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of Mercury's orbit, nearer to the other planets at some times than at other times, their diameters would appear to vary inversely as their distances.) To such a spectator Venus and the Earth would appear superior planets. These, and all the planets farther distant, would have conjunctions and oppositions. Their motions would appear sometimes direct, sometimes retrograde.) They would seem stationary at intervals. If Mercury have not a rotation on his axis, a succession of day and night would scarcely be suggested to such a spectator. He might, however, observe the diurnal rotation of the other planets.

Why would the diameter of the planets seem to vary at Mercury? In what proportion would they seem to vary? Would the other planets at Mercury appear to be superior, or inferior? Would they appear to have conjunctions only, or conjunctions and oppositions? How would their motions appear?

SECTION III. Prospect at the Earth.

The prospect at the Earth is best known to its inhabitants. The inferior planets have two conjunctions in every synodical revolution; the (superior planets have conjunctions and oppositions in succession. The planets seem to enlarge or diminish, as they are nearer or farther distant.) The inferior planets, by the position of their illuminated sides, assume all the phases of the Moon. By the rotation of the Earth on its axis, the Sun, the planets, and the fixed stars, appear to have a diurnal revolution from east to west.

Where is the prospect of the solar system best known to the inhabitants of the Earth? What planets appear to have conjunctions only, and what conjunctions and oppositions? Why do the planets appear larger at some times than at others? Which of the planets: exhibit the phases of the Moon?

SECTION IV. Prospect at Jupiter.

To beings like ourselves, with eyes unassisted by glasses, it can scarcely be known, at Jupiter, that there are inferior planets. The greatest elongation of the Earth would not exceed 11° 11'; that of Mars, 17° 13. By means of equatorial telescopes, however, planets may be seen extremely near the Sun.

Can it be known at Jupiter that there are inferior planets? At Jupiter what would be the greatest elongation of the Earth and Mars? By what means can planets be seen very near the Sun?

SECTION V. Prospect at Herschel.

An inhabitant of this Earth, transported to Herschel, would almost lose sight of the solar system. To him the Earth could never appear more than 3° 2′ from the Sun; Mars, 4° 38'; Jupiter, 15° 48'. Of these, without the assistance of glasses, he would not be likely to have a view, nor to have any knowledge, unless he had been an astronomer in this world. This, however, may depend on circumstances, particularly the atmosphere of Herschel. But Saturn, having his greatest elongation 30°, might often be seen exhibiting, his ring excepted, all the phases of the Moon. To such an inhabitant, however, Saturn might not be the only planet visible. We know not but there may be planets in the system still farther distant from the Sun, well known to the inhabitants of the Georgium Sidus; for Almighty Power is not bounded by our limited view.

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If an inhabitant of this Earth were transported to Herschel, could he have a view of the solar system? To him, how far would the Earth and the other planets appear from the Sun? May it be that the inhabitants of Herschel see other planets unknown to us?

CHAPTER IV.

Comets.

THE term comet is derived from the Latin cometa, This is a derivative from coma, a head or lock of hair. The original is a Greek primitive, comee, hair. Without doubt, comets are so called from the train or tail they exhibit when in the vicinity of the Sun.

Comets are large heavenly bodies, moving round the Sun in various directions, and in orbits very eccentric. They seem to come from some far distant region, make a short circuit round the Sun, and then retire to their unknown bound. By the unlearned, they are often called/blazing stars.) It is not strange, if, as has been represented, in the days of barbarism and superstition, comets were considered portentous; if they were regarded as the harbingers of war, famine, and pestilence; if they presented to the frighted imaginations of men the convulsions of states, the dethronement of princes, and the fall of empires. Even among the ancients, however, men of science regarded them in a very different light. Such men so far observed the motions of comets, as to form ideas of them in some measure consonant to modern philosophy. By the Chaldeans, they were considered as planets; and such they were regarded by the Pythagorean philosophers of Italy.

Astronomers of the present day view comets not only as harmless, but designed, by the all-wise Creator, for benevolent and important purposes; though most of those purposes must be unknown to us, or deduced by reasoning from analogy.

There is a great diversity in comets. When viewed through a good telescope, a comet generally resembles a

mass of aqueous vapor surrounding a dark nucleus. The shades of appearance are very different in different comets. Even the nucleus seems wanting in some. Comets of this kind were observed by Dr. Herschel ; some by the sister of that astronomer. Approaching the Sun, the nebulous light of a comet becomes more brilliant, and its luminous train increases in length At the perihelion its heat is greatest, and the length of its train the most extensive. Here the comet sometimes shines with all the splendor of Venus. Its brilliancy decreases as it retires from the perihelion, till it reassumes its nebulous appearance. "History records," says Dr. Rees, "that some comets have appeared (as large as the Sun. One of this magnitude is said to have been visible at Rome in the reign of Nero. astronomer Hevelius also observed a comet, in 1652, which did not appear to be less than the Moon, though it was deficient in splendor, having a pale, dim light, and exhibiting a dismal aspect."—Wilkins's Astronomy.

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The number of comets which have been seen within the limits of the solar system is not known. It has been stated at from 350 to 500.

Some comets have approached nearer to the Sun than any of the planets. Of ninety-eight, whose elements have been computed, twenty-four passed between the Sun and the orbit of Mercury; thirty-three between the orbits of Mercury and Venus; twenty-one between the orbits of Venus and the Earth; sixteen between the orbits of the Earth and Mars; three between the orbits of Mars and Ceres; and one between the orbits of Ceres and Jupiter.

The tails of comets sometimes occupy an immense space. The comet of 1681 stretched its tail across 104°; that of 1769 subtended an angle of 60° at Paris, 70° at Boulogne, 97° at the Isle of Bourbon.

By some, the tails of comets have been considered the rays of the Sun, transmitted through the nucleus of the comet, believed to be transparent like a lens.

This was the opinion of Appian, Cardan, and Tycho Brahe. Kepler thought the tail was formed by the solar rays driving away the denser parts of the comet's atmosphere. Euler thinks there is a great affinity between the aurora borealis, the zodiacal light, and these tails; and that the cause of them all is the action of the Sun's light on the atmosphere of the Earth, the Sun, and the comets.

The hypothesis of Dr. Hamilton, of Dublin, deserves particular consideration. He supposes the tails of comets to be streams of electrical light. The doctor supports his opinion by these arguments: "A spectator at a distance from the Earth would see the aurora borealis in the form of a tail, opposite to the Sun, as the tail of a comet lies. The aurora borealis has no effect upon the stars seen through it, nor has the tail of a comet. The atmosphere is known to abound with electric matter; and the appearance of the electric matter in vacuo resembles exactly that of the aurora borealis, which, from its great altitude, may be considered in as perfect a vacuum as we can make. The electric matter in vacuo, suffers the rays of light to pass through without being affected by them. The tail of a comet does not expand itself sideways, nor does the electric matter. Hence he supposes the tails of comets, the aurora borealis, and the electric fluid, to be the same kind of matter." It may be added, in confirmation of this hypothesis, that many astronomers have observed an undulatory motion in the tails of comets, similar to what is sometimes seen in the aurora borealis. About the close of the revolutionary war, the aurora borealis was most extensive and brilliant in the United States. This, with vast undulations, covered the whole northern half of the hemisphere, collecting into a beautiful centre in the zenith. To a spectator on a distant planet, this might give the Earth an appearance resembling, in some measure, the blazing effulgence of a comet.

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