The Perseids Meteor Shower - (August 2009)

Summer vacation is over now for most of us and it´s time to get back into the routine of autumn; work, school, extracurricular activities, etc. Another part of my family´s routine is the peak of the Perseid meteor shower, which takes place on the night of August 12-13. Fortunately our bright summer night skies become dark enough by then to see the best meteor shower of the year. If the night sky had returned to autumnal darkness just two weeks later we would have missed this annual August spectacle entirely.

Originally named "the tears of Saint Lawrence" (since they happened at about the same time as that suffering saint's feast day, which is 10 August), they are now known as the Perseids since they appear to originate from the constellation of Perseus, the Champion. Some of you may recall the legend that it was Perseus who rescued Andromeda, the daughter of Queen Cassiopeia and King Cepheus. She had been chained to a cliff at the edge of the ocean and was about to be sacrificed to a sea monster named Cetus. Heroic Perseus swooped in, riding on the back of his winged steed, Pegasus, and rescued her by turning Cetus into a giant rock with the help of the magical powers of the decapitated head of the Medusa Gorgon.

Comets cause meteor showers and the Perseids are no exception as they are associated with Comet Swift-Tuttle. When a comet goes around the sun, it sheds off small particles of dust and stone along its orbit. When Earth collides with such material it enters our atmosphere and burns up from friction. Most meteors that you see are typically about the size of a sand grain, while brighter ones are pea-sized bits. The Italian astronomer, Giovanni Schiaperelli (1835 — 1910, of Mars channel fame), was the first to discover this relationship between comets and meteor showers, but it wasn´t until 1866. It did not take too long before this connection was found between other comets and meteor showers. Astronomers´s predictions of meteor showers, based on a better understanding of how and where comets leave debris along their orbits, have become quite sophisticated. The Perseids of 2004 were predicted to encounter a “fresh" clump of such material from Comet Swift-Tuttle and amazingly enough there was a large number of meteors (a so-called “storm") seen that year.

This month's diagram shows the location of the Perseids's radiant; the point in the night sky where the meteors appear to come from. Note the directions along the horizon so you know where to look and that the time shown is midnight. Meteors seen after midnight are always brighter and more numerous than those seen before. Why? Before midnight Earth is turned in such a way that meteors have to catch up with it, so they do not impact the atmosphere with as high a speed. After midnight Earth is turned so that it runs into meteors; the combined speed of both our planet and that of the meteors causes them to travel faster through the atmosphere and are therefore brighter and more numerous.
Image: the Perseids 2009

You can expect to see about 60 to as many as 90 meteors per hour during the maximum of the Perseids. By comparison on a typical night when there is no meteor shower you may see five to ten per hour.

What about the moon this year? The Waning Gibbous Moon rises at 21.16 on 12 August, crosses at its highest elevation above the horizon (due south) at 05.58 on the morning of 13 August before finally setting at 15.04 that afternoon. Which means that the Moon, even though past Full and getting smaller every night, will be visible the whole evening.

Begin your back-from-vacation activities with a “night out." Give it a try as the Perseids are some of the brightest meteors to be seen during the entire year. Make it a family outing (bring along warm clothes, lawn chairs to sit on, blankets and something hot for everyone to drink!), and as I said, in the case of my family, it's become a traditional part of our autumn routine.

/Keep looking up!
Tom