Comets and Their Mystery

New Zealand Graphic, Volume XLIV, Issue 4, 26 January 1910, Page 42

Comets and Their Mystery

By

WALDEMAR KAEMPFFERT.

THE year 1907 was distinguished by the discovery of a comet by Professor Daniel of Princeton Observatory. Although it has been surpassed in brilliance and size by many of its predecessors, Daniel’s comet was by far the brightest object of its kind that we have seen in the northern heavens for twenty-live years. When first observed, on -Tune 9th. it was a faint nebulous spot visible only through the telescope. Rapidly increasing in brightness, it could be seen with the naked eye in Inly. During the latter end of August and the early part of September it was as dazzling as a star of the second magnitude. In the early hours of the morning, from two o'clock until dawn, it was a conspicuously beautiful object in the constellation of Gemini (the Twins), particularly during the first week in September. Its head had a diameter of nearly 236,000 miles, which means that it was nearly thirty times larger than the earth. ' Because the comet was presented to US obliquelv. its tail sponvd shorter than it really wikf;

yet astronomers figured that it must have been at least twenty million miles in length. At the time of its greatest brilliance the comet had a speed of about sixty miles a second, compared with which the swiftest projectile fired from the most powerful modern gun would seem to crawl through space. On September 4th the comet whirled around the c un. A fortnight later it retreated so far from the earth that it could be seen only with difficulty. By the end of Septeml>er the telescope alone could detect it. Thus it made its exit as modestly as it had entered. Will it ever return? Perhaps in some thousands of years it may: ami on the other hand it may not. The astronomers have not as yet completed their final computation of its period. It travelled in an orbit which, although

probably an ellipse, was to us an ellipse of such inconceivably vast dimensions that mathematically it must be regarded as an open curve. Although three observations made on three different nights will usually give three points from which the astronomer ean determine in a general way the character of a comet’s path, the problem of plotting the orbit is one of unusual complexity. The period of Halley’s comet has not yet been definitely fixed, with the result that we know only in a general way that it will appear some time in 1910. Many astronomers are working hard to will a prize offered by a German astronomical society for an exact determination of the path of Halley’s comet. The orbit of Daniel’s comet presented difficulties because the angle made by its plane with the plane of the earth’s orbit was so very small that a line drawn through three points obtained on three successive nights did not differ sensibly from a straight line. When the comet rounded the sun, however, the curve was obviously more pronounced. Once in the toils of the mathematician it becomes possible to follow the movements of the comet in the astronomer’s mind’s eye, even when it has disappeared, and to indicate the very spot in the heavens where it should reappear if it describes a closed curve. When the labour of plotting the orbit of Daniel’s comet is at last completed, it may transpire that it visited tire earth so long ago that its visit has been forgotten even by tradition. Who knows but it may have ushered in some pregnant event when mankind was young. Who knows but it may return to us when mankind is old and decrepit and the earth is entering upon that last stage of its career which will ultimately reduce it to a cold, dead, and desolate world ? Halley and his eomet are inextricably bound up not only* with the history of Europe, but with Newton rnd his law of _gravitation; for Halley was Newton’s pupil, staunch friend, and counsellor. To his persuasive insistence end to bis touching devotion to what he considered his scientific duty We owe the publication of that famous treatise of Newton’s in which the immutable laws of gravitation were first laid down. He became the prophet of gravitation. In accordance with Newton’s laws he plotted the orbit of a eomet that had alarmed the world in 1682, and concluded that it was the same that had shone in 1607 and 1531, and that it would return in 1758, fifty-four years after his utterance. Past the prime of life w hen he made his calculation, ha knew that the triumph of see*ng his prediction fulfilled would be denied him. lie died in 1742 at the age of eighty-five, certain that his forecast would bo verified, and leaving behind him a pathetically patriotic appeal, which reads: “Wherefore, if, according to wliat we have already said, it

