the starry sky, visibly changing from day to day and moving across the "fixed" constellations, striking terror into the hearts of all men, causing emperors to abdicate,* wars to be fought and sceptics to turn religious. The most splendid of these "tailed stars" were seen in the years 243, 891, 1066, 1145, 1264, 1402, 1532, 1556, 1661, 1680, 1682, 1739, etc., some of them-that of 1402 for instance-being so bright and near as to be seen in full sunlight, others remarkable for the peculiar form of their tails or the bright nucleus.

Among these there was one-the one of 1682-which was seen by a young man, Edmund Halley, then twenty-six years of age. Halley was greatly interested in the wonderful apparition, as had been millions of people before him; but he set himself a task which was nothing less than gigantic-namely, to compute the elements of all comets seen in the two centuries-from 1480 to 1680-and to lay out their paths. How he succeeded we shall see immediately.

Many people had looked at this great comet which visits us every 76.8 years

*According to good authority the superstitious Charles V. abdicated after the great comet of 1556 had appeared, because of belief in a divine sign specially sent for his benefit.

for a few months and then flies away again to the outermost confines of the solar system-beyond the orbit of Neptune-fully 3,220,000,000 miles from us. Of course, people did not know the comet to be a kind of brother to the earth, children both of the same father

the sun. All they could understand. was that a wonderful star with a swordlike tail hung gracefully but threateningly over the earth; that it appeared from nowhere and gradually vanished to nowhere, leaving terror in its wake. Again and again it appeared-its absence of 76 years between visits being sufficiently long to cause nearly all who had seen it on its previous visit to be dead and buried when it called again. So it continued to spread terror century after century until Edmund Halley started to take away the mystery and incognito and put it in its proper place among the members of the sun's family. He camped, so to say, on the comet's trail and calculated the curve of the orbit, fixing it as a parabola. He took up other comets and analyzed their motions, coming in the course of his labor across one-1607-which had the identical curve, speed and general elements of the comet of 1682. Halley was

astounded. He calculated further back and came across the comet of 1531, where he found the same elements again. He then made the daring step which forever placed his name among the greatest astronomers of modern timeshe risked his reputation and predicted the return of the great comet of 1682 for the year 1758. Feeling certain that he would not live until then he said to the scientists of Europe when making his prediction: "And when it really appears, do not forget that it was Englishman who made the first prediction of the return of any comet."

an

result that a great number were found to be periodical, nine of them revolving within the orbit of Saturn. Halley's great comet was talked of everywhere.

Its next return to perihelion in 1835 was predicted with greater accuracy and a higher degree of confidence; the date set for the event by M. Lehman, published on July 25th, 1835, was the twentysixth of November. The place of its ap-. pearance was calculated by M. Rosenberger and on the fifth of August the comet was actually discovered as a faint telescopic nebula within a degree of the place predicted. On the twentieth of August it had come so much nearer as to be generally visible and on the sixteenth of November passed its perihelion, then curved around the Sun, became first larger and larger, then dimmer and dimmer and finally disappeared on the fifth of May.

Halley died sixteen years before the time scheduled for the reappearance of the comet named after him, but as this time approached astronomers and mathematicians took up the questions of disturbances by the big planets and came to the conclusion that Saturn would retard the comet's motion one hundred days and Jupiter 518 days, or a total of 618 days beyond the date of its perihelion passage as calculated on the basis of speed, distance and attraction of the sun. Clairant, a famous mathematician, fixed this date for the thirteenth of April, 1759, allowing one month either way as a possible error. The comet was actually seen in the big telescopes on Christmas Day, 1758, became visible to the naked eye in January and passed its perihelion-the point of its orbit nearest to the sun-on March 12, 1759. This was the first ocular proof that the comets-or at least some of themreally belong to the solar system.

If any lay person had seen this comet between August twentieth and October second, 1835, he would not have known it to be one, for it appeared in the form of a slightly oval nebula, without a "tail" and having a tiny dot of bright light in the center of the nebulous mass. Soon after the date named above, the tail began to be developed and increased so rapidly that on the fifteenth of the month it stretched over fully twenty degrees of the visible sky. From that time it decreased with equal rapidity until on November fifth it was

but 21⁄2 degrees in length. During the time of the perihelion passage November 14-18the tail was entirely absent, but grew again as the comet began to recede from the sun. It remained visible until the following May and its constant changes of appearance were carefully studied by astronomers all over the world.

Ever since Halley showed that this great comet

of 1607 and 1682 was a periodical visitor to the neighborhood of Our own planet, astronomers and mathematicians have endeavored to trace its former appearances to the earliest historical times, and a really accurate list has been prepared giving every return of the comet down to May 15th, 240 B. C. The average time for each complete orbit around the sun, calculated from the 29 known returns is 28,031 days, or 76.8 years; while the quickest return was in 27,351 days and the longest trip took fully 28,990 days. revolution now nearing completion is calculated as even shorter than the shortest hitherto recorded-see table below-it will take only 27,180 days. Thus there is a variation between the

The

451 A. D.

451 A. D.

Aug. 26, 1531 A. D.

Oct. 27, 1607 A. D.
Sept. 15, 1682 A. D.
Mar. 12, 1759 A. D.
Nov. 16, 1835 A. D.

April 15-May 1, 1910 A. D.

How is such a remarkable difference to be accounted for? The causes are to be sought in the following explanation: Three hundred and seventy days after its perihelion passage in 1835 the comet crossed the orbit of the planet Jupiter, Jupiter itself was not anywhere near at that time, but one hundred and fifty-two days later reached the crossing point, while the comet was flying straight away from the sun towards the outer boundaries of the solar system. Though it had been speeding away from Jupiter's orbit for fully 150 days as if afraid of being caught, the mighty pull of Jupiter even at that great distance, added to the still mightier pull of the sun at a still greater distance served to retard the comet's speed considerably, so that it did

in February, 1839, of Uranus in March, 1844, and that of Neptune in December, 1856. Slowly, very slowly, receding still further and further, it floated far away into the void beyond the farthest known planet, reaching its aphelion-the greatest distance from the sun-in February 1873. 1873. At this point the comet's speed is very small. Gradually it swings further around the curve and commences its return journey towards the sun. In April, 1889, it crossed Neptune's orbit on the return trip; it needed thus fully thirty-two and one-third years to cover the small arc lying beyond Neptune's orbit. To race over the opposite, equally large arc near the sun it needs just 725 days. Uranus was passed in March, 1902, and Saturn in February, 1907.

not quite reach the great distance from the sun that it had flown the century before. It crossed the orbit of Saturn

The orbit of Jupiter was crossed in April, 1909.

The question most important to the average intelligent reader is naturally: "When can I see it and where?" According to the most accurate calculations the comet will appear in the big telescopes (or at least on photographs taken with them) in October, 1909, near the constellation Orion. About the beginning of February, 1910, it will perhaps become visible to the naked eye in the constellation Pisces. In March it passes behind the sun. Thereafter it will become brighter and be visible during April and the first week of May shortly before sunrise. On May 17th it will again pass the sun and from then on to about May 25th will be at its greatest brilliancy and apparent size. Its distance on May 18th being about 11,800,000 miles, or about one-eighth of the distance between sun and earth will render it a very conspicuous object in the northern hemisphere. Its flight will be swift at this time, as it crosses in six days over the constellations Taurus,