The Solar System

The solar system is composed as follows:—there is a central body, the sun, around which revolve along stated paths a number of important bodies known as planets. Certain of these planets, in their courses, carry along in company still smaller bodies called satellites, which revolve around them. With regard, however, to the remaining members of the system, viz. the comets and the meteors, it is not advisable at this stage to add more to what has been said in the preceding chapter.
For the time being, therefore, we will devote our attention merely to the sun, the planets, and the satellites.

Of what shape then are these bodies? Of one shape, and that one alone which appears to characterise all solid objects in the celestial spaces: they are spherical, which means round like a ball.

Each of these spherical bodies rotates; that is to say, turns round and round, as a top does when it is spinning. This rotation is said to take place "upon an axis," a statement which may be explained as follows:—Imagine a ball with a knitting-needle run right through its centre. Then imagine this needle held pointing in one fixed direction while the ball is turned round and round. Well, it is the same thing with the earth. As it journeys about the sun, it keeps turning round and round continually as if pivoted upon a mighty knitting needle transfixing it from North Pole to South Pole. In reality, however, there is no such material axis to regulate the constant direction of the rotation, just as there are no actual supports to uphold the earth itself in space. The causes which keep the celestial spheres poised, and which control their motions, are far more wonderful than any mechanical device.

At this juncture it will be well to emphasise the sharp distinction between the terms rotation and revolution. The term "rotation" is invariably used by astronomers to signify the motion which a celestial body has upon an axis; the term "revolution," on the other hand, is used for the movement of one celestial body around another. Speaking of the earth, for instance, we say, that it rotates on its axis, and that it revolves around the sun.

So far, nothing has been said about the sizes of the members of our system. Is there any stock size, any pattern according to which they may be judged? None whatever! They vary enormously. Very much the largest of all is the Sun, which is several hundred times larger than all the planets and satellites of the system rolled together. Next comes Jupiter and afterwards the other planets in the following order of size:—Saturn, Uranus, Neptune, the Earth, Venus, Mars, and Mercury. Very much smaller than any of these are the asteroids, of which Ceres, the largest, is less than 500 miles in diameter. It is, by the way, a strange fact that the zone of asteroids should mark the separation of the small planets from the giant ones. The following table, giving roughly the various diameters of the sun and the principal planets in miles, will clearly illustrate the great discrepancy in size which prevails in the system.

Sun	866,540	miles
Mercury	2,765	"
Venus	7,826	"
Earth	7,918	"
Mars	4,332	"
ZONE OF ASTEROIDS
Jupiter	87,380	"
Saturn	73,125	"
Uranus[3]	34,900	"
Neptune[3]	32,900	"

The earth takes about 365¼ days to revolve around the sun. This period of time is known to us as a "year." The following table shows in days and years the periods taken by each of the other planets to make a complete revolution round the sun:—

Mercury	about	88	days.
Venus	"	226	"
Mars	"	1	year and 321 days.
Jupiter	"	11	years and 313 days.
Saturn	"	29	years and 167 days.
Uranus	"	84	years and 7 days.
Neptune	"	164	years and 284 days.

From these periods we gather an important fact, namely, that the nearer a planet is to the sun the faster it revolves.

Compared with one of our years what a long time does an Uranian, or Neptunian, "year" seem? For instance, if a "year" had commenced in Neptune about the middle of the reign of George II., that "year" would be only just coming to a close; for the planet is but now arriving back to the position, with regard to the sun, which it then occupied. Uranus, too, has only completed a little more than 1½ of its "years" since Herschel discovered it.

Having accepted the fact that the planets are revolving around the sun, the next point to be inquired into is:—What are the positions of their orbits, or paths, relatively to each other?

Suppose, for instance, the various planetary orbits to be represented by a set of hoops of different sizes, placed one within the other, and the sun by a small ball in the middle of the whole; in what positions will these hoops have to be arranged so as to imitate exactly the true condition of things?

First of all let us suppose the entire arrangement, ball and hoops, to be on one level, so to speak. This may be easily compassed by imagining the hoops as floating, one surrounding the other, with the ball in the middle of all, upon the surface of still water. Such a set of objects would be described in astronomical parlance as being in the same plane. Suppose, on the other hand, that some of these floating hoops are tilted with regard to the others, so that one half of a hoop rises out of the water and the other half consequently sinks beneath the surface. This indeed is the actual case with regard to the planetary orbits. They do not by any means lie all exactly in the same plane. Each one of them is tilted, or inclined, a little with respect to the plane of the earth's orbit, which astronomers, for convenience, regard as the level of the solar system. This tilting, or "inclination," is, in the larger planets, greatest for the orbit of Mercury, least for that of Uranus. Mercury's orbit is inclined to that of the earth at an angle of about 7°, that of Venus at a little over 3°, that of Saturn 2½°; while in those of Mars, Neptune, and Jupiter the inclination is less than 2°. But greater than any of these is the inclination of the orbit of the tiny planet Eros, viz. nearly 11°.