THE RETURN OF HALLEY'S COMET BY S. A. MITCHELL, Ph.D. (Assistant Professor of Astronomy, Columbia University) IN all ages of the world's history the arrival of a comet has attracted widespread attention, and now with the return of Halley's comet we are but experiencing a revival of this perennial interest. The appearance of one of these monsters of the sky suddenly coming without warning was naturally looked upon by our forefathers as the harbinger of war and catastrophe, and we have many references in literature to their baneful influences. In Homer's "Iliad" we read of the "red star that from his flaming hair shakes down diseases, pestilence, and war." Shakespeare gives us the lines, When beggars die, there are no comets seen, The heavens themselves blaze forth the death of princes. THE OLD IDEA OF COMETS AS PORTENTS Those who remember the great comet of 1882 will recall that many said that it was the cause of the war that England was then carrying on in Egypt; and our own great Civil War was ushered in by Donati's splen HALLEY'S COMET ON FEBRUARY 3, 1910 Yerkes Observatory) did comet of 1858, and by the comets of 1860 (As photographed by Professor E. E. Barnard, of the and 1861. Such coincidences can be numbered many fold; and it is but natural that there should have grown up in the popular covered before the invention of the telescope mind down through the centuries the con- 300 years ago. At the present time five viction that a comet brought in its train dis- or six comets are discovered yearly; but asters of all kinds,-war, murder, and sud- most of these are faint and can be seen den death. Undoubtedly this old supersti- only with the aid of a telescope and are contion has been the cause of our present inter- sequently known only to the astronomer. est in comets, and the year 1910, with its If each comet brought a war along with it, return of Halley's comet, is by some looked it would. indeed, require the continued situpon with fear and dread, for does not this ting of The Hague Conference! The abcomet of Halley's come so close to the earth surdity of this notion was recognized 150 that we are to sweep right through its tail? years ago. One author of that period says: What will happen if the astronomers have "If war is caused by the bile of a sovereign made a slight mistake in their calculations becoming heated by the approach of a comet, and the comet should come into collision then in order to preserve the peace of nations with the earth? With the earth traveling a court doctor should be employed, who in space at the great speed of 181⁄2 miles should counteract the action of the comet per second, and the comet in the opposite direction with a velocity even greater, such a head-on collision would be appalling; the earth might possibly be blown to pieces! Altogether we have records of about one thousand comets, half of which were dis by the application of sundry doses of rhubarb." There have, indeed, been remarkable comets seen in years when a great war was being waged, or a comet may have appeared a year or two previous. There have also been splendid comets in the sky when there was no attendant war, and likewise there have been great wars without their attendant comet. We do not recall any serious outbreak of hostilities following in the train of Coggia's fine comet of the year 1874, and certainly no one can point to a remarkable comet at the time of the recent Russian-Japanese war; and that surely was a great war. It would, indeed, be remarkable if wars and comets did not at times appear together, but to imagine that a heavenly body of such small weight as a comet could be able to disturb the affairs of men is perfectly absurd. This has even less foundation than the superstition that the moon has an influence on weather, for all the statistics of modern science show that the weather is absolutely independent of the moon. HOW NEW COMETS ARE DISCOVERED New comets are usually discovered by an astronomer after careful and diligent search with a telescope of low power. Such a quest demands an almost infinite amount of patience in nightly scanning the heavens up and down in the hope of detecting a stranger in our midst. So close a watch is kept that seldom does an intruder escape the eager eyes of the sentries and attack the citadel as happened with the first comet of the year 1910. Comet A, 1910, eluded all eyes till it became very bright and quite close to the sun, and an astronomer was not the first to see it. Sometimes a comet is accidentally found on a photographic plate exposed for some other purpose, such a one being the Morehouse comet of 1908. If the comet is not a new one, but the return of one already known, it is possible to direct the telescope. to the point in the sky where it is expected, and a long exposure photograph may detect it. Halley's comet was discovered on September 11, 1909, on a photograph taken for the purpose by Prof. Max Wolf, of Germany. At the time the comet was very faint, and looked exactly like a very small star. Up to the time of Sir Isaac Newton nothing was known of the behavior of comets. Some thought they took their origin from the sun; still others that they might have been volcanic matter thrown off from the moon, while others imagined they might be phenomena of the earth's upper atmosphere. Comets are much too large to be the result of