Since the planets shine by light reflected from the sun, half of their globes are always bright,--and the other half is always hidden in the darkness of the shadow which trails behind. Each planet, however, rotates on its axis, which gives each side in turn a night and day. Exception to this must be made in the case of Mercury and Venus, their rotation with respect to the sun having been destroyed by the powerful tidal actions of the sun eons ago. Hence only one hemisphere in each of these planets is warmed by the light of the sun; the other forever faces the darkness and the cold of outer space. The period of a planet's rotation is determined by watching its markings, just as the period of the sun's rotation was determined by watching its spots. The larger planets rotate much more swiftly than the smaller ones; a "day" on Jupiter or Saturn is not half as long as on Mars or on Earth. The seasons of a planet depend on the angle at which its axis is tipped. The axis of the earth tips over at an angle of 23½ degrees from the perpendicular to the plane of its orbit, so its seasons are fairly well divided. Jupiter leans only 3 degrees and has little seasonal change but poor Uranus is inclined about 98 degrees and is subjected to the direst extremes. Since the year of Uranus consists of 84 of our years, each hemisphere has 42 years of darkness and 42 years of light. The four inner planets are quite solid in comparison to the four outer ones, which are largely gaseous. [Figure 142: The Axis of the Earth tips from the Perpendicular to the Plane of its Orbit.] Mercury is the most solid planet of all, being more like metal than the earth; neither Jupiter nor Saturn have stable surface crusts while Saturn is so light that it would float on water. Following are the diameters of the planets starting from Mercury, nearest to the sun, to Neptune, the farthest away. Mercury 3,030 miles Venus 7,700 " Earth 7,927 " Mars 4,230 " Jupiter 88,300 " Saturn 73,700 " Uranus 32,000 " Neptune 35,000 " The planets that are relatively near the sun have few or no satellites, Mercury and Venus having none, Earth one, Mars two; those farther distant have a goodly number, Uranus possessing four, Jupiter nine and Saturn ten.