Meteors and Meteorites

A meteor (from mexeopa—"things in the air") is an atmospheric phenomenon, and most generally refers to such phenomena as halos, rainbows, hail, whirlwinds, and so on. Here, we will use the astronomical definition and apply the term only to what has classically been called a "shooting star": a streak or sometimes a point of light due to the incandescence of a celestial object (called a meteoroid) that enters the Earth's atmosphere at high speed (typically in the range 10-20Km/s). The process of frictional heating ablates the meteoroid and causes a large volume of excited and ionized atoms in its wake. It is the glow from the subsequent deionization and deexcitation that we see in the meteorite trail. Most meteors are dust motes or small fragments, including most of those that are seen in meteor "showers."

When the Earth crosses the orbital debris of a comet, a meteor shower may be observed. The frequency of meteors seen during a meteor shower may vary from a few per hour to hundreds or thousands per hour but the latter are rare. A number of the showers are periodic. A list of the more prominent known showers appears in Table 5.8, which gives the approximate date of the onset of the shower (± ~2 days), the typical shower range of dates over which it is visible, the radiant, or point in the sky from which the meteors appear to diverge, and, if it is known, the comet associated with the shower. The names of the showers derive from the constellation or star within the constellation closest to the radiant. Both the date and the duration are approximate and may fluctuate from year to year. The dates of the shower mark the ascending or descending nodes of the nearly identical paths of the particles in the shower. Like the comets from which they most likely derive, the orbits are subject to perturbation, and the particles' orbits increasingly diverge, broadening the stream, and spreading the particles along the orbit with time. From year to year, any particular shower will vary in brilliance and duration, depending on the local time of transit of the radiant. It is to be expected that the ancient records might show showers that no longer occur as the result of planetary perturbations, or very long-term encounters as a result of limited dispersion in a long-period cometary orbit. A list by Imoto and Hasegawa (1958) contains at least three radiants that may fall into one or the other of these categories. Records of European observations of meteors between the 5th and 15th centuries by Dall'olmo (1978) indicate many cyclical shower events. Forty-seven of the observations are of certain or suspected showers, and although precise dates are not always stipulated, it is clear that April 4/5 (Julian) marked the date of a recurrent shower. Major showers were observed on these dates in the years 1040,1094,1095,1096,1122, and 1123. Stephenson and Clark (1978, pp. 11-12) suggest that the 1095 and 1122 events may be the April Lyrids. Probably, all these events are Lyrids. At least one of these occurrences had historical significance. In his book on the crusades, Treece (1962, p. 84) noted that the April 1095 shower was so impressive that Gislebert, Bishop of Lisieux, interpreted it as a sign of heavenly approval for a crusade. In November of that year, Pope Urban II convened the Council of Clermont, and, declaring that the end of the world was near, launched the "holy war" known later as the First Crusade.

Meteors in meteor showers are typically neither very bright nor very massive, but records of bright meteors and even meteorite falls observed in connection with showers are not unknown (see the Dall'olmo 1978 list), recent examples of which are the videotaped meteor impacts on the Moon during the 1999 Leonid meteor shower. Meteors that are not in showers are called sporadic meteors. Occasionally, they are bright enough to be seen even in the daytime. Such a meteor is called a "fireball" or bolide. Some have been observed (and even heard!) to explode during transit. Babylonian diaries recorded the phenomenon, referred to as kakkabu rabu, "big star," as this excerpt from the diaries of the year 419 b.c. (Sachs and Hunger 1988, p. 65) illustrates:

Year 5 of Umakus,. . . Month II,. . . The 22nd, Venus' last appearance in the West behind the chariot. In the middle part of the day, a big star which was like a torch flashed from south to north, and the land heard the noise of the sky.

Chadwick (1989) suggests that the term kisri ("lump," "knot") was sometimes used to refer to meteors as far back as 2000-1600 b.c., in the old Babylonian period.

Stephenson and Yau (1992) find a reference to what may be the earliest reliably recorded meteor shower in the Ch'un-ch'iu ("Spring and Autumn") Annals, records of the feudal state of Lu for the years 722-481 b.c., during the Eastern Chou dynasty (770-256 b.c.) of China:

7th year [of the kung (Duke) of Lu], summer, 4th month, day hsin-mao. At night the regular stars are not seen. At midnight the stars fell like rain.

Table 5.8. Selected meteor showers."

Name

Date

Duration

Radiant

Associated comet

Quadrantidsb

Jan. 3

1d

15h28m +50°

0 0

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