OF THE SOLAR SYSTEM.
It is fully proved that the planets, with the earth which we inhabit, and also the moon, revolve round the sun, which is fixed in the centre of the system. There are two kinds of planets, primary and secondary. The first move round the sun, and respect him only as the centre of their motions. The secondary planets, called also satellites or moons, are small er planets, revolving round the primary, while they, with the primary planets, about which they move, are carried round the sun. The planets move round the sun at various distances, some being much nearer to him than our earth, and others being much farther off. There are 11 primary planets, which are situated, with respect to their distances from the sun, as follows : Mercury ; Venus Q • the Earth ® ; Mars ; Ceres, Pallas, Juno, Vesta, Jupiter IL ; Saturn ; and the Herschel planet, or the Georgium Sidus . (See plate I. Astronomy.) Of these our earth is accompanied by one moon, Jupiter has four moons, Saturn has seven moons, and the Herschel planet has six moons. None of these moons, except our own, can be seen without a good tele scope. The other five planets do not ap pear to have any satellites, or moons. All the planets move round the sun from west to east, and in the same direction do the moons revolve round their primaries, ex centine those of the Herschel planet.
which seem to move in a contrary direc tion. The paths in which they move round the sun are called their orbits. These orbits are elliptical ; but the ec centricity of the ellipses is so small, that they approach very nearly to circles. They perform their revolutions also in very different periods of time. The time of performing their revolutions is called their year. The planets are evidently opaque bodies, and they shine only by reflecting the light which they receive from the sun ; for Mercury and Venus, when viewed by a telescope, often ap pear to be only partly illuminated, and have the appearance of our moon when she is cusped or horned, having the illu mined part always turned towards the sun. From the appearance of the boundary of light and shadow upon their surfaces, we conclude that they are spherical; which is confirmed by some of them having been found to turn periodically on their axes. Venus and Mercury, being nearer to the sun than our earth, are called inferior pla nets, and all the rest, which are without the earth's orbit, are called superior pla nets. That the first go round the sun is certain, because they are seen sometimes passing between us and the sun,and some times they go behind it. That their or bits are within that of the earth is evident, because they are never seen in opposi tion to the sun, that is, appearing to rise from the horizon in the east when the sun is setting in the west, which is another proof that the earth is not the centre of celestial motions. On the contrary, the orbits of all the other planets surround that of the earth ; for they sometimes are seen in opposition to the sun, and they never appear to be horned, but always nearly or quite full, though sometimes Mars appears a little gibbous, or some what deficient from full.
Since all the planets move round the sun in elliptical orbits, the sun itself is situated in one of the foci of each ellipse. That focus is called the lower focus. If we suppose the plain of the earth's orbit, which passes through the centre of the sun, to be extended in every direction as far as the fixed stars, it will mark out among them a great circle, which is the ecliptic ; and with this the situations of the orbits of all the other planets are com pared. The planes of the orbits of all
the other planets must necessarily pass through the centre of the sun ; but if ex tended as far as the fixed stars, they form circles different from one another, as also from the ecliptic ; one part of each orbit being on the north, and the other on the south side of the ecliptic. Therefore the orbit of each planet cuts the ecliptic in two opposite points, which are called the nodes of that particular planet, and the nodes of one planet cut the ecliptic in planes different from the nodes of another planet. A line passing from one node of a planet, to the opposite node, or the line in which the plane of the orbit cuts the ecliptic, is called the line of nodes. That node where the planet passes from the south to the north side of the ecliptic is called the ascending node, and the other is the descending node. The angle which the plane ofa planet's orbit makes with the plane of the ecliptic is called the inclina tion of that planet's orbit. Thus, fig. 2. Plate IL where F represents the sun, the points A and B represent the nodes, and the line AB the line of nodes formed by the intersection of the planes of the orbits C and D. The angle E F G is the angle of inclination of the planes of the two orbits to each other. A line drawn from the lower focus of a planet's orbit (viz. where the sun is) to either end of the con jugate axis of its orbit, (which line is equal to half the transverse axis) is called the mean distance of the planet from the sun. But, according to some, the mean distance is a mean proportional between the two axes of that planet's orbit. The distance of either focus from the centre of the orbit is called its eccentricity. The two points in a planet's orbit, which are farthest and nearest to the body round which it moves, are called the apsides ; the former of which is called the higher apsis, or aphelion; the latter is called the lower apsis, or perihelion. The diame ter which joins these two points is called the line of the apsides. When the sun and moon are nearest to theearth, they are said to be in perigee. When at their greatest distance from the earth, they are said to be in apogee. When a planet is situated so as to be between the sun and the earth, or so that the sun is between the earth and the planet, then that planet is said to be in conjunction with the sun. When the earth is between the sun and any planet, then that planet is said to be in opposition. It is evident that the two inferior planets must have two conjunc tions with the sun, and the superior pla nets can have only one, because they can never come between the earth and the sun. When a planet comes directly be tween us and the sun, it appears to pass over the sun's disc, Or surface, and this is called the transit of the planet. When a planet moves from west to east, viz. ac. cording to the order of the signs, it is said to have direct motion, or to be in consequentia. Its retrograde motion, or motion in antecedentia, is when it appears to move from east to west, viz. contrary to the order of the signs. The place that any planet appears to occupy in the ce lestial hemisphere, when seen by an ob server supposed to be placed in the sun, is called its heliocentric place. The place it occupies, when seen from the earth, is called its geocentric place.