Comets and Their Story.
A Talk From London. By Dr. A. C. C. Crommelin, F.R.A.S.
The most-striking feature of the heavenly orbs is the regularity of their movements, permitting us to forecast their positions centuries in advance. There is, however, a notable exception to this power of prediction. From limo to time. a brilliant object with a long tail appears, is seen for a few days or weeks to move rapidly across the heavens, and then withdraws again into invisibility.
These apparitions arc now received with enthusiasm by astronomer, and the public; but this has not always been the case. Formerly, they caused great terror and apprehension; their great nice led men to believe that they were very near the earth, probably some sort of pestilential vapour in the upper air: from this it was a natural step to look on them as forerunners of plague, famine. and war.
Newton's Discovery.
There were several stages in the attainment of more accurate knowledge about comets. The great Danish astronomer, Tycho Brahe, compared his cometary observations with than made at a distant observatory, and proved that they are celestial, not terrestrial objects, being much more remote than the moon. Then Sir Isaac Newton discovered the law of gravitation about two and a half centuries ago, and deduced from it that comets travel round the nun in very long oval curves. only becoming visible to on for a small part of their track lying near the sin.
Accurate Predictions.
He was greatly assisted in this work by Halley; who, after much labour, found the paths of all the comets that had been observed in the preceding two centuries, and proved that what appeared to be three of them were really the same body returning at intervals of about seventy-six years.
It now became quite possible to predict the appearance of certain comets; then Halley's comet came back as predicted in 1759, 1835, and 1910. Most of us remember the last return; the comet presented a grand spectacle, with an immense tail, in more southern countries; but in England the view was marred by its being low down in the twilight. We are able to calculate the returns of this comet both forward and backward; this has been done for a period of two thousand years, and records have been found of the comet's appearance at almost every return.
Taking Us by Surprise.
Besides Halley's comet, there are some sixty others whose return can be predicted; but they are mostly faint, and of little popular interest. It is only the really fine comet that can be announced beforehand; the others, when they come, take us by surprise; their periods are, for the most part, so long that they have not been seen since the dawn of exact history.
The last comet of great splendour that was visible in this country appeared in the autumn of 1882. It was remarkable for the very close approach that it made to the sun, the distance being half a million miles. At that time it shone so brightly that it could be seen with ease in full sunshine; its tail was a hundred million miles long, and remarkably straight.
Another fine comet, known as Donati's, appeared in 1858; its tail was beautifully curved like a scimitar; it passed over the bright star Arcturus, which shone undimmed through hundreds of thousands of miles of cometary matter.
Another very grand comet appeared in 1811, at the time of Napoleon's Russian campaign. This had a tail 100 million miles long, and a very bright head, which was the more remarkable because the comet was outside the earth's orbit, and did not approach close to the sun. It takes some 3,000 years to go round the sun; the periods of some comets are longer still, and they go out into space to some thirty times the distance of Neptune, the furthest known planet. I have estimated that the total number of comets can hardly be less than an eighth of a million.
Travelling Stones.
It was found in the last century that there very close connection between comets and shooting-stars; these are lumps of stone or metal that are travelling round the sun in long oval paths like the comets. When they enter the earth's atmosphere, friction, produced by their speed of several miles per second, causes them to glow. Most of them are burnt to dust in the upper air, but sometimes they descend to earth. Some lumps of this kind can be seen in the Natural History Museum. Chemists have examined the lumps, and find that they contain a great deal of gas, chiefly hydrogen and its compounds.
Meteor Showers.
The belief now is that the head of a comet consists of a swarm of multitudes of these meteoric masses; the paths of many of the well-know meteor showers, such as the November shower from the sickle of Leo, and the August shower, known as the Tears of St. Lawrence, were found to show a perfect agreement with the paths of certain comets. The conclusion is that the swarm of meteors that forms a comets head is gradually scattered under the distortion produced by the sun and the planets , so as to leave a long trail of meteoric débris, in the wake of the comet. A meteor shower takes plaice when the earth traverses one of these trails; and as they are very numerous, we get several showers every year.
The Sun's Repulsion.
The meteoric constitution of a comet's head also gives us an explanation of the formation of the tail. We have seen that meteoric lumps contain much gas; on approaching the sun, its heat draws out this gas, which forms a cloud round the comet's head. A violent repulsive force from the sun then acts on the gas, driving it with great speed away from the sun. This force does not disturb the big lumps forming the comet's head, but only the finely divided matter in the tail. A cornet's tail is not attached to it like an animal's tail; a better analogy is the jet of steam and vapour from the funnel of a locomotive. which is continually dissipated and renewed. The tail matter dose not return to the head, but is lost in space.
It in well to remember that when a comet is leaving the sun, it goes tail first. This is contrary to popular imagination, which pictures the tail as being left behind, like the tail of a rocket, but there is no analogy between them, in spite of a similar appearance.
As to the origin of comets, nothing certain is known. My own view is that a few of them arise front matter erupted from the sun, others from similar eruptions from the planets, especially Jupiter, which appears to be still in a very heated state. The remainder are, perhaps, detached fragments of the great comical cloud of dust and gas which is believed to have been the primitive form of the planetary system.
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