The author is an astronomer, science communicator and full professor of science education at UQAM.
They say that three things are inevitable in life: death, taxes and the return of the Perseids in mid-August.
For many, this annual meteor shower has become a highlight of their summer vacation, and some do not hesitate to get as far away as possible from major centers — and the light pollution that affects their skies — in order to make the most of the spectacle. With peaks of between fifty and one hundred meteors per hour predicted on the night of August 11-12, we can’t blame them!
The Perseids are as regular as a metronome because they correspond to the annual passage of the Earth through a cloud of dust and debris left there by the repeated passages of the periodic comet Swift-Tuttle around the Sun.
Every 133 years, when Swift-Tuttle approaches the Sun, the heat of our star sublimates the surface of this comet, which sows behind it along its orbit grains of dust of all sizes, like Tom Thumb sowed pieces of bread.
Since the trajectory of this comet crosses the orbit of the Earth at the point where our planet passes around mid-August, the Earth is then bombarded from all sides by particles that enter the atmosphere at more than 200,000 km/h. At such a speed, even an object the size of a grain of sand strongly compresses the air in front of it and brings it to temperatures of several thousand degrees.
Due to the heat, the smallest fragments vaporize completely at high altitudes, while particles the size of a fist or larger usually survive their passage through the atmosphere and reach the ground, where they become valuable meteorites.
Supersonic
Contrary to popular belief, it is not the molten dust grain that we see as a shooting star as it crosses the sky, but rather a column of superheated and ionized air that emits light along its trajectory.
Smaller objects produce faint, short, and fleeting trails, while larger meteors can create trails that remain visible for several seconds. Even the entry of an even larger object into the atmosphere can create a lightning-like flash and a sonic boom that can be heard for tens of kilometers around.
The year 2024 promises to be a very good year for observing the Perseids. The main factor to consider is the phase of the Moon: if the Moon is full, its light will obscure most of the shooting stars, irreparably spoiling the show. Fortunately, this year, the Moon will be in first quarter on August 12, which will bring it to the western side of the sky in the evening and below the horizon well before midnight.
This is a good thing, because meteors are generally considered to be more numerous and brighter after midnight. At that time, observers find themselves on the side of the Earth facing the particle bombardment, which increases their relative speed relative to the atmosphere and, therefore, the brightness and length of their trails.
The first-quarter Moon will also be far from the constellation Perseus, which rises in the northeast around 9 p.m. in mid-August. Because of a perspective effect similar to what happens when you drive your car at night with your headlights on through a snowfall, the Perseid meteors will all appear to come from the constellation Perseus. This phenomenon is what gives the shower its name.
Patience
To observe the Perseids, you will need a lounge chair, a few blankets, a thermos filled with the hot beverage of your choice… and that’s it! Set up your position facing east in a spot where the horizon is clear, ideally away from any source of light, and let your eyes acclimate to the ambient darkness.
After a few minutes, you should see your first shooting stars appear. As the night progresses, turn south to follow Perseus’ slow movement across the sky.
Finally, be patient: the exact moment when the Earth will cross the densest part of the dust cloud from the Swift-Tuttle comet is particularly difficult to predict and varies from year to year. If you have the opportunity, it may be interesting to also observe the sky on the night of August 10-11 and 12-13: you could have some nice surprises. All that’s left to do is hope for clear skies… and not too much smoke from the forest fires raging in Western Canada!