meteors per hour, slowly increasing during the first week of August. By about August 7-8. observed rates have usually risen to about 10-15/hr, and the core activity period over the next week or so is a particularly rewarding time for meteor observers.

Peak is usually on August 12. In 2004, maximum activity is expected around Aug 12d 07h UT, meaning that the highest observed rates for those in the British Isles may well be found in the pre-dawn hours of August 11-12 (a Wednesday night to Thursday morning) ; the evening of Aug 12-13 should also be very productive, with activity perhaps beginning to tail off noticeably by the following morning.

Peak observed rates are typically of the order of 50-60 Perseids/hr, translating to a sky- and radiant altitude corrected Zenithal Hourly Rate of about 80 in most years. Rates are at about half

this level on the nights to either side of the peak, making Aug 10-11 and 13-14 productive nights for visual observers too!

Activity is still substantial on Aug 14-15 (typically still around 10 Perseids/hr), but thereafter the shower declines quite rapidly, and is over by August 20.

Given clear skies, observers are encouraged to make visual watches by the Meteor Section’s standard methods - outlined on the web page at http://www.britastro.org/meteor - on all possible nights (and not just maximum itself!) from Aug 8-9 onwards. Initially in this period, watches will be restricted to the late evening, but as the Moon retreats further into the morning sky, longer dark-sky stints into the post-midnight interval will prove possible.

Unlike last year’s return, which was largely wiped out by strong moonlight, the peak period of the 2004 Perseids is fairly well placed. The Moon is at Last Quarter on August 7, and the waning crescent is still something of a nuisance, rising around 23h local time up to Aug 10-11. By Aug 11-12, it has moved further into the morning sky, allowing a longer dark sky ‘window’. Its dimishing phase also makes the Moon’s presence, unavoidable later in the night, less of a problem than when gibbous.

As usual, and especially if observing with the waning Moon in the sky, observers are asked to carefully note limiting magnitude conditions during their watches.

The Perseids are well-known for their abundance of bright meteors. Brighter Perseids, particularly, often leave behind brief persistent ionisation trains which result from ionisation on the high atmosphere produced by the meteoroids’ high entry velocity of 60 km/sec. The presence and duration - usually a few seconds , occasionally longer - of ionisation trains should be recorded. In most years, about 25% of Perseids leave behind trains.
 

Photographically, the Perseids have always been regarded as an ideal target thanks to the large proportion of bright meteors. Conventional film remains the medium of choice: digital cameras can’t - yet! - handle the necessary long-duration exposures! Ideally, standard 50 mm or wide-angle 28 mm lens at f/2.8 should be used to obtain a reasonable amount of sky coverage. A field about 20-40 degrees in azimuth from the radiant and about 50 degrees above the horizon is recommended - Cygnus or the Square of Pegasus are good aiming directions. Time exposures (these needn’t be driven) of 10-15 minutes’ duration with a tripod-mounted camera, loaded with ISO 400 film, can capture meteors brighter than magnitude 0.

Enhanced Perseid activity was noted around the time 109P/Swift-Tuttle’s 1992 perihelion, this taking the form of short duration ‘spikes’ of high rates ahead of the regular, established maximum. This enhancement continued to manifest, albeit diminishing in intensity, up to at least 1997. Observations in 2000 and 2002, however, suggest a return to a more ‘normal’ Perseid activity pattern. It should be bore in mind, however, that the shower is not necessarily the dependable ‘Old Faithful’ popularly presented in the American ‘glossies’: some past returns, for poorly understood reasons, have been more active than others, that in 1980 being a good case in point. Indeed, computer modelling of the Perseid meteor stream by Esko Lyytinnen and Tom Van Flandern suggests that enhanced activity comparable to that in 1980 may occur early on August 11-12, accompanied by a short-lived peak of faint Perseid activity: observations are needed to check this theory! Their inherent variability makes the Perseids a still-intriguing observational target, and one which should continue to receive attention.

Observational reports will be welcomed by the BAA Meteor Section Director: Neil Bone, ‘The Harepath’, Mile End Lane, Apuldram, Chichester, West Sussex, PO20 7DZ.