(Editor’s note: The Onizuka Center for International Astronomy Visitor Information Station on Mauna Kea will be open to the public for meteor viewing. Astronomy experts will be on hand to answer questions.)

Special to Hawaii 24/7 by Andrew Cooper | A Darker View

A warm summer evening is the perfect time to be out under a dark sky enjoying the stars. All the better is there is a meteor shower to add to the already spectacular show. On the evening of Aug. 12 the Perseid Meteor Shower will peak.

A typical Perseid shower will produce up to 60-100 meteors an hour, what regular meteor observers would call 60-100ZHR This is what a typical observer would see given a dark sky and good conditions, a metric called zenith hourly rate or ZHR. You can estimate this rate by counting the meteors seen in a shorter period.

If you count all that you see for 10 minutes and multiply by 6 you would have a reasonable estimate of ZHR. As the rate of meteor arrival is irregular it is necessary to count for ten minutes or more to achieve a decent average rate. Conditions such as light pollution or clouds will result in some faint meteors being missed and a lower count.

With new Moon on Aug. 9, there will be no moonlight to obscure the meteors, a beautiful, dark sky to enjoy through the night. The spectacular planetary conjunction in the evening sky will be at it’s peak allowing for a great night of skywatching.

Like many meteor showers, predicting the peak is a complex task fraught with uncertainty. In the case of the Perseids, there are several possible peaks associated with various streams of debris left behind by comet 109P/Swift-Tuttle. For this year there are two predicted peaks, the traditional peak and some ‘enhanced filamentary’ peaks that occur a little and after before the traditional peak.

The traditional peak will occur from 6 p.m. Aug. 12 to 7 a.m. Aug. 13 (08:00HST to 21:00HST on Aug 12 for Hawaii). While the enhanced peaks might occur from 9hUT on the 12th to 14hUT on the 13th (23:00HST on the 11th to 04:00 on the 13th for Hawaii). Emphasis should be placed on the traditional stream for this year, with possibilities of 100ZHR seeming reasonable.

Thus much of the predicted peak falls during daylight hours on the 12th for observers on the North American West Coast or across the Pacific. This leaves the dilemma of observing the night of the 11th or the night of the 12th. The timing does slightly favor the night of the 12th, but either may be a good choice for western hemisphere observers.

Either side of the peak the Perseids provide a normal rate of 15-20ZHR for several days, providing a decent showing of meteors for observers who are not lucky enough to catch a peak.

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 10 IMO INFO(2-09) 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 into the outer Solar System, and theory predicts that such outburst rates should dwindle as the comet to Earth distance increases.

However, 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. Activity seemed to be roughly normal in 2005 and 2006 (though the latter was badly moonlit).

Recent work implies only the ‘traditional’ peak is liable to recur in 2010 (most likely near the nodal crossing time given in the table above), but observers should be aware of these additional timings as possibilities, and plan their efforts accordingly, just in case! – International Meteor Organization website notes on the 2010 Perseids

A warm summer night, spectacular planetary conjunctions, a moonless dark sky, the peak of a reliable meteor shower… The 12th of August looks to be one of the best nights that our sky can provide. If you set one night aside to get out and enjoy, mark this one on the calendar.

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