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equally thiough the November one. On this account, the former star-showers are quite regular, while the latter vary in brilliancy through periods of 33£ years. (See p. 10.)
Relation Between Meteors And Comets.—The orbit of the November shower is found to be almost identical with that of the comet of 1866; while the August stream is in the track of the comet of 1862. It is a popular theory that these comets are only clusters of meteors crowded so closely together as to be visible by the reflected light of the sun. The single meteors are too small to be seen, except when they plunge into the earth's atmosphere and take fire. On the other hand, Herschel thinks that meteors are the dissipated parts of comets torn into shreds by the sun's attraction.
Radiant Point.—A star (n) in the blade of the sickle is the point from which the stars in the November shower seem to radiate, while one in Perseus (7) is the radiant point of the August shower. In the shower of 1866, two observers, who counted the falling stars at the rate of 2,500 per hour, saw only five whose paths, if traced back, would not meet in Leo.
Meteorological Effect.—The temperature of August and November is said to be considerably increased by this ring of meteoric bodies, which prevents the heat of the earth from radiating into space. A corresponding decrease of temperature in February and May is caused by the stream or ring of meteors coming between the sun and earth.
Height.—Herschel estimates the average height of shooting stars above the earth to be 73 miles at their appearance and 52 at their disappearance.
Weight.—Prof. Harkness estimates that the average weight of shooting stars does not differ much from one grain.
We come now to notice a class of bodies the most fascinating, perhaps, of any in astronomy. The suddenness with which comets flame out in the sky, the enormous dimensions of their fiery trains, the swiftness of their flight, the strange and mysterious forms they assume, their departure as unheralded as their advent—all seem to bid defiance to law, and partake only of the marvellous. Superstitious fears have always been excited by their appearance, and they have been looked upon in every age as
"Threatening the world with famine, plague, and war;.
Thus the comet of 43 B. c, which appeared jast after the assassination of Julius Csesar, was looked upon by the Bomans as a celestial chariot sent to convey his soul heavenward. An old English writet observes: "Cometes signifie corruptions of the ayre. They are signs of earthquakes, of warres, of changyng kyngedomes, great dearthe of corn, yea, a common death of man and beast." Another remarks: "Experience is an eminent evidence that a comet, like a sword, portendeth war; and a hairy comet, or a comet with a beard, denoteth the death of kings, as if God and nature intended by comets to ring the knells of princes, esteeming bells in churches upon earth not sacred enough for such illustrious and eminent performances."
Description.—The term comet signifies a hairy body. A comet consists usually of three parts;—the nucleus, a bright point in the centre of the head; the
coma (hair), the cloud-like mass surrounding the nucleus; and the tail, a luminous train extending generally in a direction from the sun. There are comets without the tail, and others with several, while some are deprived of even the nucleus. These last consist merely of a fleecy mass, known to be comets from their orbits and rapid motion. Comets are not confined, like the planets, to the limits of the zodiac, but appear in every quarter of the heavens, and move in every conceivable direction. When first seen, the comet resembles a faint spot of light upon the dark background of the sky: as it approaches the sun the brightness increases, and the tail begins to show itself. Generally it is brightest near perihelion, and gradually fades away as it recedes, until it is finally lost, even to the telescope.
The Time Of The Greatest Brilliancy depends somewhat on the position of the earth. If, as represented in the figure, the earth is at a when the comet, moving toward perihelion, is at r, the comet will appear more distinct than when it is more distant at s, although at the latter point it is really brighter. If, however, the earth is at c or b at the time of perihelion, the comet would be much more conspicuous. Again, if the earth is passing from a to b during the time the comet is near the sun, it will appear less brilliant than if it were moving from c to d, as we should then be much nearer it during its greatest illumination.
Number Of Comets.—Kepler remarks that there are as many "comets in the heavens as fish in the sea." Arago has estimated that there are 17,500,000 within the solar system, basing his calculations on the number known to exist between the sun and Mercury. Of this vast number, few are visible to the naked eye, and a still less number attract observation, owing to their inferior size and brilliancy. Many are doubtless lost to our sight by being above the horizon in the daytime. Seneca mentions that during a total solar eclipse, a large and splendid comet sud denly made its appearance near the sun.
Orbits Of The Comets.—Comets form a part of the solar system, and are subject to the laws of gravitation. Like the planets, they revolve around the sun, but they differ in the form of their orbits. While the planets move in paths varying but little from circular, and thus never depart so far from the sun as to be invisible to us, the comets travel in extremely elongated (flattened) ellipses, so that they can be observed by us only through a very small portion of their paths. In Fig. 67 are represented the three general classes of their orbits. A comet travelling along an elliptical orbit, though it may pass far from the sun, will yet return within a fixed time; one pursuing either a parabolic or hyperbolic curve cannot return, as the two sides separate from each other more and more. Many of the comets of the first class have been calculated, and they have repeatedly visited our portion of the heavens; while those of the other classes, having once formed part of our system, go away forever, seeking perhaps in the far-off space another sun, which in turn they will abandon as they have our own.