In the stages of concentration the planetary nebulae might well repeat those through which the greater solar mass proceeded. If the volume of the material were great, subordinate rings would be formed, which when they broke and concentrated would constitute secondary planets or satellites, such as our moon. For some reason as yet unknown the outer planets--in fact, all those in the solar system except the two inner, Venus and Mercury and the asteroids--formed such attendants. All these satellite-forming rings have broken and concentrated except the inner of Saturn, which remains as an intellectual treasure of the solar system to show the history of its development. To the student who is not seeking the fulness of knowledge which astronomy has to offer, but desires only to acquaint himself with the more critical and important of the heavenly phenomena which help to explain the earth, these features of planetary movement should prove especially interesting for the reason that they shape the history of the spheres. As we shall hereafter see, the machinery of the earth's surface, all the life which it bears, its winds and rains--everything, indeed, save the actions which go on in the depths of the sphere--is determined by the heat and light which come from the sun. The conditions under which this vivifying tide is received have their origin in the planetary motion. If our earth's path around the centre of the system was a perfect circle, and if its polar axis lay at right angles to the plane of its journey, the share of light and heat which would fall upon any one point on the sphere would be perfectly uniform. There would be no variations in the length of day or night; no changes in the seasons; the winds everywhere would blow with exceeding steadiness--in fact, the present atmospheric confusion would be reduced to something like order. From age to age, except so far as the sun itself might vary in the amount of energy which it radiated, or lands rose up into the air or sunk down toward the sea level, the climate of each region would be perfectly stable. In the existing conditions the influences bring about unending variety. First of all, the inclined position of the polar axis causes the sun apparently to move across the heavens, so that it comes in an overhead position once or twice in the year in quite half the area of the lands and seas. This apparent swaying to and fro of the sun, due to the inclination of the axis of rotation, also affects the width of the climatal belts on either side of the equator, so that all parts of the earth receive a considerable share of the sun's influence. If the axis of the earth's rotation were at right angles to the plane of its orbit, there would be a narrow belt of high temperature about the equator, north and south of which the heat would grade off until at about the parallels of fifty degrees we should find a cold so considerable and uniform