should return again about the year 1755, candid posterity will not refuse to acknowledge that this was first discovered by an Englishman.” With poetic fitness tiie comet Mazed forth on Christmas day, 1758. Newton’s law of gravitation teaches us that comets must describe ellipses, parabolas, or hyperbolas, all of which <urves are obtained by cutting a cone in d flerent ways. Since Halley’s time the orbits of more than three hundred comets have been plotted with more or less accuracy, and of these, sixty describe ellipses; 255 parabolas, and two hyperbolas. Of the entire number we may expect to see only the sixty travelling in elliptical orbits: for the others follow open curves which must inevitably convey them far beyond the confines of our solar systeii). The sixty comets which revolve about the sun in closed ellipses return to the same point after periods that vary from three years to several hundred years. On an average two or three periodical comets appear every

year, and three cr four of which are u> expected and will never be seen again. Mathematics in Newton’s law of gravi. tat ion have so thoroughly dispelled tha dreadful divinity which once did hedge a comet that only the possibility of a collision of the earth with some largei fiery wanderer gives us any cause for uneasiness in those unsuperstitious days. A gambler at Monte Carlo, however, la more likely to break the bank than tha earth is to encounter a comet. Two inquisitive scientists, Arago and Babinet, fave computed the possibility of such a meeting. They have soothingly concluded that such a calamity may occur once in about fifteen million years, and that the chances in favour of a collision are roughly 281.000,000 to 1. Although. Ihe earth has i.ever struck r large comet, it has frequently swept through A

comet’s tail. The last passages of this kind occurred il’ 1819 and in 1861. In neither case was anyone the wiser until, long after, the fact was announced by astronomers. It the earth ever does collide with a very large comet it has been asserted that the impact -will develop heat enough to melt granite. The effect on terrestrial life can be- imagined. So remote is the possibility, however, that speculation of this kind is childishly futile. Jules Verne and the modern newspaper are largely responsib’e for the popular belief in such a catastrophe. A conut is distinguished usually by a nucleus, by an envelope called th.; com;;, which surrounds the nue’eus, and, lastly, by its luminous tail streaming behind

the nucleus for perhaps a hundred million miles and mor? as the comet swims Howard the sun. Occupying a volume thousands of times greater than the sunthe question naturally arises, Hew can it body with so vast an appendage sweep (through the solar system without deJanging every planet? Fortunately for jtue preservation of the solar system, a

toniet, so far from being a compact mass, Is often transparent. Stars have been distinctly seen without perceptible diminution of brightness, not only through fthe tail, but even through the nucleus. In structure the tail is a gossamer of (molecules so ghostly that in comparison She filmiest of bridal veils is coarsely Sense and the thinnest haze that hovers cn. the horizon is an impenetrab'e blanket. Indeed, the earth’s atmosphere on (the clearest day is far denser. Hundreds ■Of cubic miles of a comet’s tail are probably outweighed by a jarful of air. A plume of such fairy lightness can hardly .be supposed to remain permanent, and so 5t is not astonishing to find that during its swift journey around the sun a (comet's outlines are incessantly changing. An interval of a few days, or perhaps a lew hours, ma v work wonders in its diaphanous texture. Its path is its only permanent characteristic, indeed, the only characteristic by which it can be purely identified if ever it returns. From all the known facts astronomers have concluded that the nucleus of a comet is merely a mass of meteors, easily dispersed into small groups, or distributes gradually along the orbit, until event(*> filly the comet is completely disintegrate® land extinguished. Astronomical history offers considerable evidence in support Of this hypothesis. Biela's comet, discovered in 182fi, and carefully observed On each return, split into two parts and (reappeared as a curious double comet in 3846. When it revisited the earth in 11852, the two parts had drifted away (from each other, and were separated about one million miles. Since then the Comet has disappeared. Every six and a half years the earth crosses the track of that lost comet. Meteoric showers ithen rain upon us. In these meteors we lee all that is now left of Biela's comet. Similarly, the great comet of 1882 literally lost its head by breaking into four portions, each cf which will some day form a separate comet. Another link in this chain of testimony is presented by ifl.e chemical composition of meteorites (Which have found their way to the earth, a composition which agrees exactly with ifhat of a comet. How large arc the meteorites which a comet? From all that we (Can judge, their size may vary from a grain to several tons. The shoal of ■aneteoTites or ** shooting stars,” through which the earth ploughs in autumn, are certainly but mere grains of matter heated to luminosity by the friction of the earth’.-, atmosphere. Of Buch grains a comet is probably chiefly {composed. As a comet approaches the sun violent eruptions occur in the nucleus. (The matter which is ejected is thrown back in a curve, and forms the brilliant (hollow easing which we call the coma. Sometimes several comas are formed in Succession, and are concentrically collected around the nucleus. Doaiati’s comet of 1858 was ao equipped. Doubtiw* much of the matter which is thus

