This is the name given to that collection of worlds which, with the sun as a centre, circulate at different distances from and around it. There have at present been discovered eight large planets and thirty-four smaller ones called "asteroids" or "planetoids," the larger planets revolving in the following order from the sun or centre:-- 1. MERCURY. 2. VENUS. 3. THE EARTH. 4. MARS. 5. JUPITER. 6. SATURN. 7. URANUS. 8. NEPTUNE. The Sun, the centre of the solar system, is the great source of light and heat to our planet and all others which revolve around it, as well as of another agent not so well understood, namely "actinism," or that power which produces the chemical changes in many substances exposed to the sun's rays, and which has been of late turned to such useful and wonderful account in the art of photography. The sun is an immense sphere, many thousand times larger than our earth, or indeed all the planets put together. It turns upon its axis in twenty-five days, and, like all spheres which rotate, has a slightly flattened form; it is supposed that the sun itself, carrying its planets with it, performs a journey round some other centre very far away, but for all the purposes of explaining the solar system it may be supposed to be stationary. The diameter of the sun is about 890,000 miles, or nearly 112 times the diameter of our earth, and as the bulk of spheres is as the cubes of their diameters, it follows that the sun is about 1,400,000 times the bulk of the earth. Fig. 1 shows the relative size of the sun and the planets which rotate round it; the specific gravity of the sun is about a quarter that of our earth or one-and-a-half times that of water. On its surface irregularities are seen (by the aid of the telescope) which give it a wavy appearance, and beside these, black spots or tracts (_maculæ_) which appear to traverse from west to east, across its face; these however do not travel on, but are carried round by the sun in its rotation, for by observing any one of them, it is seen to go across and make its appearance again at the other side, and the time it takes to arrive at the place where it was first seen, determines the time of the sun's rotation, which (as before stated) is twenty-five days or nearly; these spots alter in size, form, and number from time to time, some of them remaining permanent for months, others closing in and disappearing in a few hours. [Illustration: FIG. 1. PLANETS COMPARED WITH A QUARTER OF THE SUN.]