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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

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 ; Ithough most of those purposes must be unknown to ús, or deduced by reasoning from analogy.

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

some

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. “The 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.

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 fearer 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, 700 at Boulogne, 970 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, jpf 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.

From what is the term comet derived ? Why are comets so called? What are comets? Whence do they seem to come, and what to do? How have they been considered by the unlearned ? Had any of the ancients just views of comets? How do astronomers of the present day view them? What is the general appearance of comets? Is there much variety in their appearance ? How large have some comets appeared ? What number of comets has been seen within the solar system? How near the Sun have some comets approached? How large a space have the tails of some comets occupied ? What have been the different opinions of authors respecting the tails of comets ? Whose opinion seems best supported ? May the Earth ever have had the appearance of a comet? When did the aurora borealis appear most brilliant in the United States ?

CHAPTER V.

Equation of Time.

Though the apparent motion of the Sun has been used as a measure of time from the greatest antiquity, yet accurate observation has shown it is far from being uniform. The Sun is either faster or slower than a well-regulated clock or watch, during most of the year. (At four times only do they coincide, viz. the 14th of April, the 15th of June, the 31st of August, and the 23d of December. From the 14th of April to the 15th of June, the Sun is fast of clock; from the 15th of June to the 31st of August, it is slow of clock; from the 31st of August to the 23d of December, it is fast of clock; from this time to the 14th of April, it is slow of clock. From the difference of longitude, the days of coincidence are not all the same in the United States as in Europe. About the 1st of November, the Sun is 16 m. 14 or 15 s. fast of clock. This is the

1

greatest inequality. The difference is caused by the elliptical figure of the Earth's orbit, and the obliquity of the equator to the plane of the ecliptic.

The orbit of the Earth being elliptical, like the other planetary orbits, with the Sun in one of the foci, the Earth, in its annual revolution, moves more slowly in the aphelion than in the perihelion, as has been before shown. But, the motion on its axis being perfectly uniform, any given meridian will come round to the Sun sooner at the aphelion than at the perihelion. Hence the solar day will be shorter at the former, and longer at the latter, than that measured by an accurate time-keeper.

Let S be the Sun, [Plate v. Fig. 8,] E the Earth; A MP the Earth's orbit; A the aphelion, P the perihelion ; the line M S the mean proportional between the semi-axes of the orbit; m a point in the equator represented by the external circle of the Earth, E. Let the spaces A Sa, MSn, P S p, represent equal areas of the orbit. The arches of these, by the great law of Kepler, represent the Earth's motion in equal times, as a solar day. It is evident that the point m, when the Earth is at a, at n, or at p, must pass from m to the line E S to complete a solar day. It is also evident, that it must pass farther when the Earth is at р

than when it is at a, the distance at n being a mean between the extremes. A day, therefore, measured by the Sun, will agree with that shown by a good timekeeper when the Earth is at M. At A it will be shorter, and at Plonger, than the true day of the clock.

By this equation the Sun would be faster than the clock, while the Earth is passing from the aphelion to the perihelion of its orbit; slower than the clock, while the Earth is passing from the perihelion to the aphelion. At either apsis the Sun and clock would coincide. The difference between the Sun and clock would increase, while the Earth is passing from the aphelion or perihe

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