Meteor

Meteor (Gr. perEwpa, literally " things in the air," from yerb., beyond, and a€ipav, to lift up), a term originally applied by the ancient Greeks to many atmospheric phenomena - rainbows, halos, shooting stars, &c. - but now specially restricted to those luminous bodies known as shooting stars, falling stars, fireballs and bolides. Though these objects only become visible in the atmosphere they are extra-terrestrial planetary bodies, and properly belong to the domain of astronomy. The extraterrestrial bodies which happen to find a resting-place on the 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 star showers had been occasionally observed, but instead of being attentively watched they had been neglected, for their apparitions had filled mankind with dread, and superstition attributed to them certain malevolent influences. It was the brilliant exhibition in November 1833 that, in modern times particularly, attracted earnest students to investigate the subject of meteors generally, and to make systematic observations of their apparitions on ordinary nights of the year. Historical records were searched for references to past meteoric displays, and these were tabulated and compared. The attention devoted to the matter soon elucidated the phenomena of meteors, and proved them to be small planetary bodies, practically infinite in 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 bright careers and loud detonations, descend upon the earth's surface and can be submitted to close inspection and analysis (see Meteorites). The fireball or bolide (Gr. (30Xis, a missile) comes next in order from its 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 firstto a sixth-magnitude star. They exhibit a great dissimilarity in paths, motions and colours. The smallest and most numerous class are the telescopic meteors invisible to the naked eye. They range from the 7th magnitude to the smallest object perceptible in large telescopes.

	Average Heights.		Length of Path.	Velocity per sec.
	Beginning.	Ending.
Swift fireballs .	85 m.	50 m.	55 m.	38 m.
Slow fireballs.	66 „	25 „	116 „	15 „
Slow fireballs (radiants near horizon).. .	59 ,,	48 „	121 „	13 .,
Swift shooting stars.. .	81 „	56 „	42 ,,	41 „
Slow shooting stars. .. .	63 „	49 ,,	36 „	17 „

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 miles apart. These pairs of observations have shown a parallax from which the elevation of the objects above the earth, the lengths and directions of their courses, &c. could be computed. The average heights are from 80 to 40 m. A few, however, first appear when higher than 80 m. and some, usually slow-moving meteors, descend below 40 m. But altitudes beyond zoo and within 20 m. are rare: - 30 of the November Leonids give a mean height of 842 to 572 m. 40 of the August Perseids „ „ to 54 m.

When the length of a meteor's: course is known and the duration of its flight has been correctly estimated it is easy to compute the velocity in miles. The visible life of an ordinary shooting star is, however, comprised within one second, and it is only rarely that such short time intervals can be accurately taken. The real velocities derived from good observations are rarely, if ever, under 7 or 8 m. per second, or over 60 or 70 m. per second. In a few exceptional cases abnormal speed has been indicated on good evidence. The slower class of meteors overtaking the earth (like the Andromedids of November) have a velocity of about 8 or 10 m. per second, while the swifter class (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 special regard to their directions it is seen that they diverge from a common focus. The apparent scattering or diversity of the flights is merely an effect of 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 night of the year there are a great number of these radiants in action, but the large majority represent very attenuated showers. In 1876 the number of radiants known was 850, but about 5000 have been determined up to the present time. These are not all the centres of 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 long-enduring and brief radiants. The Perseids are visible from about the fil th of July to the 10th of August, the radiant having a daily motion of about 1° R.A. to E.N.E. The Lyrids also vary in the position of their radiant, but the Orionids form a stationary position from about the 9th to the 24th of October. A large proportion of the ordinary feeble showers also appear to be stationary.

Solid bodies (chiefly stone or stone and iron) enter the atmosphere from without at all conceivable angles and at a velocity of about 26 m. per second, while the earth's orbital velocity is about 184 m. per second. In thus rapidly penetrating the air heat is generated, the meteor becomes incandescent, and the phenomena of the streak or train is produced. Before the object can pierce the dense lower strata of air its material is usually exhausted, but on rare occasions it withstands the fiery ordeal, and fragments of the original mass fall upon the earth.

