ABOUT COMETS.

Patea Mail, Volume V, Issue 499, 11 February 1880, Page 2

ABOUT COMETS.

“ Stranger of heaven, I bid thee hail ; Shred from the pall of glory riven, That Qashest in celestial gale, Broad pennon of the King of heaven. “ Whate’er portends thy front of fire, And streaming locks so lovely pale ; Or peace to man, or judgments dire, Stranger of heaven, I bid thee hail.” Hogg. In t the first 1850 years of our era, says Mr Hind, 697 appearances of comets were

recorded, 80 of which occured in ille first half of this century. A large _ proportion of these latter were telescopic, and, as numbers of small comets, that return at short intervals, have rewarded the searching of astronomers in the last thirty years, iFis probable that, on an average, two comets are seen every year by scientific observers. But wonderful displays, like that which we have seen of late, are comparatively rare, averaging, perhaps, six in a century. But it is, of course, more than probable that not half of the comets that approach the sun are ever seen by man. Professor Mitchel says that there is no reason to suppose that comets are not numbered by millions! This being the case, it is amusing to bear people say, when a comet appears, that they have not seen it predicted in the Almanacs. The idea of calculating the elements of a comet’s orbit, and of predicting its return, is now in its two hundredth year, having been concieved l>3’ Newton in the end of IGBO. Since then about 200 comets have been seen, which, had they all been seized upon by compntists, would have been but few among the ‘ millions,’ especially as, in many cases, compulation would only show that the visitor had come for the first and the last time in eternity. It will he obvious, therefore, why there was nothing about the present comet in the Almanacs. When the world is live or six millions of years older, probably a considerable proportion of the visits of these mysterious wanderers may be predicted, but, should man endure upon the earth for unimaginable years, he will still be liable to surprises of the kind that the world lias just experienced. Before going further, lot ns premise that comets, like sun, moon, and stars, rise and set, and that their nightly disappearance is not due to their motion in space, but to the revolution of the earth. The motion of a comet must be marked by observing its place among the stars each night, and however rapid that motion may be, no change of place would he observed during the lime that one would care to gaze, except, in very tare cases, a kind of flickering in the tail. We must also premise that comets pass round the sun, or from side to side of him, just as Venus docs, or as the moon appears to do. When the moon is on the west of the sun she rises, and sets, ns ; old moon,’ before him. \\ hen she passes to the east, she vises and sets as ‘ new moon,’ after him. In the same way comets double heaven’s mighty cape,” but while the moon passes the stm in one direction only, comets pass him in every concievahle direction. We premise also that it is impossible to tell the course of a comet until it has been observed several nights. Sir John Hcrschal says—With the third observation we nail him.”

Having set ourselves right in these elementary matters, we will, for convenience of treatment, divide comets into two classes, namely those that belong to the Solar System, and those that do not. Comets that are not permanent members of onr System, but only stray visitors from the outer universe, are known by tire nature of the curve they describe in approaching tire sun, and in retreating from him. They move on a patn which, if followed on to infinite distance, would never curve so far as to meet itself again and enclose a space. Suppose that one of these comets approaches the sun from the North, if we trace hack its course we will find that as that course recedes from the sun it becomes continually less and less curved, and more ami more like a straight line. If wc watch this comet when it has ‘‘doubled the cape,” and is retreating from the sun on the South, the course again becomes straighter and straighter. Obviously, then, these two prongs, so to speak, of its pat.li, that by which it came and that by which it went, can never meet “ at the other cud,” or ever have another end to meet at ; for by continually straightening out they get further apart. (That of; course- is a popular statement. A scientific t reatmeat of the hyperbola and the parabola urns', be sought in some treatise on conic sections). But some reader may suggest that the comet might turn round another sun, ns it did round ours, and then come back. For arguments sake, we will suppose it to do this, but must premise that tiie probabilities are millions of millions to nothing against the supposition. Suppose, then, that it travels round another sun. the nearest one, namely, the brightest of those two stars that point to the Southern Cross, and (hen returns, when will it arrive the second time ? This nearest star, or other sun, is 210 thousand times as far olf as onr sun is. Under the law of gravitation it would, Mr Proctor declares, take the comet more than eight millions of years to pass from sun to star, and, of course, as long to return from star to sum If. therefore, men were certain ’(which is impossible) that the present comet was steering for that nearest star, ami if they were certain (which is also impossible) that it would return to our sun, our somewhat remote posterity, sixteen millions of years hence, would have the satisfaction of knowing that the comet of 1880 was within a few hundred thousand years of its return. But if, as is infinitely more probably, our comet made for some other than the nearest star, the _ period would, of course bo longer; and if it made for any, except ten or a dozen, of the myriads of stars in heaven, the period for its imagined return, or for its imagined previous visit, would be removed from the present by more hundreds of millions of years than pliyicists, or even geologists, allow for the age of the earth. But from this supposition, which he have allowed for argument sake, we return to the statement that comets which, in their journey to and from the sun, describe curves that cannot meet at “the other cud,” do not belong to the Solar System, and visit it but once.

