Besides the above enumerated primary planets, our system contains eighteen secondary planets, or moons. Of these, our Earth has one moon, Jupiter four, Saturn seven, and Herschel six. None of these moons, except our own, and one or two of Saturn's, can be seen without a telescope. The seven other planets, so far as has been discovered, are entirely without moons.

All the planets move around the sun from west to east and in the same direction do the moons revolve around their primaries, with the exception of those of Herschel, which appear to revolve in a contrary direction.

The paths in which the planets move round the sun, and in which the moons move round their primaries, are called their orbits. These orbits are not exactly circular, as they are commonly represented on paper, but are elliptical, or oval, so that all the planets are nearer the sun, when in one part of their orbits, than when in another.

But

In addition to their annual revolutions, some of the planets are known to have diurnal, or daily revolutions, like our earth. The periods of these daily revolutions have been ascertained in several of the planets by spots on their surfaces. where no such mark is discernible, it cannot be ascertained whether the planet has a daily revolution or not, though this has been found to be the case in every instance where spots are seen, and, therefore, there is little doubt but all have a daily, as well as a yearly motion.

The axis of a planet is an imaginary line passing through its centre, and about which its diurnal revolution is performed. The poles of the planets are the extremities of this axis.

The orbits of Mercury and Venus are within that of the earth, and consequently they are called inferior planets. The orbits of all the other planets are without, or exterior to that of the earth, and these are called superior planets.

That the orbits of Mercury and Venus are within that of the earth, is evident from the circumstance, that they are never seen in opposition to the sun, that is, they never appear

How many moons does our system contain? Which of the planets are attended by moons, and how many has each? In what direction do the planets move around the sun? What is the orbit of a planet? What revolutions have the planets, besides their yearly revolutions? Have all the planets diurnal revolutions? How is it known that the planets have daily revolutions? What is the axis of a planet? What is the pole of a planet? Which are the superior, and which the inferior planets?

In the west, when the sun is in the east. On the contrary, the orbits of all the other planets are proved to be outside of the earth's, since these planets are sometimes seen in opposition to the sun.

Fig. 180.

This will be understood by fig. 180, where suppose s to be the sun, m the orbit of Mercury or Venus, e the orbit of the earth, and j tha of Jupiter. Now it is evident, that if a spectator be placed any where in the earth's orbit, as at e, he may sometimes see Jupiter in opposition to the sun, as at j, because then the spectator would be between Jupiter and the sun. But the orbit of Venus, being surrounded

by that of the earth, she never can come in opposition to the sun, or in that part of the heavens opposite to him, as seen by us, because our earth never passes between her and the sun.

It has already been stated, that the orbits of the planets are elliptical, and that, consequently, these bodies are sometimes ncarer the sun than at others. An ellipse, or oval, has two foci, and the sun, instead of being in the common centre, is always in the lower foci of their orbits. Fig. 181.

The orbit of a planet is represented by fig. 181, where a, d, b, e, is an ellipse, with its two foci, s and o, the sun being in the focus s, which is called the lower focus.

When the earth, or any other planet, revolving around the sun, is in that part of its orbit nearest the

How is it proved that the inferior planets are within the earth's orbit, and the superior ones without it? Explain fig. 180, and show why the inferior planets never can be in opposition to the sun. What are the shapes of the planetary orbits? What is meant by perihelion ?

sun, as at a, it is said to be in its perihelion; and when in that part which is at the greatest distance from the sun, as at b, it is said to be in its aphelion. The line s, d, is the mean, or average distance of a planet's orbit from the sun.

Ecliptic. The planes of the orbits of all the planets pass through the centre of the sun. The plane of an orbit is an imaginary surface, passing from one extremity or side of the orbit, to the other. If the rim of a drum head be considered the orbit, its plane would be the parchment extended across it, on which the drum is beaten.

Let us suppose the earth's orbit to be such a plane, cutting the sun through his centre, and extending out on every side to the starry heavens; the great circle so made, would mark the line of the ecliptic, or the sun's apparent path through the heavens.

This circle is called the sun's apparent path, because the revolution of the earth gives the sun the appearance of passing through it. It is called the ecliptic, because eclipses happen when the moon is in, or near, this apparent path.

Zodiac.-The Zodiac is an imaginary belt, or broad circle, extending quite around the heavens. The ecliptic divides the zodiac into two equal parts, the zodiac extending 8 degrees on each side of the ecliptic, and therefore is 16 degrees wide. The zodiac is divided into 12 equal parts, called the signs of the zodiac.

The sun appears every year to pass around the great circle of the ecliptic, and consequently, through the 12 constellations, or signs of the zodiac. But it will be seen, in another place, that the sun, in respect to the earth, stands still, and that his apparent yearly course through the heavens is caused by the annual revolution of the earth around its orbit.

To understand the cause of this deception, let us suppose

What is the plane of an orbit? Explain what is meant by the ecliptic. Why is the ecliptic called the sun's apparent path? What is the zodiac? How does the ecliptic divide the zodiac? How far does the zodiac exterul, n each side of the ecliptic?

Fig. 182.

that s, fig: 182, is the sun, a b, a part of the circle of the ecliptic, and cd, a part of the earth's orbit. Now, if a spectator be placed at c, he will see the sun in that part of the ecliptic marked by b, but when the earth moves in her annual revolution to d, the spectator will see the sun in that part of the heavens marked by a; so that the motion of the earth in one direction, will give the sun an apparent motion in the contrary direc tion.

A sign, or constellation, is a collection of fixed stars, and, as we have already seen, the sun appears to move through the twelve signs of the zodiac every year. Now the sun's place in the heavens, or zodiac, is found by his apparent conjunction, or nearness to any particular star in the constellation. Suppose a spectator at c, observes the sun to be nearly in a line with the star at b, then the sun would be near a particular star in a certain constellation. When the earth moves to d, the sun's place would assume another direction, and he would seem to have moved into another constellation, and near the star a.

Each of the 12 signs of the zodiac is divided into 30 smaller parts, called degrees; each degree into 60 equal parts, called minutes, and each minute into 60 parts, called seconds.

The division of the zodiac into signs, is of very ancient date, each sign having also received the name of some animal, or thing, which the constellation, forming that sign, was supposed to resemble. It is hardly necessary to say, that this is chiefly the result of imagination, since the figures made by the places of the stars, never mark the outlines of the figures of animals, or other things. This is, however, found to be the most convenient method of finding any particular star at this day, for among astronomers, any star, in each constellation, may be designated by describing the part of the animal in which it is

Explain fig. 182, and show why the sun seems to pass through the ecliptic, when the earth only revolves around the sun. What is a constellation, or sign? How is the sun's apparent place in the heavens found? Into how many parts are the signs of the zodiac divided, and what are these parts alled?

situated. Thus, by knowing how many stars belong to the constellation Leo, or the Lion, we readily know what star is meant by that which is situated on the Lion's ear or tail.

The names of the 12 signs of the zodiac are, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. The common names, or meaning of these words, in the same order, are, the Ram, the Bull, the Twins, the Crab, the Lion, the Virgin, the Scales, the Scorpion, the Archer, the Goat, the Waterer, and the Fishes.

Fig. 183.

Sagitarius

Aquarius
**

Genini *

The 12 signs of the zodiac, together with the sun, and the earth revolving around him, are represented at fig. 183. When the earth is at A, the sun will appear to be just entering the

Is there any resemblance between the places of the stars, and the figures of the animals after which they are called? Explain why this is a convenient method of finding any particular star in a sign. What are the zames of the 12 signs?