Comets and their Tails.

Kaipara and Waitemata Echo, 12 June 1914, Page 8

Comets and their Tails.

A comet, when first seen, is generally a small, round object, with a nucleus or central condensation surrounded with a halo of light, which we call the coma. Generally, that characteristic feature, the tail, is not to be seen at first. The exact nature of a comet is not as yet filly known ; but in a general way it is supposed to be composed of a swarm of solid particles of an unknown size, and widely separated. F.ach particle carries with it an envelope of hydro-carbon gas, which gaslight is produced either by electrical discharges between the particles, or by some other light-evolv-ing action due to the sun's influence. Tgi.s is before the comet puts forth a tail which is done only as it approaches the sun. The interesting process of putting forth a tail is due to the action of the sun, which warms up and liberates the gas on that side of the comet nearest to it. The liberated gas is supposed to be electrifier! by the sun, and the similar electricity in the sun repels the gas and drives it off into space in an opposite, or nearly opposite, direction, allowing for the movement of the comet in its orbit. It will be seen that the tail, as it goes streaming off into space, requires constant replenishing. The tail at one time is not the tail of another ;it varies like the steam from a tea-kettle, which presents a fairly close analogy. Tt is an expensive affair to keep up a tail millions of miles long. For this reason, the longer a comet is a member of the Solar System, the less tail it is able /to present, and the short-period comets which keep coming around frequently, like Encke's Comet, for instance, have no tails at all. Talk of Hamlet with Hamlet left out ! The tail, as we have seen, is formed by the gas on the side nearest the sun escaping and going around a cornet and away. For this reason the tail is a hollow conoid, and the centre naturally looks fainter than the edges. And now as to the length of comets' tails. People generally like to read about big dimensions, and we can satisfy them in this particular. The length of a good large comet's tail is seldom less than ten or fifteen million miles, and frequently reaches from thirty to fifty, million miles. In many cases it has been known to exceed one hundred million miles, and at the end to be several million miles across. The great comet of 1882 had a tail ■more than one hundred million miles long, about two hundred I housand miles in diameter at the comet's head, and a diameter of ten million miles out at the end. A pretty good-sized gas-jet that ! This comet went round the .sun through perihelion—that is, the point in its orbit nearest the sun— with a velocity of more than three hundred and fifty miles a second, almost grazing the sun's surface. Speaking of the comets which have been drawn into our Solar System, it is known that Saturn has captured two, I'ranus three, Neptune six, and Jupiter some sixteen. Sometimes a comet separates or splits into parts. Such was the ; t case with Biela's Comet, discovered! in 1826—a comet whose orbit comes near that of the earth. • On its return, in 1846, it had divided into two parts, which parts travelled along side by si.io for more than four months, being about one hundred and sixty thousand miles apart. On the comet's next return, in 1853, both parts were seen ; but they were one million five hundred thousand miles asunder, ' and neither part has been seen k since.—"Spare Moments."

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