volcanic action; still modern science has no adequate explanation of where they come from, though it seems certain that their home is in the solar system. After Newton had firmly established the law of gravitation and had shown that all the planets and satellites of the solar system obeyed it, he inquired whether comets did not do likewise. While wondering over these matters the great comet of 1680 appeared and gave him the chance, and he showed that not only did comets obey gravitation and travel about the sun in obedience to it, but he explained how, from observations on three nights, it was possible to calculate the comet's path. If the comet moves in a closed curve it is called "periodic"; it returns to visit the sun at short or long intervals, depending on the size of the curve, and it then moves in an ellipse. Every child knows that an ellipse is drawn by sticking two pins firmly in a piece of paper, then taking a piece of string, joining the two ends together and tracing around with a pencil. In such a curve does a planet like the earth, or a periodic comet, move, and with the sun at one of the foci. The earth's path approximates much nearer a circle than the orbit of such a comet, but none the less they both move in ellipses. Following the method of Newton, and, as he tells us, after a prodigious amount of labor," Halley, in 1705, published the orbits of no less than twenty-four comets. Of these there were three that seemed to have the same paths about the sun,-i. e., their distances from the sun when at their closest points and the inclination to the ecliptic were the same. These were a comet observed by Peter Appian in 1531, one observed by Kepler in 1607, and one which appeared in 1682. Moreover, the celebrated comet of 1456 seemed to fit in with these three and seemed to point to the same comet returning after an interval of seventy-five or seventysix years. When would be the next return? Halley was keen-sighted enough to see that Jupiter or Saturn might accelerate or retard the motion of the comet if it came near one of these big planets, and by a guess, having in it the element of inspiration, he predicted that the end of 1758 or the beginning of 1759 would see the return of the comet; and, being proud of his nationality, he called upon "candid posterity to verify the claim. which was first made by an Englishman." Halley died in 1742. As the time for the comet approached the greatest enthusiasm was aroused in the prediction. In the meantime mathematics had been greatly improved and the art of calculating vastly facilitated. Clairaut, the Frenchman who took up the problem, found that the comet would be retarded 100 days by the action of Saturn and 518 days by Jupiter, or 618 days altogether, and he gave the date of passing closest to the sun as April 13, 1759. The comet was discovered on Christmas Day, 1758, and passed the sun just one month before the predicted time, at magnificent triumph for exact astronomy. The comet was a splendid object in the skies and likewise again at its appearance in 1835. This comet is now close to the earth and sun and is of the greatest interest to the astronomical world. THE PERIODICITY OF HALLEY'S COMET As this was the first periodic comet to be observed, the scientific interest in it has been very great, and previous visits to the earth have been carefully investigated. As is given in Table I, authentic returns of TABLE I-PAST APPEARANCES OF HALLEY'S COMET HALLEY'S COMET AS IT APPEARED IN 1835 (From a sketch by Struve, probably somewhat exaggerated) 76.5 former year William of Normandy came over to England, and the Bayeux tapestry 78.9 gives a representation of the popular belief 77.6 of the connection of the comet with the con76.2 quest of England and the death of Harold. 77.0 75.2 74.9 76.5 CALCULATING THE RETURN OF A COMET 76.7 74.5 the comet date back without a break to the year 11 B. C. Even 225 years previous there was a comet observed that was undoubtedly Halley's, but the records are not absolutely conclusive. The dates give the times of perihelion passage, or the time when the comet was nearest the sun, and instead of placing the day and month of the year the times are expressed in decimals of a year, 1910.3 meaning that the comet came to perihelion three-tenths of a year after the first of January. The past appearances of the comet most worthy of note are those of 1066 and 1456. In the At the last appearance of the comet in 1835-36 its position in the sky was measured by many astronomers. From its motions as exhibited in these measures it was possible to calculate the comet's journey off to hundreds of million of miles from the sun. As the comet traveled through space it went close to some of the bodies of the solar system. These planets pull the comet exactly in the same way as does the sun, but with less force, since they weigh much less than the sun. The planets might accelerate or retard the motion of the comet, depending on their relative positions. These "perturbations" of the comet's motion it is necessary for the from the earth. In these HOW ΤΟ FIND HALLEY'S The diagram on opposite page shows the relations of the paths of the earth, comet, Venus, and Mars about the sun. The planets all move about the sun in the same direction, which is opposite to that taken by the hands of a clock; the comet, unlike the planets and all other