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Perseids

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Perseids
Photo: Sophie DesRosiers
The Perseids

They return year after year, and for many of us, they spell holidays and warm summer nights. But who — or what — are they? The Perseid meteors, of course!

Each year around mid-August, Earth passes close to the orbit of periodic comet 109P/Swift-Tuttle, whose wake is peppered with billions of dust particles that give rise to the famous Perseid meteor shower. However, the quality of the celestial show varies dramatically from year to year, mainly as a function of Earth’s distance from the densest parts of the particle stream, but also as a function of the Moon’s presence.

The Perseids in 2020

In 2020, we’ll have to contend with mixed astronomical circumstances to observe this very popular meteor shower. As always, the presence of moonlight is the limiting factor. During the nights closest to the Perseids’ peak, the waning Moon will rise past midnight, thus leaving the first half of the night free of interference.

This year the “traditional” peak of activity is expected between 4 a.m. and 5 p.m. on August 12. Some computer models also predict that Earth could cross another filament of comet dust around 10 p.m. on the 12th, which might bring about a brief spike of activity—but this would occur around sunrise here in Quebec.

In mid-August, the radiant of the Perseids, located in the constellation Perseus, appears above the northeast horizon during the evening and climbs higher in the sky with each passing hour until dawn: in principle, the number of visible meteors increases through the night because the geometry becomes more favourable. However, the Moon eventually enters the scene to spoil the show: last quarter occurs on August 11, and its light will impede the view of meteors during the last hours of the night— just when the Perseids would be at their best. We’ll have to settle for a short window of dark sky during the hours before the peak of the shower, between 10 p.m. and 1 a.m. on the night of August 11 to 12. Despite these less-than-ideal conditions, one could expect to count about 30 Perseids per hour under clear, dark skies free from light pollution.

Because the Moon rises a bit later each night, there’s a net gain of dark time for the nights that follow the peak. But there’s a catch: the number of potential meteors drops by half for each 24-hour period that separates us from the time of peak activity.

Even though the observing circumstances are far from ideal for viewing the Perseids this year, keep in mind that this shower is active from the end of July through the third week of August. You’ll probably get a chance to spot a few shooting stars under dark, moonless skies — but one has to expect much lower numbers of meteors when observing farther from the period of peak activity. Keep your wish list it handy, just in case!

In 2021, the Moon will be just a thin crescent, so we can hope for nearly ideal astronomical conditions for viewing the Perseids.

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