Mark your calendar for August 12 to see one of the most reliable meteor showers.
By Lowell and Kaye Christie, F47246
Several times a year we rearrange our sleeping schedules to watch one of nature’s annual fireworks displays. Just like clockwork, during the second week of August, the early morning sky lights up with the castoff particles left by the passage of a comet.
Comets are the vagabonds of the solar system. Many of them travel in an ellipse, coming close to the sun at one end of their orbit, and moving toward the outer solar system at the other end. Although hundreds of comets pass through the inner solar system each year, most are so small that they go unnoticed. But occasionally a big comet travels by.
These larger comets attract attention, not only by their brightness, but by the tail that can extend far across the sky. Comets are made up of dust, rock, ice, and frozen gases. At least they are frozen during the outer portion of the comet’s journey. As the comet approaches the inner solar system, the sun’s heat causes the frozen gases to vaporize. And as the gases stream out from the comet, they carry away particles of dust, which the sun’s radiation pressure and the solar wind form into what we see as a comet’s tail. This pressure from the sun is what makes the tail point away from the sun rather than follow the path of the comet.
It’s these dust particles, left behind by the comet Swift-Tuttle, that create North America’s most popular meteor shower of the year each August. The Swift-Tuttle comet was discovered in 1862 and reappeared in 1992 on another trip around the sun. This 130-year cycle has been traced back and associated with reported meteor showers over the course of more than 2,000 years.
The Swift-Tuttle comet is one of many short-period comets, meaning those having an orbital period of less than 200 years. We won’t see this comet again in our lifetime, but it is the largest (about six miles wide) known to repeatedly pass near Earth. And large comets leave large dust trails behind.
Each year, between August 9 and August 13, the Earth passes through the densest portion of the Swift-Tuttle comet dust trail. The display is known as the Perseid meteor shower, because the light show seems to originate from the constellation Perseus.
This year the best viewing opportunities for this meteor shower will come in the early morning hours of Tuesday, August 12. You may see shooting stars (the popular name for meteors) on any night and at any hour, but there is definitely a “best” time to find meteors. It’s all based on the motion of the Earth and the amount of illumination that exists from the sun and the moon.
Since the streaks of light caused by meteors occur when tiny bits of space debris hit the Earth’s atmosphere, the leading edge of the Earth will always produce a better light show than the trailing side. It’s sort of like bugs hitting a moving vehicle. The windshield may be covered with these unlucky insects, but you seldom have to scrape them off the rear window.
In the Earth’s path around the sun, the leading edge is that portion of the planet where dawn is just breaking. That means there are actually more meteor trails just as the sun is coming up. Of course, you can’t see them because of all the light. So, it turns out that the best time for meteor watching is usually between one and three hours after midnight, local time.
This year the moon is even going to cooperate. Although there will be a full moon (and a partial eclipse) four days later, on August 16, the moon will set at about 2:00 a.m. on August 12, leaving several hours of very dark sky during the height of the meteor shower. The darker the sky, the more meteors you can see, and some astronomers predict that in excess of 100 meteors per hour will be visible this year.
Entering the atmosphere at a speed of approximately 37 miles per second (133,200 miles per hour), these small, mostly sand-grain-sized particles compress the air in front of them. The compression heats the air, which in turn heats the particle to approximately 3,000 degrees, causing it to glow.
Meteors become visible at a height of approximately 60 miles, and most break up at an altitude of between five and seven miles. The color they display on their path across the sky depends upon the composition of the particle: iron produces yellow; sodium, orange-yellow; magnesium, blue-green; and silicon, red. If you are really lucky, you may see a fireball “” a meteor that is brighter than any of the stars or planets (brighter than Venus) that may be visible in the sky.
Watching a meteor shower doesn’t require any special equipment. In fact, binoculars or a telescope just gets in the way. Of more importance is a comfortable place to sit (reclining lawn chairs are excellent), something warm to wear, and, depending upon your location, perhaps some insect repellent. If you have some star charts or are taking pictures, bring along a red-filtered flashlight so you can see things without ruining your night vision.
Even camera equipment doesn’t have to be very sophisticated. It does require that you be able to control long exposure times “” anywhere from 15 seconds to 10 minutes. Just use a normal lens (not a telephoto), and set the camera on a tripod. Start the exposure and watch the sky. When you see a good meteor streak, end the exposure.
You’ll achieve better results if you test for exposure length a few days before the actual meteor shower. The amount of ambient light at your viewing location will determine the maximum exposure time you should use. If you are far from city lights, you may be able to use a full 10 minutes or longer and still have a black sky. If a significant amount of light pollution exists, the maximum exposure times must be shorter.
Don’t worry too much about where to look. Although the beginning point of the meteor trails seems to be the constellation Perseus, you’ll probably see longer, brighter trails by watching the darkest portion of the night sky.
Major meteor showers such as the Perseids extend over multiple nights, so if you can’t make it on August 12 for the most concentrated show, take a look on the 11th or the 13th. Get to bed early the night before, and set your alarm for the early hours of the morning. Who says fireworks only occur in July?