The other class of comets, to which wc now come, are those which never go beyond tho range of the sun’s attraction. They travel in courses that enclose a space. Their curves, when continued far enough, always meet. The majority of comets probably belong to.this class. “ Of about 200 comets whose orbits have been obtained with more or less accuracy, 40 appear to have described ellipses, 7 hyperbolas, and 150 cannot be distinguished from parabolas.” The ellipses indicate that return is certain, the hyperbolas that it is impossible, and the parabola also indicates that return is impossible. But it is impossible to prove that any of the 150 orbits mentioned arc of that form ; for the parabola is the turning point between the other two curves, a fraction more “bend” and it would lie an ellipse, or true oval, a fraction less and it would be a hyperbola. The

parabola is of one form only, while the other curves have infinite varieties. The chances, then, are that the great majority of the “ 150 orbits not to be distinguished from parabolas” arc not really of that figure, but only very near it on either side. Now, as 40 ellipses have been proved, and only 7 hyperbolas, it is reasonable to suppose that most of the 150,” that are not real parabolas, are ellipses, very long ones, but still, turning ultimately into themselves and returning to the sun. Of the 40 comets proved to move in elliptic orbits, and therefore to return regularly, a number are small and of short periods. One, Encke’s, completes its orbit in three years and four months. Its least and greatest distances from the sun are, respt clively, 80 and 255 millions of miles. It is only a small patcli of vapour, and has neither head nor tail. Every revolution period is some hours shorter than the preceding one, which is one of the most interesting facts in astronomy. Another telescopic comet of short period is Biela’s. It approaches nearer to the sun than the earth does, and retreats to more than live times the earth’s distance. It’s period is six years and nine months. In 184G, this comet broke in two ; its parts were widely separated when they returned in 1852. From that time it has been lost, and tbe most careful search has failed to detect it. It crosses that part of the earth’s orbit that wo occupy at the end of November. When the earth passed tiie spot on Nov. 27, 1872, there were tremendous showers

of shooting stars, more than sixty thousand being seen in a few hours at one place. These were small masses of matter that were set on fire by tbe friction of rushing into our atmosphere. They, however, were not the comet ; for they were three months behind, but they were travelling in its track, and were in some mysterious way associated with it. As a contrast to these short periods, we may mention Halley’s comet, which appeared in IGB2, and was calculated by Halley, Newton’s assistant, to return in 75 years. “ Itemembor,” said be, “ when it returns, that an Englishman was the first to predict the return of a comet.” It has since returned twice, as Halley foretold, and a reference to history proves that it has been seen a number of times previously, at intervals of 75 years. It recedes from the sun about B,SOUmillion miles. Donati’scomet that appeared in 1858 is expected to return in 2,20U years. This wanderer will reach a station from which the sun will appear as a yellow star among the other stars of heaven ; hut that will only he a step on the journey that it could take without casting off its allegiance to the sun. A comet journeying towards the nearest star might return alter an absence of seven millions of years, and if it were to cruise in any other direction the voyage might he vastly prolonged. But when the period exceeds a few thousands of .years there is uncertainty about the return, for the curve is difficult to measure, and “cannot be distinguished from a parabola.” Side by side with these immense distances, let us hear in mind the terrible proximity to the sun which comets, at times, endure. Newton’s comet passed, he say r s, within 150 thousand miles oft lie sun, attained a velocity of one million miles per hour, and endured a heat 2,0-. , U times greater than that of redhot iron. Thu comet of 1848, says Sir John llerschel, approached within LUO thousand miles of (no sun, and was subjected to 47 thousand times the heat of the tropical sun, or to 28 times the heat at the focus of a len.se 82 inches in diameter, which melted agate and rock crystal and diffused them into gas.

Comets are probably the largest bodies in the Universe. Tbe one last mentioned bad a tail 200 millions of milos long, and readied over two-iiftlis of tbo visible sky. The tail of the present comet must, no doubt, bo measured in tens of millions of miles. Lint ail comets have not tails, and those that have, only put them on as they draw near civilisation. It is comets’ etiqnette not to carry their tails behind them, but to keep them pointing away from the snn. As they come their tail is behind them, as they turn round it is at right angles to the line of motion, and ns they recode it goes first. According to the spectroscope, the tail shines by reflected sunlight, and must, therefore, one would think, be material, or it could not reflect. But there is enormous difficulty in believing it to bo material and ponderable. Consider what is implied in having tbe tail pointing from the sun. Take the comet of 184-3, with its 200 millions of miles of tail. It —the head—whirled round the sun in two hours. The end of the tail would therefore have to sweep half round a circle the diameter of which was 400 millions of miles. In other words, the tail would have to travel about seven times as far as from the earth to the sun in two hours. This is impossible for a material tail under Hie law of gravitation ; for the nearer the sun the more rapid the motion, whereas with the comet it would be the reverse. But Sir J. Herschel says that fresh matter was continually shot out from the head of the comet, winch thus kept supplying itself with new tails, and that so rapidly that it always had one opposite the snn. This is simply incredible ; for it requires an infinitely greater velocity than tbe sweeping round of one tail would require. On this latter theory, the “shutout” matter would have to travel tho whole length of the tail in the lime that, on the sweeping round theory, it would take the end of the tail to move its own diameter. Besides the great astronomer has forgotten io tell us why the cast-off tails are never seen, and where all the “ new matter” for tails comes from. A great authority says there may not be as much solid matter in tho largest comet as would fill a snuff-box. But every astronomer has his pet theory on this inscrutable subject, and objections can be heaped up against them all. The plain truth is that nobody knows anything about it, except that tails, as tails, do not obey the law of gravitation, and that faint stars, that the least atmospheric haze would hide, have been seen through 50 thousand miles of tail. But our space and the reader’s patience warn us to have done. Wo will only add, that the superstitions and fears that tinge all tho poetry on the subject of comets —as witness the lines from the Ettrick Shepherd above—are unsuitable to this age. We shall be sorry for the eomol limb runs foul of the earth, and as for “ pestilence and war,” dry summers and death of. kings, famine and potato-blight, we are content again to agree with the younger Herschol—“ this is all wild talking.”