Mark your calendars! Next Saturday night, August 11, the Perseid meteor shower will reach its peak, displaying up to 100 meteors each hour in the early Sunday morning hours. This year will be a great opportunity for seeing the Perseids, as the Moon will be in its crescent phase and rises at 1 a.m., remaining low in the sky during the peak activity around 3 to 4 a.m.
To observe the Perseids, you need not endure staying up till 3 a.m. This is a massive meteor shower producing bright shooting stars from 10 p.m. onward at rates of one every 2-5 minutes. And with the Moon out of the picture in the evening so that we can see fainter meteors, the rate may be somewhat higher. When you go outside around 9 to 11 p.m. to look for these beautiful shooting stars, sit comfortably facing east and scan the skies. You will not be disappointed if you are patient.
Of course clouds, or just haze and fog, can cripple the ability to see the shower. But the Perseid meteor stream is so vast that the shower actually lasts well over a month, from mid-July through August 22nd, though the rate of meteors is substantially lower away from the peak on the night of the 11th. Certainly if next Friday or Sunday night is clear at least a dozen meteors will be observable in the evening of those nights, should Saturday night prove to be cloudy.
Meteor showers are caused by comets when their orbits cross or nearly-cross Earth’s orbit. A comet is composed of frozen gas, water ice, and rock – a “dirty snowball," as described by astronomer Fred Whipple in 1950. As these objects approach the sun, the ice melts creating a stream of gas and dust that forms the comet’s brilliant tail.
Long after the comet has passed round the sun and is traveling back out into the frozen regions of the outer solar system, a trail of sand and dust persists in its orbit. Even after the main body of the comet has dwindled to a size which we cannot easily see from Earth, this trail will continue in the original orbit.
In the very rare case that a comet’s orbit comes very near Earth’s orbit, the sand particles in the comet’s orbit will be attracted by Earth’s gravity as the Earth travels through that area of its orbit each year, and will fall toward the Earth at great speeds. As the particles of sand enter the atmosphere, they heat to many thousands of degrees, glow intensely, and vaporize dozens of miles above the ground. What we see is a bright point of light suddenly streaking across the sky, growing faint and vanishing. It is one of the most fascinating natural sights we can experience in our lives, and first-time observers feel the excitement deep within their souls.
Because the point of intersection between the comet’s orbit and Earth’s orbit is a fixed location in Earth’s orbit, the meteor shower associated with a particular comet peaks on the same day of every year. For the same reason, the direction from which the shower hits Earth is the same each year. From Earth, the meteors in a certain shower appear to come from a common center in the sky, and the constellation in which that center is found gives its name to the meteor shower. Hence, the Perseid meteor shower next weekend has a center (scientifically, a “radiant”) located in the constellation Perseus.
There are over 50 known meteor showers occurring throughout the year; however, many consist of very dim meteors (very small bits of sand), or the rate of the shower is very slow – maybe 5 an hour. Among the 50 or more showers, 6 peak at a time of the year when the radiant is in the daytime sky. These showers have never been seen by eye – we only recently have discovered them using radar. There are 9 major meteor showers each year easily visible from the northern hemisphere.
Other showers are highly variable in their rates, as the particles in their orbits are not evenly distributed, but are clumped into groups separated by large distances. An extreme example of this is the Leonid meteor shower, which peaks in mid-November. Usually producing a mere 15 bright meteors an hour, the Leonids have a history of producing meteor storms. During the storm of 1833, an estimated 100,000 meteors per hour were seen!! That is a rate of about 30 per second, which must have looked like the largest (silent) fireworks display ever seen.
The Perseid meteor shower is the most reliable shower over a long history (the Geminids, occurring in December, have recently been cited as more reliable, but being outside in Connecticut after midnight on December 12th is a whole lot different than staying out in August!).
The Perseids are associated with the comet currently designated “Swift-Tuttle”, named after the two gentlemen who independently discovered it in 1862. It is more properly stated to have been re-discovered by Swift and Tuttle, as the comet has a very regular orbit of 133 years. Historian astronomers have found observations of the comet back to the year 69BC in Chinese annals. The first known appearance of the Perseids was in 36AD, again as recorded in a Chinese annal.
The most recent passage of Swift-Tuttle was in 1992, and was followed by a strong surge in Perseid meteor rates in August, 1993, with 300-500 occurring per hour. Since that year the shower has become more modest, though we can still expect 60-100 meteors per hour at the peak.
I have always used the date of the Perseids to mark the start of my fall children’s course in astronomy. This fall’s course will begin on August 16th. There are still a couple spaces left in the class, should you want to join us. Please visit www.turnerclasses.com for more information.