The earth rotates uniformly about its axis. The time taken to make a
complete revolution of three hundred and sixty degrees is called a
sidereal day, for it is the interval of time between consecutive
transits of any distant star across any meridian of the earth. The time
between consecutive transits of the sun across any meridian is called a
solar day; the average of these throughout the whole year is called a
mean solar day, and is the practical standard of time adopted by
civilized nations. The ordinary proofs that the earth rotates are: (1)
Bodies falling from a great height have an easterly deviation; (2)
Foucault’s pendulum experiment; (3) a gyroscope delicately balanced so
as to be free to change the direction of its axis in any way will, if
rotated, exhibit an apparent deviation; (4) in northern hemispheres a
projectile deviates to the right, in southern hemispheres to the left;
(5) the trade winds; (6) Dove’s law of wind-change.
The speed of a body on the equator, due to the diurnal rotation, is
about 1,000 miles an hour. The centrifugal force due to this speed
diminishes the weight of bodies; if the earth rotated in an hour, they
would be thrown off from the surface at the equator.
The axis of the earth is not perpendicular to the ecliptic, but at angle
of 66° 32′ to it; the equator is, therefore, inclined to it at an angle
of 23° 28′. This unsymmetrical placing of the bulging portions of the
earth causes a slow wobbling, or precession of its axis, in the same
sort of way as a spinning top will wobble when pushed over on one side.
There is also a slight vibration or “nodding” motion of the earth’s
axis, known as nutation. The period of each precession is about
twenty-one thousand years; if the earth’s orbit occupied a constant
position in its plane, the periods would be twenty-six thousand years
each. These motions have considerable influence on climate, the modern
theories of the Ice Age being connected with the known facts of
precessional motion.