Axis, Inclination of

The equators of rotating bodies appear never to parallel their orbits. Hence there is an inclination of the axis when considered in reference to the plane of the orbit. Within the solar system these inclinations arc, at this epoch, as follows: Mercury 72°, Venus 60°, the Earth 23½°, Mars 25°, Jupiter 3°, Saturn 26°, Uranus 102°, Neptune 155°, Pluto unknown. The inclination of the Sun's axis to the plane of the Earth's orbit is about 7°. Its inclination to the plane of its own orbit is unknown, because the Sun's orbit is itself unknown. It is claimed by some that there is an additional motion of the Earth's axis amounting to 50" a century, making an orbit of about 2½ million years, in the course of which the North Pole and the South Pole successively point to the Sun instead of as at present to the Pole Star. This theory is advanced by way of explanation for successive Ice Ages.