Multitudes of meteors infest space. On a clear moonless night one person may count eight or ten shooting stars in an hour. But there are more than twice as many visible in the early morning hours as in the evenings, and during the last 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 day and would be visible to unassisted vision in the 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 orbit, appear to be separated by intervals of about 250 m. In special showers, however, they are much closer. In the 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 sun in obedience to the law of gravitation in the same manner as 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 plane of the ecliptic; hence their orbits are not like the orbits of planets and short-period comets, which are little inclined, but like the orbits of parabolic comets, which often have great inclinations.

902, Oct. 13.	1101, Oct. 17.	1602, Oct. 28.	1833, Nov. 13.
93 1, Oct. 14.	1202, Oct. 19.	1698, Nov. 9.	1866, Nov. 14.
934, Oct. 14.	1366, Oct. 23.	1799, Nov. 12.	1867, Nov. 14.
1002, Oct. 1 5 .	1 533, Oct. 25.	1832, Nov. 13.	1868, Nov. 14.

Historical records supply the following dates of abundant meteoric displays: - These showers occurred at intervals of about one-third of a century, while the day moved along the calendar at the rate of one month in a thousand years. The change of style is, however, responsible for a part of the alteration in date. The explanation of these recurring phenomena is that a great cloud or distended stream of meteors revolves around the sun in a period of 331years, 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 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 184 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 Leo and are termed Leonids. In 1867 the remarkable discovery was made that Tempel's comet (1866: I.) revolved in an orbit identical with that of the Leonids. That the comet and meteors have a close physical association seems certain. The disintegrated and widely dispersed material of the comet forms the meteors which embellish our skies on mid-November nights.

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 6.7 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 sky near the star 7 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 annual displays of meteors have often been remarked on about the 10th of August, directed from Perseus, but they do not appear to have exhibited periodical maxima of great strength. They are probably dispersed pretty evenly along a very extended 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 10th of 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 south-west of a Lyrae. The display is apparently an annual one, though with considerable differences in intensity, and the 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 year's.

Apart from the instances alluded to there seem few coincidences between the orbital elements of comets and meteors. Halley's comet conforms very well, however, with a meteoric shower directed from Aquarius early in May. But there are really few comets which pass sufficiently near the earth to give rise to a meteoric shower. Of 80 comets seen during the 20 years ending 1893, Professor Herschel found that only two, viz. Denning's comet of 1881 and 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.

Date.	Radiant R.A. Dec.	Date.	Radiant R.A. Dec.
Jan. 2-3	230°+53°	July-Sept.	470+43°
Feb. 10-15 .	750+41°	Sept. 5-15	62°+37°
March 1-4 .	166°+ 4°	Sept. 3-22	740+41°
March 24	160+58° +58°	Oct. 2 .	2300+52°
April 19-22 .	2710+33°	Oct. 4	310°-1-79°
April-May .	193°+58°	Oct. 15-24 .	92°+15°
May 1-6	338°- 2°	Oct. 20-25 .	100°-+-13°
May 11-18 .	2310+27°	Oct. 30-Nov. I	43°+22°
May-July	2520-21°	Nov. 2. .	58'4"
June 13. .	3100+61°	Nov. 14-16	1510+22°
July 15-19 .	314°+48°	Nov. 16-28 .	154°+40°
July 28-30 .	339 ° -11	Nov. 20-23 .	63°--1-22"
Aug. 9-13 .	450+57°	Nov. 17-23 .	250+43°
Aug. 10-15 .	2900+53°	Dec. 4 .	162°+58°
Aug. 21-25 .	2910+60°	Dec. 9-12	108°+33°

Radiants of Principal Showers

The following is a list of the chief radiant points visible during the year: Many meteors exhibit the green line of magnesium as a principal constituent. Professor N. von Konkoly remarked in the fireball of 1873 (July 26) the lines of magnesium and sodium. Other lines in the red and green have been detected and found by comparison with the lines of marsh gas. Bright meteors often emit the bluish-white light suggestive of burning magnesium. In addition to magnesium and sodium the lines of potassium, lithium and also the carbon flutings exhibited in cometary spectra, has been seen.

Meteoric observation has depended upon rough and hurried eye estimates in past years, but the importance of attaining greater accuracy by means of photography has been recognized. At several American observatories, and at 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. -122° 12'. But the materials obtained were few, the shower having proved inconspicuous. The photographic method appears to have practically failed during 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 combustion. The finer meteors on entering the air only weigh a few 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 sand, and still be large enough to evolve all the light presented by them. (W. F. D.)