CHAPTER LI.

PARLEY TELLS ABOUT THE SECONDARY PLANETS, OR SATELLITES, OR MOONS.

1. I HAVE told you something about all the primary planets, or planets more commonly so called; but I have not yet told you about the secondary planets, or moons, or satellites, or attendants upon some of the primary planets.

2. A satellite is anything that follows or attends upon another. The moon is called a satellite because it follows or attends upon the earth.

3. The earth is not the only planet that has a moon, or satellite, or secondary planet to accompany it. Jupiter has four, Uranus has six, and Saturn has seven.

4. These satellites or secondary planets of Jupiter, Saturn, and Uranus, are called moons, only because they resemble the moon, which is the single satellite, or secondary or attendant planet, of our primary planet the earth.

QUESTIONS.

1. What about secondary planets? Satellites? Moons? 2. What is a satellite? Why is the moon called a satellite? 3. How many satellites or moons has Jupiter? Saturn? Uranus? 4. Why are the satellites of Jupiter, Saturn, and Uranus called Moons?

CHAPTER LII.

ABOUT ALL THE PLANETS TOGETHER, AND ABOUT THE ORBITS OF THE PLANETS.

1. I HAVE now told you about all the planets belonging to the Solar System. As I have told you before, they all revolve round the sun, from west to east, in great circles, called their paths or orbits. Some of these planets are nearer the sun than others. The nearest is Mercury, which is thirty-seven millions of miles from the sun. The furthest off is Herschel, or Uranus, or the Georgium Sidus, which is eighteen hundred millions of miles from the sun.

2. These planets are of various magnitudes or sizes. The smallest is Mercury, which is much smaller than the earth; the largest is Jupiter, which is at least one thousand two hundred times as large as the earth. The picture on the opposite page shows you the comparative sizes of the planets.

3. All these planets, so far as known, turn on their axes, or perform their diurnal revolutions in different spaces of time. Jupiter turns in little less than ten hours, which is the shortest time. Mars turns in twenty-five hours, which is the longest time in which any planet performs its diurnal revolution. Jupiter has the shortest, and Mars the longest days and nights of any of the planets. It appears that the

Thus

largest planets turn, or revolve on their axes, in the shortest time.

4. As planets move at different distances from the sun, the orbits or circles in which they move are of different lengths. Consequently, they move around the sun in different periods of time. The shortest period is that of Mercury, which is about three months. The longest period is that of Uranus, which is about eightyfour years.

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5. All the planets are round, or nearly round, or spherical. The earth is about twenty-five miles further through, at the equator, than at the poles; it is not, therefore, a perfect round, or sphere, or globe. It is called a spheroid, by which is meant a globe or spherelike figure, but not a perfect sphere or globe. It is supposed that the other planets also are spheroids.

6. The planets move in their orbits with different degrees of velocity; those nearest the sun the swiftest. Mercury moves at the rate of one hundred and ten thousand miles an hour, or one thousand six hundred and sixty-six miles in a minute. While you can walk across a room, it flies two hundred miles!

7. Venus moves at the rate of seventy-six thousand miles an hour; the earth at the rate of sixty-four thousand, and Mars at the rate of fifty-five thousand miles an hour.

8. The planets being at different distances from the sun, receive different degrees of light and heat. At Mercury, the heat and light are supposed seven times as great as upon the earth. If any of our animals or plants were carried from the earth to Mercury, they would immediately perish, from the excessive heat. If there are such creatures there, they must be imagined of different natures from those of the earth.

9. The degrees of light and heat at Uranus are assumed as three hundred and sixty times less than upon the earth. Even during the day, that is, when the sun is shining upon that distant planet, the light,

as it must seem to us probable, is very dim, like our twilight. The cold must be greater than we can conceive. Any of the plants or animals of our world would be instantly frozen to death there. There can be no water there; for in such a degree of cold, it would be instantly converted to ice, as solid as our rocks.

10. You remember that several of the primary planets have moons, or satellites, or secondary planets, revolving around them. The earth has one, Jupiter four, Saturn seven, and Uranus six; in all, eighteen.

11. Thus I have told you of the seven larger primary planets, Mercury, Venus, the Earth, Mars, Jupiter, Saturn, and Uranus. I have told you of the five asteroids, or lesser primary planets, counting the planet so lately discovered by Cacciatore. I have also told you of the eighteen satellites or moons which move in their orbits round their several primary planets; and I have told you of the two great bright rings which move round Saturn.

QUESTIONS.

1. In what direction do all the planets revolve round the sun? Which planet is the nearest to the sun? Which is the furthest from the sun? 2. Which is the smallest planet? The largest? How much larger is Jupiter than the earth? 3. How long is Jupiter turning on its axis? How long is Mars? Which planet has the longest days and nights, and which the shortest? 4. How long is Mercury revolving round the sun? Is this the shortest period? What is the longest revolution round the sun? 5. Are the orbits of the planets round? What figure are they? Are all the planets round? Why is the earth called a spheroid? 6. Do all the planets move with the same velocity? Which planets move