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See also:METEOR (Gr. µerEwpa, literally " things in the See also:air,” from µera, beyond, and aeip6Lv, to lift up) , a See also:term originally applied by the See also:ancient Greeks to many atmospheric phenomena—rainbows, halos, See also:shooting stars, &c.-but now specially restricted to those luminous bodies known as shooting stars, falling stars, fireballs and bolides. Though these See also:objects only become visible in the See also:atmosphere they are extra-terrestrial planetary bodies, and properly belong to the domain of See also:astronomy. The extra-terrestrial bodies which happen to find a resting-See also:place on the See also:earth are studied under the name of meteorites (q.v.). In ancient times meteors were supposed to be generated in the air by inflammable gases, Isolated fireballs and See also:star showers had been occasionally observed, but instead of being attentively watched they had been neglected, for their See also:apparitions had filled mankind with dread, and superstition attributed to them certain malevolent influences. It was the brilliant See also:exhibition in See also:November 1833 that, in See also:modern times particularly, attracted See also:earnest students to investigate the subject of meteors generally, and to make systematic observations of their apparitions on See also:ordinary nights of the See also:year. See also:Historical records were searched for references to past meteoric displays, and these were tabulated and compared. The See also:attention devoted to the See also:matter soon elucidated the phenomena of meteors, and proved them to be small planetary bodies, practically See also:infinite in See also:numbers and illimitable in the extent and variety of their orbits. The various kinds of meteors are probably but different manifestations of similar objects. Perhaps the most important meteors are those which, after their See also:bright careers and loud detonations, descend upon the earth's See also:surface and can be submitted to See also:close inspection and See also:analysis (see METEORITES). The fireball or bolide (Gr. Is, a missile) comes next in See also:order from its See also:size and conspicuous effects. It may either be interspersed with many smaller meteors in a shower or may be isolated. The latter usually move more slowly and approach rather near to the earth. The ordinary shooting stars vary from the brilliancy of a first- to a See also:sixth-magnitude star. They exhibit a See also:great dissimilarity in paths, motions and See also:colours. The smallest and most numerous class are the telescopic meteors invisible to the naked See also:eye. They range from the 7th magnitude to the smallest See also:object perceptible in large telescopes. The altitudes at which these bodies are visibly presented to us differ in individual cases. More than a thousand observations in duplicate have been made of the paths of identical meteors seen from two stations many See also:miles apart. These pairs of observations have shown a See also:parallax from which the See also:elevation of the objects above the earth, the lengths and directions of their courses, &c. could be computed. The See also:average heights are from 8o to 40 M. A few, however, first appear when higher than 8o m. and some, usually slow-moving meteors, descend below 40 M. But altitudes beyond too and within 20 M. are rare: Average Heights. Length of Velocity Beginning. Ending. Path. per sec. See also:Swift fireballs . 85 m. 50 M. 55 m. 38 m. Slow fireballs. . 66 ,, 25 ,, 116 ,, 15 „ Slow fireballs 59 ,, 48 ,, 121 ,, 13 „ (radiants near See also:horizon) . . . Swift shooting 81 ,, 56 „ 42 ,, 41 ,, stars . . . Slow shooting 63 ,, 49 ,, 36 .. 17 ,, stars . . . 3o of the November Leonids give a mean height of 84ii to 571 M. 40 of the See also:August Perseids „ „ „ 8o to 54 M. When the length of a meteor's course is .known and the duration of its See also:flight has been correctly estimated it is easy to compute the velocity in miles. The visible See also:life of an ordinary shooting star is, however, comprised within one second, and it is only rarely that such See also:short See also:time intervals can be accurately taken. The real velocities derived from See also:good observations are rarely, if ever, under 7 or 8 m. per second, or over 6o or 70 M. per second. In a few exceptional cases abnormal See also:speed has been indicated on good See also:evidence. The slower class of meteors overtaking the earth (like the Andromedids of November) have a velocity of about 8 or to m. per second, while the swifter class (See also:meeting the earth like the Leonids of November) have a velocity of about 44 M. per second. When the members of a shower are observed with See also:special regard to their directions it is seen that they diverge from a See also:common See also:focus. The apparent scattering or diversity of the flights is merely an effect of See also:perspective upon objects really traversing parallel lines. The centre upon which the observed paths converge is called the radiant point or, shortly, the radiant. On every See also:night of the year there are a great number of these radiants in See also:action, but the large See also:majority represent very attenuated showers. In 1876 the number of radiants known was 850; but about 5000 have been determined up to the See also:present time. These are not all the centres of See also:separate systems, however: many of the positions being multiple observations of the same showers. Thus the August Perseids, the returns of which have been witnessed more frequently than those of any other meteoric stream have had their radiant point fixed on more than 250 occasions.
