Page:The Music of the Spheres.djvu/242

 frequently are called the Terrestrial Planets because they all have solid crusts like the earth.

Jupiter, Saturn, Uranus and Neptune, forming a group called the outer or major planets, are more or less in a gaseous condition and are very huge in comparison to the first four.

The orbit of Neptune, the farthermost planet, encloses the orbits of all the other planets, and is so very large that it requires 165 years for Neptune to travel around it. This is quite a contrast to the time consumed by little Mercury who flies so close to the sun that he travels around it in 88 days. This, of course, means the length of the year on each of these planets.

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