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The Perseids are one of the most prolific and best-known of the meteor showers and can be seen in August each year, with the maximum activity on or around 12 August. One advantage of the Perseids is that it happens in the warmer weather of August, which makes it ideal anyone interested in seeing their first meteor. You can see a meteor at any time of year but, for a day or so around the date of maximum, there may be a ten times better chance of seeing one.
Because of the effect of perspective meteors in a shower appear to come from a particular area of the sky. We call this area the radiant. The Perseid radiant is in the constellation of Perseus, just below the familiar ‘W’ of the constellation of Cassiopeia. In August this can be seen reasonably high in the north-eastern sky later in the evening. Unfortunately this year there will be a fairly bright moon not far away which will wash out the fainter meteors; August 12th 2010 will be ideal with no moon at all to interfere.
Diagram showing moon as well as radiant for the 12th
The best direction to look for Perseids is generally in the direction of the Square of Pegasus and the Plough (or Big Dipper). This year the moon will be nearer the Square of Pegasus so the region above the Plough might be more productive. It all depends on your local conditions – if it is very hazy and there is light pollution you may only be able to see meteors overhead anyway.
Some meteoroids originate from material ejected by comets as they pass through the inner solar system. The Perseids were the first meteor shower to be connected with a comet when astronomer Giovanni Schiaparelli noted the similarity between their orbit and that of Comet Swift-Tuttle which was observed in 1862. Swift-Tuttle orbits the Sun about every 135 years. Its last perihelion (the point where it is closest to the Sun) was in 1992 and the next is in 2126. The debris trail is thickest nearer the comet, so the Perseids were highly active in the early 1990s with several hundred meteors per hour at the annual maximum. As the Earth crosses Swift-Tuttle’s orbit it sweeps up some of the debris released by the comet on previous orbits. This burns up in the atmosphere as a meteor, but the particles in the Perseids are much too small to reach the ground as a meteorite.
Even in quiet years the Perseid rate can exceed 100 meteors per hour at maximum, as seen from a dark site and with the radiant high in the sky. The rate tends to be highest towards dawn when the observer is on the side of the Earth moving directly into the trail of comet debris. Perseid meteors tend to be brighter than most, so the shower is ideal for anyone wishing to see their first ’shooting star’. An observer watching the early morning skies will almost certainly see several over a period of, say, 30 minutes.
The density of the trail does vary so the number of meteors can change in a matter of hours. For instance, this year there are suggestions that there could be two peaks of activity. For that reason it is always worth watching a day or so either side of maximum. It also pays to treat predicted rates with caution because they refer to ideal conditions; a hazy summer evening with moonlight and light pollution can obscure the fainter meteors and dramatically reduce the numbers you will see. Fortunately the Perseids tend to have quite a lot of bright meteors so there is still a very good chance of seeing some this year.
The Perseids are Coming
Splat! There goes another bug on the windshield.
Anyone who's ever driven down a country lane has seen it happen. A fast moving car, a cloud of multiplying insects, and a big disgusting mess.
The next time that happens to you, instead of feeling grossed out, try thinking of the experience as an astronomy lesson. Your car is Earth. The bugs are tiny flakes of comet dust. The carnage on your windshield ... it's a meteor shower!
Earth, like a speeding car, races around the Sun sweeping up everything in its path. There are no insects in space, but there are plenty of meteoroids, little flakes of dust from comets and asteroids. They hit Earth's atmosphere and--splat!--they disintegrate as fiery streaks of light called meteors.
This week lots of meteors will appear over Earth's northern hemisphere when our planet plows through a swarm of dust shed by periodic comet Swift-Tuttle. It's the annual Perseid meteor shower, which peaks on August 11th and 12th.
Just as bugs tend to accumulate on the front windshield of a car, Perseids accumulate on the front windshield of Earth.
Earth has a windshield? It's the atmosphere, which protects us from solar wind and comet dust much as a car's windshield protects passengers from wind, rain and bugs. Earth's front windshield is the early morning sky. Earth circles the Sun dawn-side first, scooping up whatever lies on that side of the planet. That's why it's usually best to look for Perseids just before dawn.
A good time to see Perseids this year is before dawn on Wednesday morning, August 12th, when Earth's front windshield is overhead.
Side windows, the ones to the left and right of passengers in cars, are good, too. Zooming down a bug-infested lane, side windows don't intercept many insects, but the ones they do gather are worth examining. Bugs that strike side windows do so at a shallow angle, leaving long and colorful streaks.
This also happens to meteors. When the constellation Perseus (the source of the Perseids) hangs low near the horizon, meteors streaming from Perseus will skim the the top of Earth's atmosphere, much like a bug skimming the side window of an automobile. Astronomers call these meteors "Earthgrazers." They tend to be long and colorful.
Look for Perseid Earthgrazers on Tuesday night, Aug. 11th, between 9:00 and 11:00 p.m. local time.
This year, evening Earthgrazers on August 11th could be the best part of the show, because the Moon and its interfering glare will be below the horizon in early evening. Dark skies = good meteor watching. Later, when Earth's "front windshield" rises, the Moon will rise with it, reducing the number of Perseids you'll be able to see before sunrise on August 12th.
Earthgrazers don't come in large numbers. The special geometry required to produce them keeps counts low, but even one or two is enough. A breathtaking Earthgrazer is the sort of meteor you're likely to remember for years.
Best of all, there's no gooey residue.
Dr. Tony Phillips
Science@NASA
