SOMETHING ABOUT COMETS

Hawera & Normanby Star, Volume LIII, Issue 9434, 5 September 1907, Page 2

SOMETHING ABOUT COMETS

Jn his astronomical notes for September in. the Christchurch Press the Rev. P. W. Fairclough says: — "The comet naturally claims our chief attention this month, so we will neglect the stars, for them we have always. So far, the information supplied by astronomers is scanty. We know that it is approaching the sun, and has several tails, and that is- about all. When it has passed its perehelion and turned round the sun it may appear in the evening, but information about its course is not sufficient to assure us of that. Should it pass near the sun it may become a splendid object, especially if its path should lie reasonably near the earth. All the comets that have made a great display t have been excited by passing near to the sun — brushing the embers with their wings, as Hogg puts it. "Comets swarm like fishes in the sea," said Kepler. During the Christian era from 700 to 800 comets have been observed. Nearly 200 of these were in the 19th century, and the great majority were telescopic. It is certain that only about half the comets that approach the sun are ever seen, their whole path being buried in the solar rays. Probably, also, only a minority of comets come near enough to the sun to be seen under any circumstances whatever. Suppose, then, that ten. comets make their perehelion passage every year — a very moderate estimate, seeing that with telescope and camera half that number are now being observed — how many comets is the sun likely to have within its control at any one time? About 80 per cent, of observed comets visit us only once. They have never been here before, and will never, come back. Such comets are within the tun's attraction about five million years before they reach their nearest point and begin to retreat. Hence, to keep up ,the supply of eight foreign comets a year, the sun must have a train of 40 million comets coming and an equal host going. A comet voyaging from Alpha Centauri, the nearest star,»;to the sun, with just sufficient momentum to carry it over the netitral line between the two suns, would Be 'ai' least ten million years on the journey. For fully four of the ten it would be under the control of the sun. Seeing that only two or three stars are as near as twice the distance of Alpha, and only a score or so within ten times his distance, what is the 1 average voyage of a comet from star to ; Sun? It would seem to be incredible j that some of our visitors have been ( on ! their way throughout all the geological ages. "Comets may be called foreign and domestic. About twenty per cent of those observed are domestic, and move I round the sun in closed^- orbits, or elI lipses. Most of these retreat as far as the orbit of one or other of the great, planets. They are supposed to have ' been captured by the planets. That' is to say, on their first visit they passed the planets in such a way as to be seriously slowed down by the planet's attraction. After this loss of momentum they could not get up speed enough to carry them away ffom the sun, but only enough to take them back to where they started- afresh from the planet's grip. Then, coming to the sun, and being often excited, have had the tail producing matter winnowed out of them. They seldom make any display. The gorgeous displays are made by visitors that have voyaged for ages through absolute zero, and are then raised to 'a great temperature by the sun. Our present visitor was at first said to have had a tail seven millions of miles long, but that is a mere bobtail. The comet of 1843 had 200 million miles of tail, and the cornet of 1880, seen for a few evenings only in^our skies, could not have been less lavishly equipped. "The tail is the most interesting part to the majority of mankind, so we must try to explain it. But first, what is a j cometS Often it is a mere, whfep of vapour, of great bulk — larger than the sun, perhaps, but of extreme tenuity. A faint star has been seen through 50,000 miles of cometic vapour. Hence the ponderable matter contained in such a comet is very .trifling. Other comets appear to be dense flights of meteors. The November and other celebrated systems of* meteors certainly travel in cometic orbits and are thought to be wrecked and dispersed comets. In no case is thei;e believed to be any considerable solid b*ody in a comet. There is often a nucleus, .or star-like centre of light. • This, possibly, arises from friction and collision as the armies of meteors are perturbed in their headlong race to the sun. As probably the very atoms of matter are disintegrated, and there is given off, a vapour-like emanation, such as the !•»• searches in radium have made us familiar with. The corpuscles fly in every directcion. Those toward the sun are combed back by some repelling energy from him. The nose of a comet is often like a gas jet held against the wind, or like a fountain falling back. In the direction . opposite to the sun stretches out _the wondrous tail. Lei this be constantly borne in . mind, the tail always points, away from the sun. When the comet is leaving the sun the tail goes foremost. The crude idea of - a burning world leaving its trail of smoke behind does not answer. The wind that carries this smoke is some . energy rushing out of the sun with the velocity of light. "We speak .of the tail of a comet as we speak of the jet of water from a nozzle. It is the same jet but not the same water. It is the same tail, but .not the same matter. The end of the ! visible tail, being, say, 200 millions of miles further from the sun than the head, could not possibly accompany the head round the" sun. The tail is constantly dispersed in space. If you sway your hose nozzle round, you observe that the jet is curved. The water keeps the direction in which the nozzle pointed at the moment of its emission. So the tail of a comet is curved. Again, if you have several nozzles, held parallel, each with a different pressure of water,, and if you sway them - all round together, you will observe that the highest pressure yields the greatest jet, and tin lowest pressure the ,uio t. curved 1 -- jet. .So with the comet. The matter that is most finely disintegrated and is carried away most swiftly, yields a straight tail;- coarser and slower matter is winnowed, or "separated," according to its velocity, and 50 we get several tails of a varying curvature. "Now, what is the repelling force, the wind, as it were, from the sun?

It is light itself. There is no space left to explain. We may return to the subject. Meanwhile read Duncan's 'New Knowledge.' "