There appear to be moving and stationary radiants, contracted and diffused radiants, and See also:long-enduring and brief radiants. The Perseids are visible from about the 11th of See also:July to the loth
of August, the radiant having a daily See also:motion of about 10 R.A. to E.N.E. The Lyrids also vary in the position of their radiant, but the Orionids See also:form a stationary position from about the 9th to the 24th of See also:October. A large proportion of the ordinary feeble showers also appear to be stationary.
Solid bodies (chiefly See also: Before the object can See also:pierce the dense See also:lower strata of air its material is usually exhausted, but on rare occasions it withstands the fiery See also:ordeal, and fragments of the See also:original See also:mass fall upon the earth. Multitudes of meteors infest space. On a clear moonless night one See also:person may See also:count eight or ten shooting stars in an See also:hour. But there are more than twice as many visible in the See also:early See also:morning See also:hours as in the evenings, and during the last See also:half of the year there are also more than twice as many visible as during the first half. It is computed that twenty millions of meteors enter the atmosphere every See also:day and would be visible to unassisted See also:vision in the See also:absence of sunlight, moonlight and clouds, while if telescopic meteors are included the number will be increased twentyfold. Ordinary meteors, in the region of the earth's See also:orbit, appear to be separated by intervals of about 250 M. In special showers, however, they are much closer. In the See also:rich display of the 12th of November 1833, the average distance of the particles was computed as about 15 m., in that of the 27th of November 1885 as about 20 m., and in that of the 27th of November 1872 as about 35 M. The meteors, whatever their dimensions, must have motions around the See also:sun in obedience to the See also:law of See also:gravitation in the same manner as See also:planets and comets—that is, in conic sections of which the sun is always at one focus. The great variety in the apparent motions of meteors proves that they are not directed from the See also:plane of the See also:ecliptic; hence their orbits are not like the orbits of planets and short-See also:period comets, which are little inclined, but like the orbits of parabolic comets, which often have great inclinations. Historical records See also:supply the following See also:dates of abundant meteoric displays: 902, Oct. 13. I10I, Oct. 17. 1602, Oct. 28. 1833, Nov. 13. 931, Oct. 14. 1202, Oct. 19. 1698, Nov. 9. x866, Nov. 14. 934, Oct. 14. 1366, Oct. 23. 1799, Nov. 12. 1867, Nov. 14. 1002, Oct. 15. 1533, Oct. 25. 1832, Nov. 13. 1868, Nov. 14. These showers occurred at intervals of about one-third of a See also:century, while the day moved along the See also:calendar at the See also:rate of one See also:month in a thousand years. The See also:change of See also:style is, however, responsible for a See also:part of the alteration in date. The explanation of these recurring phenomena that a great See also:cloud or distended stream of meteors revolves around the sun in a period of 33i years, and that one portion of the elliptical orbit intersects that of the earth. As the meteors have been numerously visible in five or six successive years it follows they must be See also:pretty densely distributed along a considerable arc of their orbit. It also follows that, as some of the meteors are seen annually, they must be scattered around the whole orbit. Travelling at the rate of 26 m. per second, they encounter the earth moving 18; m. per second in an opposite direction, so that the apparent velocity of the meteors is about 44 M. per second. They radiate from a point within the Sickle of See also:Leo and are termed Leonids. In 1867 the remarkable See also:discovery was made that Tempel's See also:comet (1866 : I.) revolved in an orbit identical with that of the Leonids. That the comet and meteors have a close See also:physical association seems certain. The disintegrated and widely dispersed material of the comet forms the meteors which embellish our skies on See also:mid-November nights. See also:Fine meteoric showers occurred in 1798 (Dec. 7), 1838 (Dec. 7), 1872 (Nov. 27), 1885 (Nov. 27), 1892 (Nov. 23) and 1899 (Nov. 23 and 24), and the dates indicate an average