Page images
PDF
EPUB

round him at different distances, and at various periods, from 1 to 80 days.

Saturn is distinguished from the other planets by his ring, as Jupiter is by his belt. When this planet is viewed through a telescope, he appears surrounded by an immense luminous circle, which is represented by fig. 186.

Fig. 186.

There are indeed two luminous circles, or rings, one within the

[graphic]

other, with a dark

space between them, so that they do not appear to touch each other. Neither does the

inner ring touch the body of the planet, there being, by estimation, about the distance of 30,000 miles between them. The external circumference of the outer ring is 640,000 miles, and its breadth from the outer to the inner circumference, 7,200 miles, or nearly the diameter of our earth. The dark space, between the two rings, or the interval between the inner, and outer ring, is 2800 miles.

This immense appendage revolves round the sun with the planet,-performs daily revolutions with it, and according to Dr. Herschel, is a solid substance, equal in density to the body of the planet itself.

The design of Saturn's ring, an appendage so vast, and so different from any thing presented by the other planets, has always been a matter of speculation and inquiry among astronomers. One of its most obvious uses appears to be that of reflecting the light of the sun on the body of the planet, and possibly it may reflect the heat also, so as in some degree to soften the rigour of so inhospitable a climate.

As this planet revolves around the sun, one of its sides is lluminated during one half of the year, and the other side during the other half; so that, as Saturn's year is equal to thirty of our years, one of his sides will be enlightened and darkened, alternately, every fifteen years, as the poles of our earth are alternately in the light and dark every year.

How many moons has Saturn? How is Saturn particularly distinguished from all the other planets? What distance is there between the body of Saturn and his inner ring? What distance is there between his inner and outer ring? What is the circumference of the outer ring?

Fig. 187.

[graphic]

Fig. 187 represents Sat urn as seen by an eye pla ced at right angles to the plane of his ring. When seen from the earth, his position is always oblique, as represented by fig. 186. The inner white circle, represents the body of the planet, enlightened by the sun. The dark circle next to this, is the unenlightened space between the body of the planet and the inner ring, being the dark expanse of the heavens beyond the planet. The two white circles are the rings of the planet, with the dark space between them, which also is the dark expanse of the heavens.

Herschel.

In consequence of some inequalities in the motions of Ju piter and Saturn, in their orbits, several astronomers had suspected that there existed another planet beyond the orbit of Saturn, by whose attractive influence these irregularities were produced. This conjecture was confirmed by Dr. Herschel, in 1781, who in that year discovered the planet, which is now generally known by the name of its discoverer, though called by him Georgium sidus. The orbit of Herschel is beyond that of Saturn, and at the distance of 1800 millions of miles from the sun. To the naked eye this planet appears like a star of the sixth magnitude, being, with the exception of some of the comets, the most remote body, so far as is known, in the solar system.

Herschel completes his revolution round the sun in nearly $4 of our years, moving in his orbit at the rate of 15,000 miles in an hour. His diameter is 35,000 miles, so that his bulk is about eighty times that of the earth. The light and heat of

How long is one of Saturn's sides alternately in the light and dark? In what position is Saturn represented by fig. 187? What circumstance led to the discovery of Herschel ? In what year, and by whom, was Herschel discovered? What is the distance of Herschel from the sun? In what period is his revolution round the sun performed? What is the diameter of Herschel ?

ue sun at Herschel is about 360 times less than it is at the earth, and yet it has been found by calculation, that this light is equal to 248 of our full moons, a striking proof of the inconceivable quantity of light emitted by the sun.

This planet has six satellites, which revolve round him at various distances, and in different times. The periods of some of these have been ascertained, while those of the others remain unknown.

Fig. 188.

Herschel

AComet

Having now given a short account of each planet composing the solar system, the relative situation of their several orbits, with the exception of those of the Asteroids, are shown by fig. 189.

In the figure, the orbits are marked by the signs of each What is the quantity of light and heat at Herschel, when compared with that of the earth?

planet, of which the first, or that nearest the sun, is Mercury, the next Venus, the third the Earth, the fourth Mars; then come those of the Asteroids, then Jupiter, then Saturn, and lastly Herschel.

The comparative dimensions of the planets are delineated at fig. 189.

Fig. 189.

[merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small]

It is said that when Sir Isaac Newton was near demonstrating the great truth, that gravity is the cause which keeps the heavenly bodies in their orbits, he became so agitated with the thoughts of the magnitude and consequences of his discovery, as to be unable to proceed with his demonstrations, and desired a friend to finish what the intensity of his feelings would not allow him to complete.

We have seen, in a former part of this work, that all undisturbed motion is straight forward, and that a body projected into open space, would continue, perpetually, to move in a right line, unless retarded or drawn out of this course by

some external cause.

To account for the motions of the planets in their orbits, we will suppose that the earth, at the time of its creation, was thrown by the hand of the Creator into open s¡ .ce, the sun having been before created and fixed in his present place.

Under Compound Motion, it has been shown, that when a body is acted on by two forces perpendicular to each other, its motion will be in a diagonal line between the direction of the two forces.

[blocks in formation]

But we will again here suppose that a ball be moving in the line m x, fig. 190, with a given force, and that another force halfas great should strike it in the direction of n, the ball would then describe the diagonal of a parallelogram,

whose length would be just equal to twice its breadth, and the line of the ball would be straight, because it would obey the impulse and direction of these two forces only.

Fig. 191.

[blocks in formation]

Now let a, fig. 191, represent the earth, and S the sun; and suppose the earth to be moving forward, in the line from a to b, and to have arrived at a, with a velocity sufficient, in a given time, and without disturbance, to have carried it to b. But at the point a, the sun S acts upon the earth with his attractive power, and with a force which would draw it to c,

in the same space of time that it would otherwise have gone to b. Then the earth, instead of passing to b, in a straight line, would be drawn down to d, the diagonal of the parallelogram a, b, d, c. The line of direction, in fig. 190, is straight, because the body moved obeys only the direction of the two forces, but it is curved from a to d, fig. 191, in consequence of the continued force of the sun's attraction, which produces a constant deviation from a right line.

When the earth arrives at d, still retaining its projectile or centrifugal force, its line of direction would be towards n, but while it would pass along to n without disturbance, the attracting force of the sun is again sufficient to bring it to e,

Suppose a body to be acted on by two forces perpendicular to each other, in what direction will it move? Why does the ball, fig. 190, move in a straight line? Why does the earth, fig. 191, move in a curved line? Explain fig. 191, and show how the two forces ac to produce a circular line of motion.

« PreviousContinue »