If you spend half an hour under a dark sky this week, you are very likely to see a meteor. Thursday morning at 2 a.m. EDT, the Perseid meteor shower reaches its peak. Remember that the date changes at midnight, so we’re talking about Wednesday night.
Meteors, popularly called “shooting stars,” are caused by small pieces of interplanetary matter known as meteoroids. These are smaller than asteroids, ranging in size from 40 inches (1 metre) in size down to objects the size of a grain of sand. These are heated to fluorescence when they encounter the Earth’s atmosphere, and observers on the ground see a rapidly moving bright point of light.
If the meteor is large enough, it may leave a train of glowing particles in its wake, which usually fades within a few seconds. Meteors do not have tails; this is because of confusion with comets, which move far more slowly and do have tails. A comet is visible for weeks or months, whereas a meteor is gone in a second or two.
Meteoroids are distributed throughout the Solar System, so it’s possible to see a meteor on any night in the year; these are called “sporadic meteors.” But some areas have a greater concentration of meteoroids, usually moving in the same orbit as a comet or asteroid which has gradually lost material along its orbit. The Perseids are associated with Comet Swift-Tuttle, which was discovered in 1862 and last observed in 1992. It will next be close to Earth in 2126.
When the Earth passes through a concentration of meteoroids, we get a greater number of meteors, what we call a “meteor shower.” These meteors may appear anywhere in the sky, but if you trace their paths backwards, they will all appear to come from a point in the sky known as the “radiant” of the shower.
These showers are usually named for the constellation in which the radiants lie, so the shower this week is called “the Perseids” after the constellation Perseus, which lies between Cassiopeia and the bright star Capella.
If you look straight at the radiant of a meteor shower, you will see the meteors appearing to come towards you and pass by on either side. Because of perspective, the meteors will tend to be short in length and brief in time. If you look well off to the side of the radiant, the meteors you’ll see will be longer. Because of the direction the Earth faces moving through space, you will usually see far more meteors after midnight local time (1 a.m. Daylight Saving Time).
With the radiant of the Perseids in the northeastern sky, the best directions to look are southeast and northwest. Better still is to lie on your back and look straight overhead.
Many people misunderstand what is meant by a meteor shower. It is a slightly higher frequency of meteors coming from a particular radiating point. On a night without a shower, you will see perhaps five meteors an hour if you watch continuously. At the peak of the Perseid shower under very dark country skies, you should expect to see 90 meteors an hour if you concentrate on looking straight overhead. That’s about three meteors every two minutes.
This year’s Perseid shower is particularly good because the peak falls just after midnight local time for observers in the eastern half of North America, and the moon is close to new and doesn’t rise until after 5 a.m.
The darker your skies, the more faint meteors you are likely to see. Under light-polluted skies, you will see far fewer meteors, as only the brightest ones can be seen.
It’s important to dress warmly, since even in summer the temperature can drop quite low after midnight. I usually wear winter clothes and lie back on a deck chair so that I can see close to overhead. It’s important to spend at least 20 minutes under a dark sky so that your eyes get fully adapted to the dark. Don’t bother with a telescope or binoculars as these take in too little sky. Your best equipment to see meteors is the plain old human eye.
An interesting project is to equip yourself with a star chart of the direction you plan to observe, and then plot the meteors you see on that chart using a red flashlight so that you don’t lose your dark adaptation. If you observe continuously for more than an hour, you should be able to confirm that the radiant is in Perseus.
The Perseids also provide a great opportunity for photography. Point your camera towards the northeast, and leave the shutter open for at least a minute. Do this several times in succession and you’re almost certain to catch a few Perseids.
If you’re clouded out on the night of the peak, try again over the next few nights, as the Perseids have quite broad peak.