The time of axial revolution which determines the length of the day varies with different planets.  The periods of the four planets nearest the sun vary only half an hour from that of the earth, while the enormous bodies of Jupiter and Saturn revolve in ten and ten and a quarter hours respectively.  This high rate of speed, and its resultant, centrifugal force, has aided in preventing these bodies from becoming as dense as they would otherwise be—­Jupiter being only 0.24 as dense as the earth, and Saturn only 0.13.  This extremely rapid revolution produces a great flattening at the poles.  If Jupiter should rotate four times more rapidly than it does, it could not be held together compactly.  As it is, the polar diameter is five thousand miles less than the equatorial:  the difference in diameters produced by the [Page 102] same cause on the earth, owing to the slower motion and smaller mass, being only twenty-six miles.  The effect of this will be more specifically treated hereafter.

The difference in the size of the planets is very noticeable.  If we represent the sun by a gilded globe two feet in diameter, we must represent Vulcan and Mercury by mustard-seeds; Venus, by a pea; Earth, by another; Mars, by one-half the size; Asteroids, by the motes in a sunbeam; Jupiter, by a small-sized orange; Saturn, by a smaller one; Uranus, by a cherry; and Neptune, by one a little larger.

Apply the principle that attraction is in proportion to the mass, and a man who weighs one hundred and fifty pounds on the earth weighs three hundred and ninety-six on Jupiter, and only fifty-eight on Mars; while on the Asteroids he could play with bowlders for marbles, hurl hills like Milton’s angels, leap into the fifth-story windows with ease, tumble over precipices without harm, and go around the little worlds in seven jumps.

[Illustration:  Fig. 39.—­Orbit of Earth, showing Parallelism of Axis and Seasons.]

The seasons of a planet are caused by the inclination of its axis to the plane of its orbit.  In Fig. 39 the rotating earth is seen at A, with its northern pole turning in constant sunlight, and its southern pole in constant darkness; everywhere south of the equator is more darkness than day, and hence winter.  Passing on to B, the world is seen illuminated equally on each side of the equator.  Every place has its twelve hours’ darkness and light at each revolution.  But at C—­the axis of the earth always preserving the same direction—­the northern pole is shrouded in continual gloom.  Every place [Page 105] north of the equator gets more darkness than light, and hence winter.

The varying inclination of the axes of the different planets gives a wonderful variety to their seasons.  The sun is always nearly over the equator of Jupiter, and every place has nearly its five hours day and five hours night.  The seasons of Earth, Mars, and Saturn are so much alike, except in length, that no comment is necessary.  The ice-fields at either pole of Mars are observed to enlarge and contract, according as it is winter or summer there.  Saturn’s seasons are each seven and a half years long.  The alternate darkness and light at the poles is fifteen years long.