4. From the moon the eye glances to those bodies, which are known to youth as stars. The observations of a short period are sufficient to establish the apparent truth, that most of them are fixed and stationary, always preserving the same apparent distance and direction from each other, but that some of them are wandering, continually changing their position with regard to other bodies apparently in their neighbourhood. The former are considered as stars, or fixed stars; the latter are planets. Occasionally a stranger appears, which, unlike other heavenly bodies, is accompanied by a train or tail more or less luminous, and which, in a longer or shorter period, becomes again invisible. These are comets.

5. These observations, which are now familiar to the mind in youth, not to say in childhood, show that all the heavenly bodies, except the stars, and perhaps the sun, are in motion. From this single fact result all the changes in nature. To produce day and night, either the sun goes round the earth, or the earth turns so as to present different parts to the sun, in a day. To produce the seasons, either the sun actually moves northward and southward, or the earth has such a motion as to present the northern part to the sun in one season, and the southern part in another. The moon, planets, and comets, by changing their position with regard to the stars, and also to each other, must obviously have a motion. In manhood, the mind inquires into the nature and motions of the heavenly bodies; obseryes the various phenomena, which they present; and, as

far as it is able, educes the laws, by which their motions are regulated. The Science, which explains these particulars, is called ASTRONOMY. It is divided into descriptive Astronomy, and physical Astronomy. The first includes an account of the phenomena of the heavenly bodies; the last explains the theory of their motions.

BOOK I.

DESCRIPTIVE ASTRONOMY,

CHAP J.

Sect. V. Of the Solar System in general. 6. The true Solar system, or, as it is sometimes called, the Copernican system, consists of the sun and an unknown number of bodies opaque, like our earth ; all of which bodies revolve round the sun, and some of which at the same time revolve round others. Those which revolve round the sun only, are called primary planets and comets. Those which revolve round a primary planet, at the same time that they are revolving round the sun, are called secondary planets, moons or satellites. The number of primary planets is 11, viz. Mercury, Venus, the Earth, Mars, Vesta, Juno, Pallas, Ceres, Jupiter, Saturn, and Uranus. The number of the secondary planets, moons or satellites is 18; the Earth has 1, Jupiter has. 4, Saturn has 7, and Uranus has 6. The number of the comets is unknown.

7. The sun is in the centre of the system. (See Frontispiece.) The primary planets move round him in the order above named, at different distances and in different times, from west to east. (It is to be noticed, that in all the figures referred to in this treatise, the upper part is south, the lower part north; the right hand west, and the left hand east.) They are often distinguished, éspecially in almanacks, by the signs used in the Frontispiece, viz. Mercury, f Venus,

Earth, Mars, Vesta, Juno, Pallas, 9 Ceres, 2 Jupiter, ņ Saturn, Uranus. The path, which a heavenly body describes in its revolution, is called its orbit. The secondary planets generally move round their primaries in the same direction, in which the primaries move round the sun. (The small circle round the earth, represents the moon's orbit. Each of the satellites of Jupiter, of Saturn, and of Uranus, describes an orbit round its primary, similar to that of the moon round the earth.) Comets move in all directions. A part of a comet's orbit is represented in the Frontispiece.

8. Though in the Frontispiece the orbits of the planets are circles, yet this is not their true form. All the revolving bodies in the solar system move in orbits oval or elliptical. (Pl. 1, fig. 2.). ABDE is an ellipse, and represents the orbit of a planet, say of the earth. The points S, s, are called foci of the ellipse. The sun, instead of being in the centre C, is in one of the foci, as S. In like manner, when a secondary planet revolves round a primary, the primary is not in the centre of its orbit, but in one of its foci. That focus of an orbit, in which the sun or a primary planet is, is called the lower focus; and the other is called the upper focus. When any body, revolving round the sun, is nearest to him, as at A, it is said to be in its perihelion; and when it is most distant, as at B, it is said to be in its aphelion. When the moon is nearest the earth, it is said to be in perigee; when at its greatest distance, it is said to be in apogee. The line ŠD is the mean distance of the orbit from the lower focus; SC is its eccentricity,