Comets

Comets. BY CHARLES H. SKIDMORE, B. L., M. A., PROFESSOR OF MATHEMATICS, BRIGHAM YOUNG COLLEGE. It is hoped that a few of the many everyday inquiries .on the subject of comets in general, and Halley's comet in particular, may be answered in the following article. But, at most, little can be said on a subject as broad as the universe. Comets are easily distinguished from other heavenly bodies. Each looks very much like a star enveloped in a luminous fog, which usually streams forth on one side with a long, hazy light. The word comet is derived from the Greek word home, which suggests the common meaning "hairy star." Milton, in Paradise Lost, says, Satan, incensed with indignation and unterrified, stood like a great comet burning in the arctic sky, shaking pestilence and war from "his horrid hair." In this, too, Milton portrays the superstitious belief in the malign influence of comets, which prevailed for centuries. Their visits were rare, but altogether too frequent to suit. Among other historical events, the downfall of Nero, the destruction of Jerusalem, and the invasion of William the Conqueror, were heralded by the appearance of comets. What terror these visitors must have awakened in the hearts of their beholders is partly told in the following quotation from Ambrose Pare, a learned writer of the 16th century, concerning the comet which appeared in 1528: This comet was so horrible, so frightful, and it produced such great terror in the vulgar, that some died of fear and others fell sick. It appeared to be of excessive length, and was of the color of blood. At the summit of it was seen the figure of a bent arm, holding in its hand a

646 IMPROVEMENT ERA. great sword, as if about to strike. On both sides of the rays of this comet were seen a great number of axes, knives, and bloodcolored swords, among which were a great number of hideous human faces, with beards and bristling hair. Another strange notion was proclaimed by old preachers that comets were the sins of mortals set on fire by the wrath of God. In the light of modern science, on the contrary, these "fireballs flung by an angry God" are quite harmless wanderers in space, whose beautiful forms and wonderful features are eagerly watched with rare delight. Even a collision with one of them may not prove disastrous. Sir Robert Ball says: In 1861 we had a novel experience. On a Sunday evening, in the midsummer of that year, we dashed into a comet. * * * It was the end of the tail which we encountered. * * * Most of us never knew that anything had happened at all, and the rest only learned of the accident long after it was all over. For a couple of hours that night it would seem that we were actually in the tail of a comet, but so far as I know, no one was injured. * * * Indeed, I have only heard of one calamity arising from the collision. A clergyman, on this particular occasion, however, said the sky was overcast with a peculiar glow, while the ordinary light was so much interfered with that the sexton had to provide a pair of candles to enable him to get through the sermon. The expense of these candles was, 1 believe, the only loss to the earth in consequence of its collision with the comet of 1861. When one contemplates the large number of comets there are in existence, their enormous size generally, and their great velocity, he wonders why there are not more collisions than there are, and why they do not work greater disaster than they do. After considering these points briefly, however, one should not fail to examine the size, shape and position of the orbits of comets, their mass, their density and constituent parts. The number of comets recorded in historical and scientific annals is about eight hundred, perhaps fifty per cent of which were discovered before the invention of the telescope, in 1609. Up to this time there were not more than an average of thirty comets seen in the century. In the 18th century there were seventy observed, in the 19th century three hundred. Of the last number not more than thirty could be seen with the naked

COMETS. 647 eye. Few men have seen more bright comets than could be counted on the- fingers of one hand, unless by the aid of a telescope. A telescope usually brings to view from three to ten per year. These are so distributed in the sky that there is seldom a day when one cannot be seen. If Kleiber is right in maintaining that not more than three per cent of those entering the solar system can be seen in our telescopes, then Kepler's statement that comets are as plentiful as fishes in the sea is not greatly exaggerated. Indeed, if one transcends the solar system Kepler's statement is sufficient. The dimensions of comets are often inconceivable. The head of the comet of 1811, the largest comet on record, was at one time 1,200,000 miles in diameter, or approximately one and one-half times the diameter of the sun. The tail of the comet of 1882, the longest recorded, was more than 10,000,000 miles longer than the distance from the earth to the sun. Comets' heads are usually from 40,000 to 150,000 miles in diameter, and their tails are from 5,000,000 miles long to ten times that length. Most comets whose heads are less than 10,000 miles in diameter pass by unobserved, even through telescopes, unless they come unusually near the earth. When one of these huge bodies comes within the effective gravitational influence of our massive sun, its motion is accelerated, and varies almost inversely as the square of its distance from the sun, until frequently it passes by and loops around the sun with a velocity a thousand times faster than the swiftest cannon ball. Indeed, the swiftest rifle bullet travels little more than 2,000 feet per second, while it is estimated that the comet of 1680 attained the incomprehensible speed of 370 miles per second, when nearest the sun, or a velocity twenty times as fast as that with which the earth moves in its orbit. Most comets acquire so great a speed in their course around the sun that they are forced into distant space along paths whose form will never permit them to return. Their orbits, or paths, are open curves, either hyperbolas or parabolas. These are obtained by heavenly bodies which travel, because of the sun's attractive force, with a velocity exceeding 26 miles a second, when at a distance of 93,000,000 miles from the sun. Again, when these bodies move with less speed than this