period of 6.s years for fifteen returns. The meteors move very slowly,as they have to overtake the earth, and their apparent velocity is only about 9 M. per second. They are directed from a point in the See also:sky near the star y Andromedae. Biela's comet of 1826, which had a period of 6.7 years, presented a significant resemblance of orbit with that of the meteors, but the comet has not been seen since 1852 and has probably been resolved into the meteoric stream of Andromedids. Rich See also:annual displays of meteors have often been remarked on about the loth of August, directed from See also:Perseus, but they do not appear to have exhibited periodical See also:maxima of great strength. They are probably dispersed pretty evenly along a very extended See also:ellipse agreeing closely in its elements with comet 1862 : III. But the times of revolution are doubtful; the probable period of the comet is 121 years . and that of the meteors 1051 years. This shower of Perseids is notable for its long duration in the months of July and August and for its moving radiant. There was a brilliant exhibition of meteors on the 20th of See also:April 1803, and in other years meteors have been very abundant on about the 19th to the 21st of April, shooting from a radiant a few degrees See also:south-See also:west of a Lyrae. The display is apparently an annual one, though with considerable See also:differences in intensity, and the See also:cycle of its more abundant returns has not yet been determined. A comet which appeared in 1861 had a very suggestive agreement of orbit when compared with that of the meteors, and the period computed for it was 415 years. Apart from the instances alluded to there seem few coincidences between the orbital elements of comets and meteors. See also:Halley's comet conforms very well, however, with a meteoric shower directed from See also:Aquarius early in May. But there are really few comets which pass sufficiently near the earth to give rise to a meteoric shower. Of 8o comets seen during the 20 years ending 1893, See also:Professor See also:Herschel found that only two, viz. Denning's comet of 188x and See also:Finlay's of 1886, approached comparatively near to the earth's path, the former within 3,000,000 M. and the latter within 4,600,000 m. Radiants of See also:Principal Showers.—The following is a See also:list of the See also:chief radiant points visible during the year: Radiant Radiant Date. R.A. Dec. Date. R.A. Dec. See also:Jan. 2-3 230°+53° July-See also:Sept. . 47°+43° Feb. 10-15 75°+41° Sept. 5-15 . 62°+37°
See also: 20-25 . 100°+13° May II-18 231°+27° Oct. 3o-Nov. I 43°+22° May-July 252°-21° Nov. 2 . 58°+ 9° See also:June 13 . 310°+61° Nov. 14—16 . 151°+22° July 15-19 3 14°+48° Nov. 16-28 . 154°+40° . J uly Nov. 20—23 . 63°+22°
Aug. . 128—30 1 5 233990°—II° Dec. 47-23 162°+58°
AAug.
Aug. 21—25 291°+60° Dec. 9—12 108°+33°
Many meteors exhibit the See also:green See also:line of See also:magnesium as a principal constituent. Professor N. von Konkoly remarked in the fireball of 1873 (July 26) the lines of magnesium and See also:sodium. Other lines in the red apd green have been detected and found by comparison with the lines of See also:marsh See also:gas. Bright meteors often emit the bluish-See also: At several See also:American observatories, and at See also:Vienna, fairly successful attempts were made in November 1898 to photograph a sufficient number of meteor-trails to derive the Leonid radiant, and the mean position was at R.A. 151° 33' Dec. + 22° 12'. But the materials obtained were few, the shower having proved inconspicuous. The photographic method appears to have practically failed during See also:recent years, since there has been no brilliant display upon which to test its capacity. Really large meteors can be satisfactorily photographed, but small ones leave no impression on the plates. Meteors look larger than they are, from the glare and flaming effect due to their momentary See also:combustion. The finer meteors on entering the air only weigh a few See also:hundred or, at most, a few thousand pounds, while the smallest shooting stars visible to the eye may probably be equal in size to coarse grains of See also:sand, and still be large enough to evolve all the light presented by them. (W. F. Additional information and CommentsThere are no comments yet for this article.
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