Science from an Easy Chair

by Sir E. Ray Lankester

Edmund Halley was the intimate friend and passionate admirer of Newton. He paid out of his own pocket for the publication of Newton’s Principia by the Royal Society in 1686, the society having expended all its available funds in printing a great work on Fishes (which shows how at the first, as now, the society cared for the whole range of the study of Nature). Halley was able to show that comets move regularly round the sun, in obedience to the same law of gravitation which controls the movements of the planets and of our earth itself; so that many of them are regular members of the solar system. Halley especially calculated out the form of the orbit of the comet of 1682 as an ellipse, and the time of its journey and recurrence, or “period,” as it is called, which he showed to be about seventy-five or seventy-six years. He predicted its recurrence in 1758. Halley died in 1742, at the ripe age of eighty-six, having, amongst other good deeds, founded the Royal Society Club, which still dines every Thursday in the session. His comet reappeared in 1759, a few months later than he had, owing to incomplete details used in his calculation, expected; but the accuracy of his scheme of its movement was demonstrated. It duly appeared again in 1835, and it is now awaited in the spring of 1910. Halley himself had identified his comet with that of 1607 and of 1531, and lately, by the aid of records from an ancient seat of astronomical observation—actually from China—it has been traced back to the month of May in the year 240 B.C. It has caused consternation and terror times enough since then, of some of which we have record. Finally, it has become the leading instance of the triumph of scientific knowledge and accuracy over ignorance and superstition. Halley’s comet caused great alarm in Rome in the year 66 A.D. A thousand years later (1066) it was seen when William the Conqueror was preparing to descend on the coast of England, and is actually represented in the Bayeux tapestry. A number of men are drawn (or rather “stitched”), with fingers pointed and eyes raised to a shape in the sky which resembles a star-fish with a large triangular-ribbed petticoat attached to it, ending in eight flames or tongues ([Fig. 45]). The picture is labelled “Isti mirant stella.” There is now no doubt, as accurate calculations have demonstrated, that William the Conqueror’s “star” was Halley’s comet—a fact which must give its reappearance in 1910 an additional interest in the eyes of Englishmen.

The shape given to the representations of stars in old pictures and engravings is a puzzle. Why do they represent a star by the shape of a star-fish? No star ever looks like that, or produces a picture of that shape on the retina. The thing is purely conventional. The shape which we call “star-shaped”—a term we apply to flowers and other things—is not in the least like a real star as seen by an unprejudiced person. What one really sees is an ill-defined point of light. The pretended conventional star of ancient drawings perhaps arose from the simple artifice of picturing tongue-like flames around or upon any representation of a fire or a source of light—“to show what it was meant to be.” Then the notions of perfection and symmetry in regard to the celestial bodies led to the “tongues” being arranged for the purposes of draughtsmanship as perfectly symmetrical-pointed rays of a six- or eight-limbed geometrical design—and latterly it is possible that the mystical figure known as the “pentacle” was utilised by astrologers and others as the emblem of a star. However they arose, neither the weird and astonishing representations of mediæval times nor the geometrical decorative “stars” of later date seem to have any relation to an attempt to represent a star as it really appears to the human eye and the interpreting brain behind it.

Fig. 45.—Copied from a portion of the Bayeux Tapestry, showing Harold’s men looking with superstitious wonder at the comet which appeared in the heavens immediately before the departure of William the Conqueror’s expedition to England in A.D. 1066. That comet was none other than Halley’s comet, and has now returned, and is visible from the earth for the twelfth time since the Conquest. (From a lecture on Halley’s comet, delivered in 1908, by Professor H. H. Turner, F.R.S., Halley’s successor in Oxford, published by the Clarendon Press.)

The orbits of comets, says Professor Turner, of Oxford, in a delightful lecture delivered in Dublin in the summer of 1908, from which I have culled many interesting facts and presented them to my readers, “differ from those of the planets in being far more highly elliptical. Our own path round the sun is nearly a circle, so that our distance from him remains nearly the same all the year round; but the distance of a comet from the sun varies greatly from ‘perihelion,’ when it is near, and consequently bright, to ‘aphelion,’ when he is so distant and faint that we lose sight of him.” The sun is not at the centre of the ellipse described by a comet’s path, but is quite near to one end of it, so that comets approach the sun far more closely than do the planets, some taking so close a turn round the sun that the heat from it to which they are exposed is 2000 times as great as that which the earth receives. If the orbit of a comet is really elliptic, then there at last comes a time, though it may be only after thousands of years, when the comet, having rounded the sun at close quarters, and journeyed off into space, has his journey brought to a turning-point at the other end of the ellipse, and begins to draw near again, advancing towards the sun. The length of the orbit of Halley’s comet is about 3255 million miles, and the breadth at its broadest is about 800 million miles, and he takes about thirty-eight years to travel the full length (along the curve) and thirty-eight years to come back again! Other comets have other lengths and breadths of orbit, and take longer or shorter periods to go round. But the conditions of attraction affecting a comet may be such that the return journey never occurs. They may be such that the comet goes on indefinitely travelling away from our sun, until he is caught by some other star, and his orbit changes its shape, with the new sun as attracting centre. These are the “wandering comets” as distinct from the “periodic comets,” which have been shown to conform to Halley’s scheme of their movement and recurrence.

And now some one will ask, perhaps impatiently, “What, after all, is a comet?” We have seen that many are continuously, and others casually, members of the solar system. What do they consist of? Spectrum-analysis shows that they consist chiefly of the chemical element carbon.[6] Though they have weight, and are attracted by the sun, yet they seem to be for all their size and terrifying shape and glare incredibly light and airy things. Herschel declared that the tail of a big comet probably consisted of but two or three pounds of solid matter—diffused, rarefied, and luminous. And the head or nucleus certainly does not weigh many hundreds of tons. In the eighteenth century astronomers observed a comet pass right in among the moons of the planet Jupiter. You might expect the moons to be terribly knocked about by such an impact. They were not; they were not deflected in the smallest appreciable degree from their position and regular movement! One is naturally inclined to look upon the tail of a comet as something like the smoke of a railway engine trailing behind the advancing “head.” As a matter of fact, it does not always trail behind, but is always turned away from the sun, so that when the comet is travelling away from the sun the tail is in front! It is now held that the tail is caused by the radiant energy (light and heat) of the sun, blowing, as it were, the lighter particles from the incandescent head, and causing them to spread out in a long track of variable shape. The photographs of the third comet of the year 1908 show that the tail can vary to an astonishing extent and with great rapidity—that is to say, in four or five hours. It is seen in those photographs as a scimitar-like curved blade, then with a second head or nucleus behind the leading one, then actually bent like the letter Z, and then divided into seven distinct diverging “plumes,” and then it returns to its former simple shape—all in the course of a few days. Astronomers have now shown that there is a close connection between comets and the showers of “shooting stars” or meteors which frequently strike the earth’s atmosphere. It is considered probable that comets eventually break down into streams of meteors, and that their “life” (if one may use that term) is, relatively to that of other heavenly bodies (which are all undergoing change and, in many cases, decay), not a very long one. But there are no facts at present known which enable us to tell whether a given comet is young or old, and it would have been a decided shock had it been found that Halley’s comet, which has so happily spent every seventy-sixth year with us for so many centuries, had “burst up,” or by “indisposition” had been unable to pay his usual visit as expected in 1910.

FOOTNOTES:

[5] The pirates of the Flowery Land.

[6] I am indebted to Mr. Rolston, of the Solar Physics Observatory, South Kensington, for some information on this matter.