ejected from the nucleus helps to form lire comet’s tail, but that supposition, justifiable thought it is, fails to explain the startling eccentricities of that tail. A comet is first seen as a hazy patch of light, frequently without any appendage. As it speeds toward the sun it throws out first jets or streamers,

and eventually its luminous tail, which increases in length and brightness as the sun is approached and which trails behind like the smoke of a steamer. When the comet whirls around the sun something very amazing happens. The tail no longer floats behind, lut actually precedes the nucleus, just as if a mighty wind were blowing it from the sun. By all the laws of gravitation it should always point toward the sun. Yet some strange solar force, more powerful even than gravitation, must -e--pel it from the sun. Only within the last few years has the riddle of th it unknown force been solved. Two un-dreamed-of sources of power have been discovered, to which we may attribute all the vagaries of i comet’s tail. Of these the one is the pressure of light, and the other the electrical repulsion of the sun.

The pressure of a sunbeam is never manifested to our eves in the sense that we actually see bodies swayed by its means. Yet a Russian physicist, Lebedev, and two Americans, Nichols ami Hull have proved by actual experiment that light and all other forms of energy radiated from the sun exert a pressure which, on the entire earth, amounts to the considerable total of seventy-five thousand tons. Light-pressure overcomes gravitation- because it acts on surfaces rather than on masses.' Divide a ball of lead weighing one pound into one thousand leaden balls. The entire mass still weighs one pound but the surface exposed to light is enormously increased. If each small leaden ball is »n turn divided into a thousand parts, the weight still remains the same, but the surface subjected to light-pressure is again enlarged. By carrying this subdivision to microscopic minuteness, particles of lead will finally be obtained so vast, in area compared with their mas; that the pressure of light will exactly counter-balance the attraction of gravitation. Consequently each particle will be poised in space absolutely motionless. When that critical point is passed, and subdivision is carried still further, the pressure of light tears each particle from the clutch of gravitation and hurls it out into space. A very distinguished Swedish physicist, Svante Arrhenius, bases an ingenious theory of cometary phenomena on this principle —a theory, moreover, which has gained credence among the more progressive scientists of our time. In order to explain. that theory somewhat more fully, we must know something of the chemical composition of a comet’s tail. By means of an instrument called the spectroscope, which enables a chemist to identify any element by its light when heated to incandescence, comets have been magically transported to our laboratories and analysed with nearly as much accuracy as if they were stones picked up in the road. This scientific sorcery has taught us that the composition of a comet is not unlike that of the blue flame of our gas-stoves. In a word, a comet consists chiefly of hydrogen and carbon combined —what chemists term hydrocarbons. As the comet dashes toward the sun and its temperature consequently rises, the spectroscope reveals the presence of iron, magnesium, and other metals in the nucleus. With a closer approach to the sun, the hydrocarbons split up into hydrogen gas and hydrocarbons of a

higher boiling point. Finally, a time comes when these more refractory hydrocarbons in turn decompose into free carbon in the form of soot. Because the interstellar spaces are airless the

