METEOR SHOWERS
Several times a year we can see meteor showers, the swarms of shooting stars that dart from a small area in the sky. These showers can persist for hours or days. On such occasions the earth is passing through a large group of particles moving in ribbonlike fashion along an orbit around the sun. Perspective makes their tracks seem to diverge from a small spot in the sky called the radiant. The shower is named after the constellation in which the radiant appears. Some of the better-known showers are listed in Table 8.3.
Long ago astronomers found that some meteoroids travel in orbits much like those of some comets. They had found a link between meteor showers and the short-period comets (to be discussed in Chapter 9). The particle swarms may be debris left by evaporation and tidal disruption of comets. For example, on the night of November 13, 1833, watchers in the southern part of the Atlantic seaboard were awestruck as over 100,000 shooting stars per hour plummeted from the constellation Leo for 3 hours. The great display was produced when the earth encountered a swarm of meteors orbiting the sun in a period of 33 years and associated with comet Tempel (1866 I). The comet itself has long since vanished leaving the meteor shower as a remainder of its existence. The meteoric displays of 1866, 1899, and 1932 were progressively weaker; then on November 17,1966, a fairly spectacular meteor shower was observed in the southwestern part of the United States.
With the passage of time the meteor streamwhich is made up of conglomerates of fine dust, ices, and ice-covered particles- is strung out along the comet's orbit. This ribbon of particles typically averages about 50,000 kilometers in cross section. Thus the earth must come fairly close to the meteor stream in order for us to see a meteor shower.
