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 towards 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 make up the tail that creates the most popular meteor shower of the year each August. Swift-Tuttle was discovered in 1862, and re-appeared in 1992 for another trip around the sun. This 130 year cycle has been traced backwards and associated with reported meteor showers extending over 2,000 years into the past.
The Swift-Tuttle is one of many short-period comets; those having an orbital period of less than 200 years. We may not see it again, but it is the largest comet (about six miles across) known to repeatedly pass near the earth. And large comets leave large dust trails behind.
Each year, between August 9th and 13th, the earth passes through the densest portion of the dust trail left by Swift-Tuttle. 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 will come in the early morning hours of August 12th. 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 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 car. 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. 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 going to cooperate. Although there will be a fairly bright moon (about 70% illumination) on the evening of August 11th, on the 12th the moon will set about 1:30 AM, 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 predictions for this year are as many as 150-200 meteors per hour. We can hope they are right.
Entering the atmosphere at a speed of about 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, and the result is meteor dust which may remain in the atmosphere for months. There are estimates that over 200 tons of meteor dust is added to the earth’s surface each day.
Meteors become visible at a height of about sixty miles, and most break up at an altitude of between five and seven miles. Their color as they turn to dust 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. This is 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 get 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 repellant. 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 being able to control long exposure times, anywhere from fifteen seconds to ten minutes. Just use a normal lens (not a telephoto), and set the camera up on a tripod. Start the exposure and watch the sky. When you see a good meteor streak, end the exposure.
You’ll get 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 ten minutes or longer and still have a black sky, although that length will create short star trails as the earth rotates. If there is much light pollution the maximum 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.
The major meteor showers, such as the Perseids, extend over multiple nights, so if you can’t make it on August 12th 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 wee hours of the morning. Who says fireworks only occur in July?