Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/120

Rh 110 METEOR that all the meteor paths as seen on the sky would if pro duced backward cross the ecliptic above the horizon. In fact there is no tendency of this kind. Hence the meteor- oids do not move in orbits that are near the ecliptic as the planets do, but like the comets they may and usually do have orbits of considerable inclinations. Numbers through the Night. There are more meteors seen in the morning hours than in the evening. If the meteors had no motion of their own in space, the earth would by its motion receive the meteors only on the hemi sphere that was in front. There would be no meteors seen in the other hemisphere. On the other hand, if the meteors had such large velocities of their own as that the earth s velocity might be neglected in comparison, and if the directions of the meteors motions were towards all points indiscriminately, then as many would be seen in one part of the night as another. In fact there are about three times as many seen in the morning hours as in the evening. The law of change from evening to morning gives a means of proving that the mean velocity of meteors is so great that they must in general be moving in long orbits about the sun. In this respect also the meteoroids resemble comets, and are unlike planets, in their motions. Of the stone-furnishing meteors more are seen in the day than in the night, and more in the earlier hours of the night than in the later. This is probably due to the fact that more persons are in a position to see the stone-falls at the periods of greater abundance. Star Showers. While the average number of shooting stars for a single observer at midnight may be regarded as tolerably constant, there have been special epochs when many more have been seen. In certain instances the sky has been filled with the luminous trains, just as it is filled by descending snowflakes in a snowstorm, making a veritable shower of fire. One of the best-observed, though by no means the most brilliant, of these showers occurred on the evening of the 27th of November 1872. Some of the observers of that shower, counting singly, saw at the rate of eight or ten thousand shooting stars in the course of two hours. The distances of the meteoroids in the middle of the swarm which the earth then passed through, each from its nearer neighbours, would be 30 or 40 miles. The following quotations show the impression made by star showers in times past : &quot;In the year 286 [of the Hegira] there happened in Egypt an earthquake on Wednesday the 7th of Dhu-1-Ka dah, lasting from the middle of the night until morning ; and so-called flaming stars struck one against another violently while being borne eastward and westward, northward and southward, and no one could bear to look toward the heavens on account of this phenomenon.&quot; &quot; In the year 599 [of the Hegira], on the night of Saturday, on the last day of Muharram, stars shot hither and thither in the heavens, eastward and westward, and flew against one another like a scattering swarm of locusts, to the right and left ; people were thrown into consternation, and cried to God the Most High with confused clamour.&quot; &quot;These meteors [November 12, 1799] might be compared to the blazing sheaves shot out from a firework. &quot; &quot;The phenomenon was grand and awful; the whole heavens appeared as if illuminated with sky rockets.&quot; November 13, 1833. &quot;Thick with streams of rolling fire; scarcely a space in the firmament that was not filled at every instant. &quot; &quot;Almost infinite number of meteors; they fell like flakes of snow.&quot; November Meteors or Leonids. These quotations all refer (except possibly the first) to a shower which has appeared in October and November of many different years since its first known occurrence on the 13th of October 902 A.D. Dates of these showers are given in the following table : Oct. 13, 902. Oct. 17, 1101. Oct. 28, 1602. Nov. 13, 1833. Oct. 15, 931. Oct. 19, 1202. Nov. 9, 1698. Nov. 14, 1866. Oct. 14, 934. Oct. 23, 1366. Nov. 12, 1799. Nov. 14, 1867. Oct. 15, 1002. Oct. 25, 1533. Nov. 13, 1832. Nov. 14, 1868. On several years after 1833, and before and after 1866-68, there were unusual numbers of those meteors seen on the mornings of November 13, 14, and 15, though per haps they would have been unnoticed had there not been special watching for them. It will be seen that all these showers are at intervals of a third of a century, that they are at a fixed day of the year, and that the day has moved steadily and uniformly along the calendar at the rate of about a month in a thousand years. The change of twelve days in the 17th century is due to the change from old to new style. The only explanation of this periodical display that is now seriously urged, and the one which is universally accepted by astronomers, is that there is a long thin stream of meteoroids, each of which is travelling about the sun in a conic section. These conic sections are all nearly parallel, and have nearly the same major axis, extending out about as far as to the orbit of Uranus, and each requir ing the common period of thirty-three and a quarter years. The length of the stream is such that the most advanced members are six or eight years ahead of the hindermost, and they all cross the earth s orbit with a velocity of about 26 miles a second. Since the earth plunges through the group nearly in the opposite direction, the velocity with which they enter the air is 44 miles a second. One of the facts which have greatly aided us in arriving at this explanation is that these meteors in all the years and through all hours of the night cross the sky as we look at them in lines which diverge from a point near the centre of the sickle in the constellation Leo ; hence the paths in the air are parallel. This implies that their velocities relative to the sun are all parallel and equal to each other. The radiation from Leo has given to them the name Leonids. Orbit of the Leonids. This orbit, common to all the Leonid meteors, is inclined to the ecliptic at an angle of 17 (or rather 163, since the motion is retrograde), has a major axis of 10 34, a periodic time of 33 - 27 years, and a peri helion distance a little less than unity. The above orbit, and that alone, explains the several appearances of the November meteors, the annual and the thirty-three year periods, the radiation from Leo, and the change of day of the month in the course of the centuries. This it does so completely that the result has never been questioned by astronomers. Shortly after the publication by Professor Adams in 1867 of the last link in the chain of the proof of this orbit, there was also published the definitive orbit of the comet 1866 I. That the comet was running almost exactly in the orbit of the meteors was at once recognized. In fact the comet is itself, in a sense, a meteoroid, and the principal member, so far as we know, of the group. Leonids had been seen in 1863, two years and two months in advance of the comet, while those of 1866 were ten months behind it. Those of later years (a few Leonids were seen even in 1870) were extended along the line of the comet s path behind it. The leaders of this long file of meteoroids had passed up beyond the orbit of Jupiter long before those which brought up the rear had crossed that planet s orbit going down toward the sun. The thickness of the stream is less than the ten-thousandth part of its length. In the densest part that we have recently passed through namely, that traversed in 1833 the density of the stream may be expressed by saying that each meteoroid must in the mean have been 10 or 20 miles from its nearest neighbours. What makes this Comet and these Meteors describe the same Orbit about the Sun? Its path might have been inclined to the ecliptic at any angle instead of 163. Or, with this inclination, its plane might have cut the earth s orbit at any other place than where the earth is on the 1 4th