Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/887

Table of Comets having a Mean Distance less than that of Saturn.

Name. Mean distance from Sun. Eccentricity. Inclination. Period in years. Encke s 2-2181 0-8464 13 4 15 3-303 Blainpain s Burckhardt s Clausen s 2-8490 2-9337 3-0913 0-6867 0-8640 0-7213 9 11 6 8 1 45 1 53 43 4-809 5-025 5-435 De Vice s . 3-1028 0-6173 2 54 45 5 4C9 Winnecke s Brorsen s 3-1343 3-1463 0-7547 0-7945 10 48 4 30 57 51 5-549 5-581 Lexell s 3-1560 0-7861 1 34 28 5-607 Pons s 3-1602 G 7552 10 42 48 5 618 D Arrest s 3-1618 0-6609 13 56 6 6-380 Biela s 3-5306 0-7563 12 33 17 6-635 Faye s 3-8118 0-5576 11 22 7 7-414 Pigott s 4-6496 0-6787 47 43 10-025 Peters s.. . 6-3206 0-7567 13 2 14 15-990

Motion in all cases direct.

Table of Comets having a Mean Distance exceeding Saturn's, and less than that of Uranus.

Name. Mean distance. Eccentricity. Inclination. Period in years. Westphal s Pons s 16-6200 17-0955 0-9248 0-9545 4*0 58 32 73 57 3 67-770 70-068 De Vice s 17-5386 0-9544 84 57 13 73-250 Olbers s 17-6338 0-9312 44 29 55 74-050 Brorsen s 17-7795 0-9726 19 8 25 74-970 Halley s 17-9875 0-9674 17 45 5 76-680

Motion of the first five direct, of Halley's retrograde.

The fine comet of 1858-59, called Donati s, was subjected to a rough spectroscopic analysis by Donati, and the spectrum shown to be discontinuous, consisting of a few broad bands. Comet I., 18GG (called Tempel s), examined by Iluggins in 1SGG, showed three bands in the spectrum of the nucleus. Brorsen s comet, examined in 1868, showed .similar appearances. But it was not until the examination of Winnecke s comet in 1868 that any successful attempt was made to compare the band spectrum of a comet s nucleus with the spectrum of a known terrestrial element. The result was somewhat remarkable. It was found that the spectrum is that obtained from carbon when the electric spark is taken through olefiant gas. The same result has been obtained, both by Huggins and by Professors Hark- ness and Young of America, from the spectroscopic analysis of Encke s comet in 1872. Professor Harkness sums up the result of his own series of researches into the condition of this comet as follows : (1.) Encke s comet gives a carbon spectrum. (2.) From November 18 to December 2, the wave length of the brightest part of the comet s spectrum was continually increasing. (3.) No polarisation was de tected in the light of the comet. (4.) The mass of Encke s comet is certainly not less than that of an asteroid. (5.) There is some probability that the electric currents which give rise to auroras are propagated in a medium which per vades all space, and that the aurora is in reality the spectrum of the medium. G. It is not improbable that the tails of all large comets will be found to give spectra similar to that of the aurora, though additional lines may be present.

It is only of late years that the study of meteors or falling stars has come to be regarded as belonging to astronomy. Even now the discovery of the connection of meteors and comets is so recent, and the aspect under which the new department of astronomy presents itself varies so continually as fresh relations are recognised, that it seems desirable to limit our remarks to the statement of a few broad facts.

It has been shown, then, that meteors travel in systems round the sun, and that displays of falling stars occur when the earth passes through a meteor-system.

There are already recognised more than one hundred meteor-systems, known by the direction in which the corresponding falling stars encounter the earth.

The best-known meteor-systems are those which produce a shower on November 13-14 in certain years, and those which usually produce a display on August 10-11. The former have been called the Leonides, because the part of the stellar sphere whence the meteors appear to radiate is in the constellation Leo. The latter, for a like reason, appearing to radiate from Perseus, have been called the Perseides.

It has been found that these two meteor-systems accord, so far as their orbital motions are concerned, with two comets, viz., respectively Tempel s comet, L, 1866, and the comet known as III., 1862. The association is indicated in the following table:—

Known Elements. Orbit of November Meteors. Comet L 1866. Orbit of August Meteors. Comet III. 18C2. Perihelion pas sage . . . ) Nov. 13, $ 1866 j 0-9893 1 - j 51 28 0-9033 j 10-340 18 3 j 33-25 Retrograde Jan. 11, ) 18C6 ] 0-9765 51 26 -l 0-9054 10-324 1718 -1 33-176 Retrograde July 23-62 0-9643 343 38 a 138 16 64 3 Retrograde Aug. 22 9 0-9626 344 41 137 27 66 25 123-74 Retrograde Perihelion dis tance Longitude of Perihelion . . Longitude of Node Eccentricity . . Semi - axis major Inclination Period (in Years).... Motion

Other comets have been associated, though not quite so distinctly, with meteoric systems. But perhaps the most remarkable evidence in this matter is that afforded by Biela s comet. At the return in 1866, and again in 1872, this comet was not seen ; and as it had become divided into two distinct comets in 1846, the inference was, that it had further subdivided since it was last seen in 1852. Now the orbit of this comet nearly intersects that of the earth, at about the place occupied by the earth on November 27. Accordingly, after the fruitless telescopic search for the comet in 1872, Professor Alexander Herschel pointed out that, during the last week in November, meteors following in the train of this comet might be expected to make their appearance. Oil the night of November 27, 1872, a remarkable shower of meteors was observed, the meteors radiating from a region of the heavens indicating that they were travelling in the track of Biela s comet.

The telescopic study of some of the remarkable comets of the last quarter of a century has been rewarded by results of interest. This has been the case in particular with Donati s comet of the year 1858. During the comet s passage past its perihelion, several envelopes formed around the head, continually passing outwards, and disappearing after a certain distance from the nucleus had been reached, while new envelopes appeared within. Besides the prin cipal tail, which was curved like an aigrette plume, another thin straight tail was visible for a while ; and for a short time yet another was seen. A striation of the chief tail could also be recognised, which, according to the researches of Bond and Norton in America, corresponded to the action of a repulsive force exerted by the sun, and sweeping matter away from the main tail with a velocity exceeding that with which this tail itself was formed, but not exceeding the velocity indicated by the straightuess of the subsidiary tails.

The great comet of 1874 is shown in fig. 51, as

pictured by Mr Iluggins. It exhibits well a peculiarity comet,

which has characterised many comets, the well-defined dark