soot cannot burn, bur must accompany the comet in the form of a very fine dust This dust, propelled away from the sun by radiation pressure. constitutes the tail of many a comet. Natural'y, the soot particles will vary considerably in size. Some will be smaller than the little leaden particles of the critical size to which reference has al--rcaily been made. They will be flung back from the comet to form the tail. Soma of the soot particles may be large? than the critical size. They will be jerked forward toward the run in advance of the comet to form what is known as the comet's beard, a rathet rare phenomenon. Because the particles which are small enough to be repelled by sunlight, may not all have the same diameter, and because there are in all probability particles other than those of carbon, it is inconceivable that the dust will be thrown back from Hie nucleus with equal force in all its parts, llenen it may happen that more than one tail will be formed. Thus Arrhenius explains the wonderful comet of 1741, which had no less than five tails, and the three-tailed comet of Donati, which astonished the world in 1858. Newton saw the great comet of 1(180 throw out a tai! sixty million mile; long in two days. Can the pressure of light impel cometary dust with sufficient speed to cover that enormous distance in so short u time? Arrhenius has mathematically demonstrated that p partiels of one-half the critical diameter would travel at a speed of 805,00® miles an hour. Since the dus-t particles under discussion are only one-eighteenth of that critical diameter they will bo vast over the same distance in less than four minutes. The particles which are thus ejected from the nucleus arc neves-s-irily minute; yet their estimated diameter, which may vary from one twentythousandth to one one hundred and-twenty-five-thousandth part o-f an inch, is not less than that of many bacteria. The doctrine of Arrhenius applies only to comets having tails which are repelled with an enengy not exceeding twenty (times the force o-f gravitation. A thir-teen-inch gun charged with the best modern smokeless [sawder cannot l« expects®

to fuc a projectile mote than a certain Dumber of miles. So the light-pressure of the sun has its limitations. In order to explain the occurrence of tails which are ejected from the nucleus with a force that may lie as much as forty times more powerful than gravitation, we must rely vu the tremendous electrical energy of the sun. The modern school of English physicists headed by Prof. J. J. Thomson, Sir Oliver Lodge, and Sir William Crookes, has taught ui> that a hot body, a metal upon which ultraviolet rays are allowed to fall, a Crookes’ tube, and radium, discharge corpuscles with enormous velocity, and that the corpuscles are charged with negative electricity. Indeed, there is some evidence that the corpuscles are themselves what may be termed material electricity. Each corpuscle is about one thousand times smalkir than an atom —the smallest body ■which chemists hitherto supposed could exist. About three hundred thousand chemical atoms laid side by side would measure an inch; yet one hundred thousand of these corpuscles can lie in the diameter of an atom. Compared with atoms they are as a buckshot to a Gothic cathedral. It is generally agreed

The sword and severed heads described l y Ambroise Pare. that the sun is constantly bombarding the universe with countless millions of these infinitesimal charges of negative electricity. Every schoolboy knows that

when the negative pole of one magnet is presented to the negative pole of another magnet, the one repels the other, lienee the sun, in order to repel negatively charged eorpuseles, must itself be a negatively charged globe. When the corpuscular charges from the sun encounter the molecules constituting the gas which surrounds a comet’s head,

they charge the molecules negatively. The result is obvious. Evidently the negatively eharged gas molecules and the negatively charged corpuscles will both be repelled by the negatively charged sun. So terrific is the corpuscular energy lavishly expended by the sun that it is amply adequate to form tails of comets for which the pressure of light cannot be invoked. That the theories of Arrhenius and Thomson are not mere scientific moonshine, but have some basis in fact, the spectroscope testifies. The gases which are so indispensable, if the corpuscular theory is to be accepted, have been traced in many a comet’s tail for vast distances from the nucleus. In Swift’s comet they persisted for three million miles from the head, which means that electrical forces were there at play. On the other hand, the -presence of cometary dust impelled by radiation pressure is indicated by reflected sunlight; for each

little particle of soot is a miniature nrirror that reflects the image of the sun. It may be that other causes contribute their share in the creation of a comet’s elements. Two at least have been definitely discovered <whieh adequately explain what was long a mystery but is really a very simply explained manifestation of cosmic forces.