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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 Orbit Op Comet. 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 suddenly made its appearance near the sun.

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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.

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Calculation Of A Comet's Eeturn.—As we can observe so small a proportion of the entire orbit, it is very difficult, indeed oftentimes impossible, to decide whether it is an ellipse, hyperbola, or parabola. A few are known to move in clearly elliptical paths, and their movements have been so accurately estimated that it is possible to predict their exact place in the starry vault on any given day and hour. The other comets may never return, or at least not for centuries hence. They may be paying our sun their first visit; or if they have swept through the solar system before, it may have been at so remote a time that no record is preserved, even if it were not before the creation of man. Under these circumstances it is obviously extremely difficult to determine the times of these apparently erratic wanderers; yet, in spite of all these obstacles, some have been tracked far into space beyond the telescopic view. For example, the comet of 1844 is announced to pay a visit to the astronomers of the year of our Lord 101,844. The period of the comet of 1744, is fixed at 122,683 years.

Distance From The Sun.—The comets at their perihelion sweep very near the sun. Thus the one of 1680 came where the temperature was estimated by Newton to be about 2,000 times that of red-hot iron. The nearest approach known is that of the comet of 1843, whose perihelion distance was but about 30,000 miles from the surface of the sun; in fact, it doubled around that body in two hours' time. (Guillemin.) The greatest aphelion distance yet estimated is that of the comet of 1844, which is over 400,000,000,000 miles. The velocity varies, of course, with the position in the orbit. The comet of 1680 moved in perihelion at the rate of over two hundred and seventy-seven miles per second; while in aphelion its velocity is only about six miles per hour.

Density Of Comets.—The quantity of matter contained in a comet is exceedingly small. Telescopic stars even are visible through them. The comet of 1770 became entangled among Jupiter's moons, and remained there four months without interfering with their movements in the least; indeed, so far from that, its own orbit was so much changed by the proximity, that from a periodical return of 5^ years, it has not been seen since. The same comet came within 1,400,000 miles of the earth without producing any sensible effect. In 1861, we have good reason to suppose that the earth actually passed through the tail of a comet, its presence being indicated only by a peculiar phosphorescent mist. So that even should our earth run full-tilt against a comet, the shock would be quite imperceptible.* Still, however lightly we may speak of the probability of such a collision, we must remember that there are comets of greater solidity. Donati's, for instance, is estimated to be about -^ the bulk of the earth. The concussion of such a body, moving

* " However dangerous might be the shock of a comet, it might be so slight that it would only do damage at that part of the earth where it actually struck; perhaps even we might cry quits, if, while one kingdom were devastated, the rest of the earth were to enjoy the rarities which a body coming from so far might bring to it. Perhaps we should be very surprised to find that the dibris of these masses that we despised were formed of gold or diamonds; but who would be the more astonished—we or the comet-dwellers who would be cast on our earth? What strange beings each would find the other!" Lettre »ir la GomHe—(M De Maupertuis.)

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