Rotation
in astronomy, the turning round of a planet on its imaginary axis, like that of a wheel on its axle.
In the infancy of astronomy it was assumed that the earth was at rest, and that the sun and stars moved round it from E. to W. After note had been taken of the fact that when a boat is gently gliding along a canal or tranquil lake, the sensation to one on board is as if the boat was stationary, and the objects on the bank moved past in the opposite direction, a second hypothesis became worth consideration, viz., that the apparently stationary earth might be like the moving boat, and the heavens resemble the really stationary bank. It I-Cyc Vol 8 ROTATION OF CROPS gathered strength when it was considered that the earth was not a sphere but an oblate spheroid, as if rapid whirling had bulged it out at the equator, that Jupiter was yet more flattened at the poles than the earth, and that the direction of the trade winds, cyclones, etc., seemed the result of rotation. In 1851 Foucault completed the proof by making visible to the eye that a pendulum with a very long string alters its direction in a way which cannot be accounted for except by rotation (see GYROSCOPE). The rotation of the earth is performed with a uniform motion from W. to E. and occupies the interval in time which would elapse between the departure of a star from a certain point in the sky sky and its return to the same point again. The only motions which interfere with its regularity are those of the precession of the equinoxes and nutation (see PRECESSION). The time taken for the rotation of the earth measures the length of its day. So with the other planets. The sun also rotates as is shown by the movement of spots across its disk (see SUN). The earth's rotation slightly increases the force of gravity in moving from the equator to the poles. Sir William Thomson reasoning from some small anomalies in the moon's motion, inferred that 10,000,000 years ago the earth rotated one-seventh faster than it does now, and that the centrifugal force then was to that now as 64 to 49.
In botany, a rotary movement of a layer of protoplasm, investing the whole internal surface of a cell, as well seen in Chara, etc. It was first investigated by Corti in 1774. Called more fully intercellular rotation. In physiology: (1) The movement of a bone round its axis, without any great change of situation. (2) The moving of the yolk in an ovum at a certain stage of development on its axis in the surrounding fluid. This was first observed by Leuwenhoeck in 1695.