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Perseids

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Perseids
Photo: Sophie DesRosiers

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 2019

General circumstances won’t be favourable for the Perseids in 2019—blame the moon, which will be full on August 15! The “traditional” peak of activity is expected between 10 p.m. on August 12 and 11 a.m. on August 13, when Earth passes closest to the dust stream from comet Swift-Tuttle. (Some meteor specialists forecast that Earth will cross another filament of comet dust around 10 p.m. on the 12th, which could cause a brief but detectable spike of activity.) The nearly full moon will therefore compromise observations of meteors during peak night. Only the very brightest meteors (which are few in number) will be able to pierce that luminous veil.

One option is to observe the Perseids during the few nights that lead up to that maximum, after the moon has set (around 3 a.m. during the night of August 11-12, around 4 a.m. during the night of August 12-13) and the start of dawn. But that window is quite short, because the first light of dawn can be detected as early as 4:30 a.m. at this time of year.

Observing a meteor shower a few nights ahead of its maximum also implies an important penalty: the number of potentially visible meteors drops by half for each 24-hour period that separates us from the time of peak activity.

On the other hand, the latter part of the night is the time when the radiant reaches its optimal height and the Perseids put on their best show due to a better geometry, so all is not completely lost.

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. Prepare your wish list, just in case, and keep it handy!

Astronomical conditions will be a bit easier in 2020, with the waning moon rising after midnight, leaving dark conditions during the evening hours for viewing the Perseids.

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