first discovered by Huygens; and when viewed through a good telescope, appears double. Dr. Herschel says,

that Saturn is encompassed by two concentric rings, of the following dimensions.

Breadth of the vacant space, or dark zone be

tween the rings

2839

The ring of Saturn revolves round the axis of Saturn, and in a plane coincident with the plane of his equator, in 10 hours 32 min. 15.4 sec. The ring being a circle, appears elliptical, from its oblique position; and it appears most open when Saturn's longitude is about 2 signs 17 degrees, or 8 signs 7 degrees. There have been various conjectures relative to the nature and properties of this ring.

IX. OF THE GEORGIUM SIDUS, OR HERSCHEL H, AND * ITS SATELLITES.

The Georgian is the remotest of all the known planets belonging to the solar system; it was discovered at Bath by Dr. Herschel on the 13th of March 1781. This planet is called by the English the Georgium Sidus, or Georgian, a name by which it is distinguished in the Nautical Almanac. It is frequently called by foreigners Herschel, in honour of the discoverer. The royal academy of Prussia, and some others, call it Ouranus, because the other planets are named from such heathen deities as were relatives: thus, Ouranus was the father of Saturn, Saturn the father of Jupiter, Jupiter the father of Mars, &c. This planet, when viewed through a telescope of a small magnifying power, appears like a star of between the 6th and 7th magnitude. very fine clear night, in the absence of the moon, it may be perceived, by a good eye, without a telescope. Though the Georgium Sidus was not known to be a planet till the time of Dr. Herschel, yet astronomers

In a

generally believe that it has been seen long before his time, and considered as a fixed star.*

In so recent a discovery of a planet at such an immense distance, the theory of its magnitude, motion, &c. must be in some degree imperfect. Its periodical revolution round the sun is said to be performed in 30445 days, or upwards of 83 years: the ratio of its diameter to that of the earth, is as 4.32 to 1; consequently its magnitude is upwards of eighty times that of the earth. If the periodical revolution of the Georgian, as above, be truly ascertained its distance from the sun may be determined by Kepler's rule, as for the other planets.

The Georgian planet is attended by six satellites; their periodical revolutions and times of discovery are as follow:

d. h. m.

I. or nearest, revolves in 5 21 25

II.

III.

IV.

V.

VI.

S.

0,

8 17 1 19,

discover'd in 1798. discover'd in 1787. discover'd in 1798. 13 11 5 14,discover'd in 1787. 38 1 49 0, discover'd in 1798.

10 23 4 0,

107 16 40 0, discover'd in 1798.

All these satellites were discovered by Dr. Herschel; their orbits are said to be nearly perpendicular to the ecliptic, and, what is more singular, they perform their revolutions round the Georgian planet in a retrograde order, viz. contrary to the order of the signs.

According to F. de Zach's account of this planet, in the Memoirs of the Brussel's Academy, 1785, there was then in the library of the Prince of Or: nge, four observations of this planet considered as a star, in a catalogue of observations written by Tycho Brahe: but, as Tycho was not acquainted with the use of telescopes, some writers contend that he could not see it; while others maintain that, as he has marked stars which are not greater than this planet, he might certainly have seen it.

CHAPTER II.

On the Nature of Comets; the Elongations, Stationary and Retrograde Appearances of the Planets; of the Fixed Stars; and the Eclipses of the Sun and Moon.

I. ON COMETS,

THOUGH the primary planets already described, and their satellites, are considered as the whole of the regular bodies which form the solar system, yet that system is sometimes visited by other bodies, called comets, which are supposed to move round the sun in elliptical orbits. These are supposed to have the sun in one foci, like the planets; and to be so very eccentric, that the comet becomes invisible when in that part of its orbit which is the farthest from the sun, It is extremely difficult to determine the elliptic orbit of a comet, with any degree of accuracy, by calculation; for, if the orbit be very eccentric, a small error in the observation will change the computed orbit into a parabola or hyperbola; and from the thickness and inequality of the atmosphere with which a comet is surrounded, telescopic observations on them are always liable to

Hence, the theory of the orbits, motions, &c. of comets, is very imperfect; and though many volumes have been written on the subject, they are chiefly founded on conjecture. The unexpected appearance of the comet in 1807 fully confirms this assertion, and will, doubtless, give rise to a variety of new calculations and new hypotheses, which, like former ones, for want of sufficient data, will disappoint the expectations of succeeding astronomers. The comets, Sir Isaac Newton † observes, are compact, solid, and durable bodies,

The latest writings on the subject of comets are M. Pingre's Cometographie, in 2 vols, 4to, and sir Henry Englefield's work, entitled, "On the Determination of the Orbits of Comets" A well written article on Comets may be seen in Dr. Rees New Cyclopedia, together with the elements of ninety-seven of them, and the names of the authors who have calculated their orbits.

