Perseids

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Perseids (PER)

Active:	July 17 — August 24;
Maximum:	August 12 11h30m — 14h00m UT (λsol = 140°0 — 140°1) but see text
ZHR =	100
Radiant:	α = 49° δ = +58°
Radiant drift:	see Table 6
vinf =	59 km/s; r = 2.6
TFC:	α = 019° δ = +38° and α = 348° δ = +74° before 2h local time
	α = 043° δ = +38° and α = 073° δ = +66° after 2h local time (β > 20° N)
IFC:	α = 300° δ = +40° and α = 000° δ = +20° or
	α = 240° δ = +70° (β > 20° N)

The Perseids were one of the most exciting and dynamic meteor showers during the 1990s, with outbursts at a new primary maximum producing EZHRs of 400+ in 1991 and 1992. Rates from this peak decreased to ~ 100 — 120 by the late 1990s, and in 2000, it first failed to appear. This was not unexpected, as the outbursts and the primary maximum (which was not noticed before 1988), were associated with the perihelion passage of the Perseids' parent comet 109P/Swift-Tuttle in 1992. The comet's orbital period is about 130 years, so it is now receding back into the outer Solar System, and theory predicts that such outburst rates should dwindle as the comet to Earth distance increases. However, additional predictions suggested 2004 — 2006 might bring a return of enhanced rates ahead of the usual maximum, and in 2004 a short, strong peak happened close to that anticipated pre-peak time, though activity seemed to be roughly normal in 2005, and the 2006 return was badly moonlit and poorly-observed.

An average annual shift of +0°05 in the solar longitude of the `old' primary peak had been deduced from 1991 — 99 data, and allowing for this could give a possible recurrence time around 16h40m UT on August 12 (λsol = 140°21), if so a few hours after the most probable maximum, that of the 'traditional' peak always previously found, given above. The timing of a tertiary peak, not seen in IMO data since 1999, would be around λsol = 140°4, 21h30m UT on August 12. While recent observations imply only the 'traditional' peak is liable to recur in 2008, observers should be aware of these additional timings as possibilities, and plan their efforts accordingly, just in case!

The waxing gibbous Moon will be setting between local midnight and 01h30m on August 12/13 for the mid-northern latitudes best-placed to follow the shower (moonset is progressively earlier for places further north), leaving some dark skies to cover whatever happens. For these same locations, the Perseid radiant is viably observable from 22h — 23h local time onwards, gaining altitude throughout the night, so circumstances overall are quite favourable. The 'traditional' maximum timing would be best-viewed from places in and around the northern Pacific Ocean, including the extreme west of North America west as far as extreme eastern Japan and China, assuming it happens as expected.

All forms of observing can be usefully carried out on the shower. For example, video data has been used in the latest IMO analyses to clarify and refine the radiant position for the shower — and to confirm that the occasional visual suspicions the radiant may be multiple are almost certainly only illusory. The only negative aspect to the shower is the impossibility of covering it from the bulk of the southern hemisphere.