On a clear night, we can often glimpse a shooting star streaking across the sky. After sailing through the solar system, this bit of rock crosses Earth’s orbit in the wrong place at the wrong time. Poof! It vaporizes as it plunges through the upper atmosphere. Though the rock burns out, the atmospheric gases along its trajectory produce a fleeting glow, re-emitting the kinetic energy from the cosmic collision as light and heat. Quick, make a wish!
On any night of the year, under ideal conditions, we can spot the glow from a dozen of these unpredictable collisions every hour. These shooting stars (a.k.a. meteors) are called sporadic.
But from late July to the third week of August, Earth travels through a part of its orbit strewn with the debris left behind by repeated passages of Comet 109P/Swift-Tuttle. The number of meteors jumps considerably, resulting in a true meteor shower. The movement of Earth, combined with the common trajectory of the particles of rock and dust, creates the illusion that these shooting stars all originate from the same spot in the sky (just as snowflakes seem to radiate from a point in front of your car as you drive through a snowstorm). In the case of the famous meteor shower in August, this point of origin lies in the constellation Perseus, and so the shower is known as the Perseids.
This year, the Perseids should provide a good show, especially for meteor watchers in North America. Though Perseid activity lasts several weeks, it’s the time of peak activity that gives all its meaning to the expression “meteor shower.” This year, the peak is expected on the evening and night of August 12–13 between 4 p.m. and 4 a.m. EDT. In a crystal-clear sky free of light pollution, you might spot up to 90 shooting stars an hour. With moderate light pollution, you’ll see half as many, which is still quite good.
Yet activity isn’t limited to just one night. Thanks to the new Moon on August 11, good conditions also prevail the week before the peak and several days after it. The number of visible meteors drops, however, the further from the peak we go.
On all these nights, activity usually begins slowly as darkness falls, but it speeds up from dusk to dawn. To better understand why, let’s go back to the example of the car in a snowstorm. You’ll notice that more snowflakes strike the front windshield than the rear window. Earth’s rotation on its axis during the night causes us gradually to face the direction of Earth’s orbital motion—in other words, to turn toward the front windshield. To conclude this parallel between celestial mechanics and auto mechanics, remember that when observing the sky, you should always turn off your headlights and let your night vision guide you through total darkness. Just don’t try this in an actual car!
The planets
A lovely string of planets will brighten up your evenings in August. Venus, Jupiter, Saturn and Mars (along with Neptune and Uranus for eagle-eyed observers) share the ecliptic almost equally. Given the geometry between the tilt of Earth and the tilt of the ecliptic, the latter unfortunately remains at very low altitude in the evening sky in late summer. To glimpse these “wandering stars,” you need a very unobstructed southern horizon.
Venus shines brighter than the other planets at twilight but gradually sinks in the sky and sets earlier and earlier. Though it reaches its greatest elongation relative to the Sun on August 17, the planet slips below the ecliptic throughout the month. As a result, dazzling Venus shows up quite low in the west-southwest, only about 10 degrees above the horizon, before it finally sets about 75 minutes after the Sun. Also note that the crescent Moon is only 6 degrees from a half Venus on August 14.
Jupiter is the brightest heavenly body in the southwest, symmetrically positioned between the first-magnitude stars Antares and Spica. Use a telescope to observe several double-shadow transits of Jovian moons on the giant planet’s cloudy surface. Among these, the transit on August 16 between 8 p.m. and 10:15 p.m. is the easiest to see from Quebec. Our Moon’s proximity (7 degrees) on the 16th and 17th adds an interesting perspective to this celestial ballet.
Farther south, Saturn ends its retrograde motion in the heart of the Milky Way and the constellation Sagittarius. In late August, the ringed giant lies less than 2 degrees from the Lagoon and Trifid nebulae. The Moon also draws near (less than 4 degrees) on the evening of August 20, providing a good opportunity to identify the planet.
Finally, like a red lantern, Mars shines low on the southeastern horizon at sunset. After an opposition that brought it to within 58 million kilometres of us on July 28, the red planet slowly falls behind Earth and loses a bit of its lustre and altitude over the month. Still, you can easily pick out the red planet on the border of the constellations Capricornus and Sagittarius since it outshines all planets and stars in the south, even Jupiter.
Clear skies!