comets of short period, moves about the sun in the diametrically opposite direction. The diagram shows that the comet is nearest the sun on April 20. To find the distance with respect to the earth, join simultaneous positions. For instance, on March 26 the line from earth to comet passes through the sun, and if we would place ourselves at the earth in the diagram, and look at the sun, we would see that the comet on the date moved from the left of the sun to the right of it. We all know that a body to the left of the sun as we face it sets after the sun, and if to the right of the sun the body is visible in the morning sky before sunrise. Hence on March 26 Halley's comet passed the sun and became a morning object. But it gets away from the line of the sun very slowly, and it will be the middle of April before it is at a far enough angle from the sun to be well seen. Following successive positions it will be evident that on May 1 comet and Venus are very close together, and consequently they should present a magnificent spectacle in the morning sky, especially so as Venus is then almost at her greatest brilliancy. Following still along with simultaneous dates we see that on May 18 the comet comes directly between the earth and the sun. Its motion carries it by the sun into the evening sky, where it HALLEY'S COMET, THE SUN AND THE EARTH astronomer to calculate, and if the comet happened to pass close to a great planet the perturbations might be very great. By referring to the table, it will be seen that the time occupied by the comet in returning changes considerably, the difference between the least and greatest is as much as five years. At the 1835 appearance the calculations were very simple, but for the 1910 appearance were exceedingly difficult due to the close approach to Jupiter. In spite of these difficulties two Englishmen, Cowell and Crommelin, calculated the time of perihelion passage within three days of the actual time, and this, too, when the comet took nearly seventy-five years to make its return! By pointing the telescopic camera to the position in the sky calculated by them, Wolf discovered the comet seven months before it should be closest to the sun, when still at a distance of three hundred millions of miles The brilliancy of a comet as it appears to our eyes depends on two factors: first, its distance from the sun, and, second, its distance from the earth. The nearer to the sun the more does our sun act on the comet and increase its intrinsic brightness, and, of course, the nearer any body is to our eye the brighter will, it be. Halley's comet is nearest the sun on April 20, when of itself it will be most luminous. But after that date it gets closer and closer to us till May 18. Immediately after that date the comet should be very brilliant, and it will be favorably situated in the evening sky for us to see it. The comet in 1835 was less bright than it had been in 1759, when it had a tail 50 degrees in length, not because it had intrinsically decreased in brilliancy but because the earth was unfavorably situated in its orbit to give us a near view. While nothing is known absolutely of how bright the comet will be, astronomers are generally agreed that it will be a splendid object readily visible to the naked eye, with a tail at least 30 degrees in length. In fact, the comet will be seen better without a telescope than with one, and for those who are not astronomers a look through a big glass would be disappointing. THE PATH OF HALLEY'S COMET THROUGH OUR distinctive about its appearance in the sky, but that its tail may change radically from day to day. Since 1882 the modern spectroscope has been developed wonderfully; and this instrument of research used upon Halley's comet will solve many interesting problems. It has told us that comets' tails shine partly by reflected sunlight and partly by light of the gas composing the tail, and already we know that the tail of Halley's comet has in it the poisonous gas cyanogen. PASSING THROUGH A COMET'S TAIL What will happen when we pass through the tail on May 18? Will the cyanogen be enough to poison us? Or if we escape will vegetation be blighted or disease be brought? We shall be directly in line at 9 P.M., Eastern standard time. The Japanese people will be directed towards the sun and comet and will form the central part of the bombardment of the particles of cyanogen gas. However, the modern theory of the tails of comets tells us that though their size is enormous their weight is exScience has taught us much concerning cessively small, and as a result the number comets, but there is still much to learn. The of particles per cubic mile in the comet's tail comet of 1882 was the first in which the is almost vanishingly small. The comet of photographic plate was used, and it showed 1882 was so situated that we could see the wonderful possibilities of photography through ten million miles of its tail, yet when applied to the heavens. Since then the stars shone through it with undimmed greatest of all photographic astronomers, luster. Hence, though there may be cyanoProf. E. E. Barnard, of the Yerkes Observa- gen gas in the tail it is there in such small tory, has given us superb pictures, which quantities that could we have a cubic mile. show that a particular comet has nothing of the tail concentrated into a glass beaker |