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Many—perhaps most—of the meteors that strike Earth are associated with specific comets . Some of these periodic comets still return to our view; others have long ago fallen apart, leaving only a trail of dust behind them. The dust particles from a given comet retain approximately the orbit of their parent, continuing to move together through space but spreading out over the orbit with time. When Earth, in its travels around the Sun, crosses such a dust stream, we see a sudden burst of meteor activity that usually lasts several hours; such an event is called a meteor shower .
The dust particles and pebbles that produce meteor showers are moving together in space before they encounter Earth. Thus, as we look up at the atmosphere, their parallel paths seem to come toward us from a place in the sky called the radiant . This is the direction in space from which the meteor stream seems to be diverging, just as long railroad tracks seem to diverge from a single spot on the horizon ( [link] ). Meteor showers are often designated by the constellation in which this radiant is located: for example, the Perseid meteor shower has its radiant in the constellation of Perseus. But you are likely to see shower meteors anywhere in the sky, not just in the constellation of the radiant. The characteristics of some of the more famous meteor showers are summarized in [link] .
| Major Annual Meteor Showers | |||
|---|---|---|---|
| Shower Name | Date of Maximum | Associated Parent Object | Comet’s Period (years) |
| Quadrantid | January 3–4 | 2003EH (asteroid) | — |
| Lyrid | April 22 | Comet Thatcher | 415 |
| Eta Aquarid | May 4–5 | Comet Halley | 76 |
| Delta Aquarid | July 29–30 | Comet Machholz | — |
| Perseid | August 11–12 | Comet Swift-Tuttle | 133 |
| Orionid | October 20–21 | Comet Halley | 76 |
| Southern Taurid | October 31 | Comet Encke | 3 |
| Leonid | November 16–17 | Comet Tempel-Tuttle | 33 |
| Geminid | December 13 | Phaethon (asteroid) | 1.4 |
The meteoric dust is not always evenly distributed along the orbit of the comet, so during some years more meteors are seen when Earth intersects the dust stream, and in other years fewer. For example, a very clumpy distribution is associated with the Leonid meteors, which in 1833 and again in 1866 (after an interval of 33 years—the period of the comet) yielded the most spectacular showers (sometimes called meteor storms ) ever recorded ( [link] ). During the Leonid storm on November 17, 1866, up to a hundred meteors were observed per second in some locations. The Leonid shower of 2001 was not this intense, but it peaked at nearly a thousand meteors per hour—one every few seconds—observable from any dark viewing site.
The most dependable annual meteor display is the Perseid shower, which appears each year for about three nights near August 11. In the absence of bright moonlight, you can see one meteor every few minutes during a typical Perseid shower. Astronomers estimate that the total combined mass of the particles in the Perseid swarm is nearly a billion tons; the comet that gave rise to the particles in that swarm, called Swift-Tuttle, must originally have had at least that much mass. However, if its initial mass were comparable to the mass measured for Comet Halley, then Swift-Tuttle would have contained several hundred billion tons, suggesting that only a very small fraction of the original cometary material survives in the meteor stream.
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