backwards, which movement they have also; but their motion, in their orbits, are absolutely in the order in which the planets move, and contrary to this order: a direct comet will appear to move contrary to the order of the signs when it is passing between the earth and sun, and its absolute motion exceeding that of the earth; this was the case with the comet of Encke in 1828, it passed from Aries towards Pisces, Aquarius, and Capricornus; when it arrived on the opposite side of the sun, had it then have been visible, it would have appeared in its direct course. To the planets the disc of the sun appears nearly of the same magnitude throughout the whole of their respective annual courses; whereas, some comets, when at their aphelion, would have the sun not much larger than a star of the first magnitude, and when returning and arrived at the orbit of Uranus, it would see the sun about the size that Venus is when largest; at the orbit of Saturn, the solar angle would be 3'; Jupiter, 6'; Mars, 21'; Earth, 32'; Venus, 46′; Mercury, 80'; and to the comet of 1680, when in perihelion, the sun would appear 100° in breadth, or 40,000 times as large as the sun appears to us. The planets move with nearly uniform velocity throughout the whole of their respective courses: this is very far from being the case with comets; that of 1680, when in perihelion, or that point nearest the sun, moved 1,768,000 feet in a second of time, and when in aphelion, or most remote from the sun, only 83 feet in the same time. The great comet of 1811, in aphelion, is eleven months describing a second of space,-a rate of motion little exceeding the swiftness of a man walking! The most remote planet is, when compared to the distance to which comets range, very near the sun: the distance of Uranus is 1800 millions of miles; the comet of Halley extends beyond this planet 1620 millions of miles, or 3420 millions of miles from the sun; that of 1680, at its aphelion, is 8200 millions of miles, and other comets extend their wondrous path considerably beyond this remote point! The periods of planets vary from 88 days of our Earth's, which is the year of Mercury to that of Uranus, which is 30,686 days, or 84 years: the periods of comets vary from 3 years, (that of the comet of Encke,) to 75 years, the period of the Halley comet; 575 years the comet of 1680; 3383 years that of 1811; and 7334 years, which is the period of the comet of 1763. The number of planetary bodies, including satellites, is twenty-nine: the number of comets actually observed exceeds four hundred; and the whole number that traverse the system is supposed to amount not merely to thousands, but to millions! this statement seems too vast for credibility, but a rigid calculation intimates that there are millions of comets whose perihelia are within the orbit of Uranus, and we have no reason to doubt but that there are many whose perihelion distance is without the orbit of this remote planet! The planets, as they appear to our earth, either always present the same invariable disc, or have their orbs subject to a periodical variation, called phases; this is owing to the earth and planets always holding on the same courses relative to one another: those that are between the earth and sun are called inferior, or below us; those that are beyond, which include the terrestrial orbit in theirs, are called superior, or above us,- the inferior, shewing phases like the moon, the superior, exhibiting a full disc, excepting Mars, who, when in quadrature, is a little defective of a full orb: but this uniformity of appearance is widely different to the changes which would be observed from the orb of a comet as it traverses the paths of the planets; a comet would, according to the positions of the planets, see them either horned, half, gibbous, or full-orbed; from being all at first inferior, on the comet's approach to the planetary system, they would, in succession, all become superior; this is on the supposition that the approach of the comet would be in nearly the planetary plane; if descending at right angles, the comet would have the solar system beneath, and presenting a most sublime appearance! The orbits of the planets never intersect each other : the orbits of some comets intersect those of the Asteroids, Mars, Earth, Venus, and Mercury,-as the comet of Encke; the comet of Biela takes in the orbit of Jupiter; that of 1770, Saturn; while by far the greater number intersect the orbits of all the planets. There is, however, an exception in the Asteroids, their paths do cross each other, but these small bodies may be considered a species of comets. The periods of planets are ascertained with the greatest precision; it is an indelible disgrace to err one minute of time or space in their revolutions: the periods of comets are involved in uncertainty, the periods of the two comets, that will return this year, are an exception; their places have been determined with the precision of those of planets. Even the perturbations of the planetary system are reduced to a law, by which their mutual attractions being known, the requisite corrections are applied, so that their places in the heavens can be predicted for any remote period of time. All constellations known In earthly sky were far behind us now. And many a streaming comet we glanced by As the adventurous' man In search of unknown seas and lands puts forth Stars densely thronging still. |