It was not until 1891 that the American astronomer, Seth Carlo Chandler, made measurements showing that there was a periodic motion of 14 months in the Earth s wobble and suggesting that this was the polhode motion.
12.
Its rotation combines two separate periodic motions into a non-periodic result; to someone on the surface of Toutatis, the Sun would seem to rise and set in apparently random locations and at random times at the asteroid's horizon.
13.
Within a dynamical system such as the ocean-atmosphere oscillations may occur regularly, when they are forced by a regular external forcing : for example, the familiar winter-summer cycle is forced by variations in sunlight from the ( very close to perfectly ) periodic motion of the earth around the sun.
14.
Everyday examples are seen when the variable is " time "; for instance the hands of a clock or the phases of the moon show periodic behaviour . "'Periodic motion "'is motion in which the position ( s ) of the system are expressible as periodic functions, all with the " same " period.
15.
From the time of Hipparchus and Ptolemy, the year was based on two equinoxes ( or two solstices ) a number of years apart, to average out both observational errors and the effects of nutation ( periodic motions of the axis of rotation of the earth, the main cycle being 18.6 years ) and the movement of the Sun caused by the gravitational pull of the planets.