648 IMPROVEMENT ERA. they are compelled by law to travel periodically around the sun along closed curves, elliptic in form. If these curves are quite circular, as in the case of the earth's orbit, the sun resides near their center, and consequently bodies moving along them will travel with an almost uniform speed. However, if these curves are greatly flattened, with the sun near the middle of one end, as in the case of the orbits of most periodic comets, the velocity of bodies moving along them will become very great in the sun's end of the curve, and very slow in the other end. There are thirty comets which travel along curves of this kind, and which make their regular periodic trips around the sun in less than one hundred years each; and, too, there are thirty more comets, moving periodically, which take a longer time than one hundred years to fall from space and sweep around the ruler of our solar system. As in geometry a circle can be determined by any three points not in a straight line, so in celestial mechanics the orbits of heavenly bodies can be determined by three different observations. It is found that the planes of orbits of comets are tilted at a greater or lesser angle from the plane of the earth's orbit, which fact makes a collision with the earth more or less difficult. The chances for such a collision are fairly told by Professor Newcomb, as follows: So small is the earth in comparison with celestial space, that if one were to shut his eyes and fire at random in the air, the chances of bringing down a bird would be better than that of a comet of any kind striking the earth. Should such a collision occur, the results would not be so disastrous as one might expect, on account of a comet's mass and density being insignificant. Comets are not dense enough to shut off from view the stars which lie behind them. And comets passing close to some of the moons of our planets have hardly disturbed them, and yet have been materially influenced themselves. Professor Young is of the opinion that if the largest comet were condensed into an iron ball, its diameter would not exceed one hundred and fifty miles. He further suggests that the mean density of a comet is only about one-six-thousandth part of that of the air at the earth's surface, and that a comet's tail is less dense than the best vacuum we can make by any means of science.

COMETS. 649 Although the mean density of a comet is extremely low, yet it is possible for it to be made up of dense particles at great distances apart. It is commonly thought, though not definitely known, that this is true, and that a comet is largely composed of "meteoric bricks or pin-heads" some distance apart, engulfed in hydrogen and other gases, "in which light is produced either by electric discharges, or by some different action due to the rays of the sun." This action becomes more vigorous as comets approach the sun, and may account, in part, for their greater light at that time, as well as for the various forms and positions of the comets' tails. Again, if these meteoric particles would clash with the sun, would there be a tremendous crash? Professor Young answers this question in the following language: The cometary particles would pierce the photosphere (apparent surface of the sun) and liberate their heat mostly below the solar surface, simply expanding by some slight amount, the sun's diameter, and so adding to its store of potential (energy about as much as it ordinarily expends in a few hours and postponing by so much the date of its solidification. There might, and very likely would, be a flash of some kind at the solar surface as the shower of cometary particles struck it, but probably nothing that the astronomer would not take delight in watching. An experience of this kind nearly happened in 1843, when a great comet passed nearer the sun than has any known body, all but grazing it surface. Encke's comet (1819) is the only one at present that shows any great tendency to fall into the sun. Its period is only three and one-third years, the shortest period known. Up to 1868 each revolution has been two and one-half hours shorter than the next preceding one; since, the change has continued at only half that rate. It is now at a distance of 32,000,000 miles, which is less than one-fourth its distance ninety-one years ago; its diameter is now only 3,000 miles, while then it was more than ten times as great. This gives an excellent example, too, of how comets waste away on their extensive journeys among the various celestial bodies, each of which exerts its gravitational force upon their unsubstantial forms. Some of their wandering gases have, without doubt, been added to the earth's atmosphere, and some of their lost particles to the dust sometimes collected on lofty Alpine snows,