Many interesting particulars respecting the nature of comets, &c. may be learned by referring to the latter end of the third book of Newton's Principia.

or a kind of planets which move in very oblique and eccentric orbits every way with the greatest freedom, and preserve their motions for an exceeding long time, even where contrary to the course of the planets. Their tail is a very thin and slender vapour, emitted by the head or nucleus of the comet, ignited or heated by the sun.

II. OF THE ELONGATIONS, &c. OF THE INTERIOR

PLANETS.

Let TE e (Plate IV. Fig. 8.) represent the orbit of the earth; a wvxfgh the orbit of an interior planet, as Mercury or Venus, and S the sun.

Let Trepresent the earth, S the sun, and a Venus at the time of her inferior conjunction; at this time she will disappear, like the new moon, because her dark side will be turned towards the earth. While Venus moves from a towards w she appears to the westward of the sun, and becomes gradually more and more enlightened (having all the different phases of the moon.) When she arrives at v, her greatest elongation, she appears half enlightened, like the moon in her first quarter, at this time she shines very bright. From her inferior to her superior conjunction, viz. from her situation in that part of her orbit which is directly between the earth and the sun as at a, to her situation in that part of her orbit in which the sun is between her and the earth; she rises before the sun in the morning, and is called a morning star. From her superior to her inferior conjunction she shines in the evening, after the sun sets, and is then called an evening star.

From the greatest elongation of Venus when westward of the sun as at v, to her greatest elongation when eastward of the sun, as at g, she will appear to go forward in her orbit, and describe the arch VW HG amongst the fixed stars; but from g to v she will appear retrograde, or return to the point V in the heavens in the order G H WV. For when Venus is at f, she will be seen amongst the fixed stars at H, and when at g she

The stationary and retrograde appearances of the inferior planets, are neatly illustrated by a small orrery, made and sold by Messrs. Wm. and S. Jones, No. 30, Holborn.

will appear at G: when she arrives at h she will again appear at H in the heavens. Hence, in a considerable part of her orbit between ƒ and h, and between w and x, she will appear nearly in the same point amongst the fixed stars, and at these times, is said to be stationary.

When a planet appears to move from the neighbourhood of any fixed stars, towards others which lie to the eastward, its motion is said to be direct; when it proceeds towards the stars which lie to the west, its motion is retrograde; and when it seems not to alter its position amongst the fixed stars, it is said to be stationary.

If the earth stood still at T, the planet Venus would seem to make equal vibrations from the sun each way, forming the equal angles g T S and v TS, each 47° 48', her greatest elongation, and the stationary points would always be in the same place in the heavens; but it must be remembered that, while Venus is proceeding in her orbit from a towards x, the earth is going forward from T towards E; hence, the stationary points, and places of conjunction and opposition, vary in every revolution. What has been observed with respect to Venus, may be applied with a little variation to Mercury.

III. OF THE STATIONARY AND RETROGRADE APPEARANCES OF THE EXTERIOR PLANETS.

Because the earth's orbit is contained within the orbit of Mars, Jupiter, &c. they are seen in all sides of the heavens, and are as often in opposition to the sun as in conjunction with him. Let the circle in which Tis situated (Plate IV. Fig. 8.) represent the orbit of the earth, and that in which M is situated the orbit of Mars. Now, if the earth be at T when Mars is at M, Mars and the sun will be in conjunction, but if the earth be at ✰ when Mars is at M, they will be in opposition, viz. the sun will appear in the east when Mars is in the If the earth stood still at T, the motion of the planet Mars would always appear direct; but the motion of the earth being more rapid than that of Mars, he will be overtaken and passed by the earth. Hence, Mars will have two stationary and one retrograde appearance. Suppose the earth to be at E when Mars is at M, he will be seen in the heavens among the fixed

west.