Under a beautiful nighttime sky, it’s not unusual to notice out of the corner of your eye the stream of light of a shooting star. A tiny pebble drifting through the solar system enters the Earth’s orbit – wrong place, wrong time – and poof!, it vaporizes in the upper atmosphere. Thus the pebble is extinguished. But the atmospheric gases along its trajectory will produce a final extended flash of light, reemitting the kinetic energy of the cosmic collision in the form of light and heat. Quick, make a wish!
Every hour, at all times of the year, in perfect conditions, the glow of about ten of these unpredictable collisions can be observed; they’re what’s called sporadic shooting stars (or meteors).
But, from the end of July to the third week of August, the Earth describes part of its orbit in a zone filled with debris from the comet 109P/Swift-Tuttle. The movement of the Earth, combined with the common trajectory of the particles, creates the illusion that these shooting stars all come from the same region in the sky, like snowflakes that seem to come from in front of the car when we drive through a storm. In the case of the famous August shower of shooting stars, that point in the sky is located in the constellation of Perseus – and therefore we talk about Perseids.
The 2018 Perseids
This year, the harvest promises to be special for Perseids hunters, especially in North America. Although the Perseids stretch over several weeks, the height of activity is normally the period that gives meaning to the expression shooting star “shower.” This year’s peak is expected the evening and night of August 12 and 13 between 4 p.m. and 4 a.m. (eastern daylight time). As many as 90 shooting stars per hour might be observed in an unobstructed sky without light pollution.
But the activity isn’t restricted to that one night, and thanks to the new moon coming on August 11, the week preceding the maximum as well as the several days following also offer ideal conditions.
On all those nights, activity generally begins quietly with the beginnings of darkness, but speeds up from the end of dusk until dawn. For a better understanding of why, let’s go back to the analogy of the car driving in a snowstorm. If you look behind you, fewer flakes are striking the back window than the windshield. The rotation of the Earth in the course of the night leads us to face the direction of the Earth’s movement – to turn towards the windshield, as it were. Let’s wind up this parallel between celestial mechanics and automobile by just remembering that, when we observe the sky, it’s preferable to turn off the headlights and let our nighttime vision drive us into the most complete darkness.