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 accoinpanied 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; observes 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. I. SECT. 1. 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, especially in almanacs, by the signs used in the Frontispiece, viz. Mercury, Venus, Earth, Mars, Vesta, Juno, Pallas, Ceres, 2 Jupiter, 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. I, 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 SD is the mean dis * To describe an Ellipse, pin down the ends of a string upon a table or piece of paper, at any two places, as S, s. The string should not be drawn, but be left slack. Then with a pencil stretch the string as far as it will extend in every direction, and the point of the pencil will describe an ellipse. The points S, s, where the string is fastened, are the foci. The ellipse will always be more or less eccentric in proportion as the string is drawn more or less tightly. tance of the orbit from the lower focus; SC is its eccentricity. Though the orbits of the planets in the frontispiece are circles, yet they are not concentric, that is, have not the same centre. The centre of each orbit is placed out of the centre of the sun at a distance equal to the cccentricity of its true orbit. Each planet is placed in its aphelion. The relative distances of the primary planets from the sun could not be well preserved in this figure, but are represented in the margin. 9. The sun and all the planets, primary and secondary, are globular, though not perfect globes. This is known of all, except the earth, by their always appearing nearly round to the naked eye, or through a telescope. It is known of the earth, by its shadow on the moon in an eclipse, which is always circular. Pl. I, fig. 4, represents the relative magnitudes of seven of the primary planets and the moon, together with the ring of Saturn, which will be described hereafter. The diameter of the sun in relation to that of the planets, as here represented, is about one foot. The relative sizes of the same planets are represented on the accompanying wood-cut. 10. The sun and the primary and secondary planets, as far as astronomers have means and opportunity of ascertaining, turn on imaginary lines passing through their centres, which are called axes. The time, in which the heavenly bodies turn on their axes, is various; but generally the largest turn quickest. A wire passing through the centre of an apple properly represents the axis of a planet. The extremities of an axis are called POLES. 11. If the earth were seen from the sun, (Pl. I, fig. 1,) it would appear to describe a circle among the stars, while it revolves in its orbit. For while it is passing from A to B, it would be seen to move among the stars from a to b. And in like manner through its |