On the evening of November 17, many Williams students gathered in the dark in the hope of viewing the Leonid meteor shower. Spectators were disappointed, as Williamstown is a very poor location from which to view the shower, but some meteors were visible after midnight.
The hopes of astronomers were particularly high for this Leonid shower because it is the height of the 33-year cycle of Leonid activity. The name of the meteor shower comes from the constellation in the sky where the radiant is located. In this case it was the constellation Leo, hence the named Leonids. Since the Earth passes through the broken-up comet’s orbit once a year, the Leonid shower occurs every year on the same date. The next shower, the Geminids, resulting from the debris of a different comet, will occur on Dec. 14.
To better view the shower, Field Memorial Professor of Astronomy Jay Pasachoff went to El Saler, Spain, one of the best places from which to view the shower. During the height of the shower, Pasachoff reported a fantastic show.
“We saw a magnificent shower, by far the best I have ever seen, but not quite the meteor storm that was at the top of our hopes. It began about an hour before the predicted maximum, and few meteors had been visible earlier in the evening,” Pasachoff said. “It is easy to see why people in locations, like the U.S., where Leo was not up in the sky at the 9:08 EST time of the peak hardly saw any Leonid meteors at all.”
The importance of time and location for viewing a shower can be understood by examining what goes on during a shower. A meteor shower is caused when the Earth passes through the orbit of a comet that has broken up and left behind many tiny grains of matter, which are small examples of the interplanetary pieces of matter known as meteoroids.
When we pass through this clump of debris, the meteoroids, which are no bigger than grains of sand, enter our atmosphere at speeds averaging around 30 km/sec (the speed of the Earth relative to the Sun), although the speed is variable from meteoroid to meteoroid. At heights near 100 km, the friction of the meteoroid going through the atmosphere causes it to heat up and vaporize. Meteoroids that burn up in this way are called meteors.
The variable size and composition of the meteoroid can cause different colors and levels of brightness to be seen in the streak of light in the sky that results from the burning. The most brilliant ones may show green and blue light and streak across the sky for several seconds. The brightest meteors are known as “fireballs.”
The meteors can be seen all over the sky, because they are equally likely to enter the Earth’s atmosphere to the north, south, east, or west of the location from where you happen to be observing. However, their direction will be such that they appear to be coming from a single point known as the radiant. Just as parallel lines converge in the distance, the meteors, which appear to be traveling parallel to each other as we move through them, appear to have a single common source in the distance.
Spain was a good viewing place because it was on the side of the Earth that was facing the direction of the Earth’s velocity as it plowed into the densest regions of the comet debris. By thinking about the Earth’s orbit and direction of spin, it is easy to see that the orientation along the direction of Earth’s velocity occurs everywhere on the Earth only once a day, in the early morning hours.
Since the time when we passed through the densest region of meteors was 9:08 p.m. EST, (when Leo is not even present in our sky) and 3:08 a.m. in Spain, Spain had a much better show.
Pasachoff reports: “We saw Leonids at the rate of 12 a minute, one every five seconds, for much of the time in the crucial half hour centered on about the predicted time of 3:08 local time. Sometimes we saw a meteor each second, but when I did 60-second counts, I didn’t go above 12 for the direction of sky I was looking at. 12 a min is 720 per hour, and correcting for the fact I was looking in only one direction, the total hourly rate must have been about 2000. Few fireballs appeared, but sometimes meteors appeared simultaneously in double, almost parallel streaks across Orion, as the radiant in Leo continued to rise in the eastern sky over the sea.”