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apparent revolution is caused by the rotation of the earth about its axis, from west to east, and this real motion occasions the sun, moon, planets, comets, fixed stars, and the whole frame of nature to appear to revolve in an opposite direction, or from east to west.
Comets perform their revolutions round the sun, as the centre of their motions, as also the planets ; but the orbits of comets are very elliptical, and those of planets nearly circular.
Planets and comets both obey the same laws in pursuing their respective courses round the sun, and though the shapes of their orbits are very dissimilar, yet each revolves round the central body in an ellipse, the sun occupying one of the foci of the ellipse: also that lines drawn from the sun to the planet or comet, sweep over equal areas, in equal portions of time, and that the squares of the times of revolution are as the cubes of the mean distances of the comet or planet from the sun.
Planets and comets are opaque bodies, and shine only by the light emitted from the san: this is evident from the phases exhibited by the moon, Venus, Mercury, and Mars, when in situations where parts of their unenlightened orbs are turned to the earth. This appearance has also been observed in comets, particularly the one which is expected in 1835. Comets, also, have been observed to transit the sun's disc, which interesting phenomenon was observed in one that appeared in 1819; its appearance, when on the disc, was that of a nebulous spot; this comet also exhibited phases; these are sufficient proofs, that comets are not self-luminous, but like the planets, shine only by reflected light.
Planets and comets have atmospheres; those of the latter, however, are very considerable, and stream off in a direction opposite to the sun. In these, and some other particulars, there is sufficient resemblance between planets and comets to prove that they are both members of the same family, of which the centre is the sun.
In many points, however, there is a great dissimilarity between planets and comets ;-a planet may be traced through the whole of its path round the sun,-through every degree of its course, without losing sight of it, excepting for a short time before and after its conjunction with the sun, and even this has been done with the planet Venus,--but comets, on the contrary, enter at one part of the heavens, and disappear at a place not very remote from the first place of observation. This was the case with the comet of Encke ; it was dimly recognized in Pegasus, passed through Equuleus, entered Antinous, and then became invisible in the solar rays. The longest track of any comet on record is that of 1820, observed by Captain Basil Hall at Valparaiso, for three months, in which time it described a curve equal to 300°.
Planets, as they keep through the whole of their annual courses, at nearly the same distances from the centre of the system, vary but little in their apparent magnitudes, as seen from the sun, and with the exception of Venus and Mars, not very considerably as seen from the earth :-Comets, on the contrary, at first appear very small, and gradually increase in size: the comet of 1769 will afford an example,- it appeared at first like a minute nebula, rapidly increased in size as it advanced, and when at its maximum, the coma surrounding the head was equal to a degree, or twice the apparent diameter of the moon; the comet that was seen in January, 1831, will furnish an illustration of their diminution ; on the 7th day it shone with the brilliancy of a star of the second magnitude; on the 26th it exhibited a pale nebulous appearance, and could only be seen with the telescope. It must, however, be here observed, that this increase and diminution arise not merely from the advance or retreat of the comet, in relation of distance, but from this and the solar action, which raises the nebulous matter, and in some way influences the nucleus as the comet approaches, which matter is again condensed as the comet retreats from the sun; distance diminishes, and proximity increases the angle under which a body is seen; so that in comets there are two principles combining to affect their apparent and real magnitudes. · Planets move nearly in the same plane with one another, their orbits not being much inclined to the earth's path or ecliptic; the larger planets, Jupiter and Saturn, being each respectively 1° 19' and 2° 30'; wbereas, comets make all possible angles with the ecliptic, from below 10° to paths at right angles to the courses of the planets: the orbit of the comet of Encke is inclined 13° 20', that of Biela, 13° 32', that of Halley, 17° 39', the comet of 1680, 61° 22', and one that was. observed in 1818, had its orbit inclined to the ecliptic in an angle of 89° 47'.
Planets move all the same way in the order of the signs, that is from west to east; comets move both ways, direct and retrograde; nor are these contrary motions merely apparent, being combined with the earth's motions, which make even the planets appear to move