650 IMPROVEMENT ERA. Several famous comets exhibit phenomena of unusual interest, but it may suffice to conclude this article with a few miscellaneous statements regarding "Comet A 1910," and the famous Halley's Comet. The name "Comet A 1910" designates that this is the first new and unexpected visitor entering the solar system this year. If named according to custom, and in the order in which they are discovered, other new comets of this year will be called Comet B 1910, Comet C 1910, etc. On the morning of January 17 came the announcement of the existence of Comet A, from Johannesburg, South Africa. The next day it was observed at the Lick Observatory, where at its brightest it was visible to the naked eye in the day time. Only one or two comets a century have been so bright. For about ten evenings, near the setting sun, it was an easy and attractive object to the naked eye, except when hidden behind a cloudy sky. As it continued in its sudden flight, its total length ranged from a few degrees up to one-fourth the distance across the visible heavens. At first it was straight, with its head downward, and its tail extending upward directly away from the sun. Later it resembled a large feather with its upward end bending to the south. It came up from behind and within 10,000,000 miles of the sun, looped around it quickly, and then receded in a line almost directly away from the earth. The earth's motion placed it behind the sun again. It will continue there until the earth has crossed its orbit, when it may possibly in several months to come, be detected again in large telescopes. Edmund Halley, from whom the famous comet of 1682 gets its name, believed as strorgly as any one could in Sir Isaac Newton's theory of gravitation. He persuaded Newton to prepare his immortal Principia, and bore the expense of its publication. In accordance with its principles he computed the orbits of at least two dozen comets. On comparing the orbits of the comets of 1531, 1607 and 1682 he was struck with their similarity and began an investigation to see if they were but one comet, whose period was seventy-six years. Convinced later of this fact, he boldly staked his reputation upon it by predicting the return of his comet in 1759. Knowing that he would be in his grave at that time, he expressed a hope that "posterity would not refuse to acknowledge that this was discovered by an Englishman." Nothing was known at that time of the existence of Uranus and Nep-

COMETS. 651 tune, but in his calculations he allowed more than a year for the retarding effect which the planets Jupiter and Saturn would have upon his comet. Astronomers began to watch for its return in 1757. A French mathematician, Clairaut, showed by skilful investigations, that it might appear on April 13, 1759, but as he had omitted some small terms in his calculations, the true time he said may differ from this by as much as a month. The actual time of its perihelion, or its nearest approach to the sun, was on March 13, 1759, just within the limit named. Thus a great truth was verified, and thus were hushed forever the opponents of Newton's theory of universal gravitation. Historical records are now at hand which show that this comet has visited the solar system at least twenty-five times, the first of which was in the year 11 B. C, and the last in 1835. At its last visit its total length was about as long as the handle of "the dipper." In some peculiar way it lost its tail at perihelion, but soon was supplied with a new one, which originated from a different place from the first, and which possessed at different times various interesting characteristics. The present visit promises to be a more brilliant and a more interesting one, as natural conditions are more favorable, and as highly improved instruments are in use. Dr. Wolf, of Heidelberg, had the honor of photographing Halley's comet on August 28, 1909, and of announcing its twenty-sixth visit, September 11, at which time he had fully verified his previous observations. After Wolfs announcement, two plates at Greenwich of the date of September 9 were inspected again, and sure enough showed slight impressions of the comet's existence. Perhaps the first sight of it visually was through the 40-inch Yerkes telescope, on September 17. The observed place of the comet differed from the computed one by only 24" in right ascension by 4' in declination. The comet passed behind the sun in March, 1910, placing the earth, the sun and the comet almost in line on the 25th. In April it came out from behind the sun's rays, curved around in its orbit at its greatest speed, and swept down between the earth and the sun, again to place these three objects in line on or about May 18. On that date its head will pass within 14,300,000 miles of the earth. Its tail di. rected away from the sun, may then engulf us. As we merge into this experience of a lifetime, we find a series of eager astronomers,