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 2021
The outlook for this year’s Perseids is very positive. In 2021, peak activity is expected between 3 and 6 p.m. (Eastern) on August 12—late afternoon for observers in Eastern North America. However, remaining uncertainties in the exact moment Earth comes closest to the densest parts of the meteor stream mean this window could in fact extend from 10 a.m. to 11 p.m. For observers in Eastern Canada, the nights of August 11-12 and 12-13 will be equally good for observing the Perseids.
Once the date and time of the meteor shower’s peak are established, the moon becomes the most important factor to consider. The presence of a bright moon creates light pollution that overpowers the faintest meteors, thereby decreasing the total number that can be seen. This year, the situation happens to be very favourable: the Perseids reach their peak activity when the moon is just a thin crescent (new moon is on August 8, first quarter occurs on the 15th), and it sets before 10:30 p.m. This means the sky will be moon-free during the second half of the night: that’s when the radiant of the Perseids, located in the constellation Perseus, climbs higher in the sky, thereby increasing the number of visible meteors—until the encroaching light of dawn puts an end to the show. During the two nights closest to peak activity, one can expect to count up to 50 Perseids per hour under clear, dark skies, free from light pollution.
Indeed, because the moon won’t be a limiting factor, it’ll be worth the extra effort to flee the light pollution from the cities and seek the darkest possible skies to enjoy the show to its fullest. And if the weather doesn’t cooperate for peak nights, don’t despair: conditions will still be interesting for two or three nights before and after August 12.
Keep in mind that the Perseids are active (though at much lower rates) from the end of July through the third week of August. Even though the number of potential meteors drops by half for each 24-hour period that separates us from the time of peak activity, you’ll probably get a chance to spot a few shooting stars under the dark, moonless skies of early August. Is your wish list ready?
In 2022, the Perseids will coincide with the full moon, which will most certainly spoil the show. All the more reason to make the best of this year’s display!