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The Solar System is mainly comprised within the limits of the Zodiac. It consists of—
1. The Sun—the centre.
2. The major planets—Vulcan (undetermined), Mercury,
Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. 8. The minor planets, at present one hundred and seventeen in number. (The paths of some extend a little outside the Zodiac.)
4. The satellites or moons, eighteen in number, which re
volve around the different planets.
5. Meteors and shooting-stars.
6. Nine comets whose orbits have been computed, and
over two hundred of which little is known.
7. The Zodiacal Light.
HOW WE ARE TO IMAGINE THE SOLAB SYSTEM TO OUB
Seltes.—We are to think of it as suspended in space; being held up, not by any visible object, but in accordance with the law of Universal Gravitation discovered by Newton, whereby each planet attracts every other planet and is in turn attracted by alL First, the Sun, a great central globe, so vast as to overcome the attraction of all the planets, and compel them to circle around him; next, the planets, each turning on its axis while it flies around the sun in an elliptical orbit; then, accompanying these, the satellites, each revolving about its own planet, while all whirl in a dizzy waltz about the central orb; next, the comets, rushing across the planetary orbits at irregular intervals of time and space; and finally, shooting-stars and meteors darting hither and thither, interweaving all in apparently inextricable confusion. To make the picture more wonderful still, every member is flying with an inconceivable velocity, and yet with such accuracy that the solar system is the most perfect timepiece known.
Sign, ©, a buckler with its boss.
Distance.—The sun's average distance from the earth is about 91^ million miles. Since the orbit of the earth is elliptical, and the sun is situated at one of its foci, the earth is nearly 3,000,000 miles further from the sun in aphelion than in perihelion. As we attempt to locate the heavenly bodies in space, we are immediately startled by the enormous figures employed. The first number, 91,500,000 miles, is far beyond our grasp. Let us try to comprehend it. If there were air to convey a sound from the sun to the earth, and a noise could be made loud enough to pass that distance, it would require over fourteen years for it to come to us. Suppose a railroad could be built to the sun. An express-train, travelling day and night, at the rate of thirty miles an hour, would require 341 years to reach its destination. Ten generations would be born and would die; the young men would become gray-haired, and their great-grandchildren v ould forget the story of the beginning of that wonderful journey, and could find it only in history, as we now read of Queen Elizabeth or of Shakspeare; the eleventh generation would see the solar depot at the end of the route. Yet this enormous distance of 91,500,000 miles is used as the unit for expressing celestial distances —as the foot-rule for measuring space; and astronomers speak of so many times the sun's distance as we speak of so many feet or inches.
The Light Of The Sun.—This is equal to 5,563 wax-candles held at a distance of one foot from the eye. It would require 800,000 full-moons to produce a day as brilliant as one of cloudless sunshine.
The Heat Of The Sun.—The amount of heat we receive annually is sufficient to melt a layer of ice thirty-eight yards in thickness, extending over the whole earth. Yet the sunbeam is only -yrnr/innr part as intense as it is at the surface of the sun. Moreover, the heat and light stream off into space equally in every direction. Of this vast flood only one twenty-three hundred millionth part reaches the earth. It is said that if the heat of the sun were produced by the burning of coal, it would require a layer ten feet in thickness, extending over the whole
sun, to feed the flame a single hour. Were the sun
Dimensions.—Its diameter is about 850,000 miles.*
A mountain upon the surface of the sun, to bear the same proportion to the globe itself as the Dhawalaghiri of the Himalayas does to the earth, would have to be about six hundred miles high.
Again: Suppose the sun were hollow, and the earth, as in the cut (Fig. 4), placed at the centre, not only would there be room for the moon to revolve in its regular orbit within the shell, but that would stretch off in every direction 200,000 miles beyond.
Its volume is 1,245,000 times that of the earth—
* Pythagoras, whose theory of the universe was in so many respects veiy like the one we receive, believed the sun to be 44,000 miles from the earth, and 75 miles in diameter.