Period and Periodicity. One of the most striking features of the ordinary phenomena of nature is their tendency to recur and repeat themselves apparently indefinitely; and in general this repetition takes place at successive and practically equal intervals of time. The day, the month, the year are familiar examples of such periods, corresponding respectively to the earth's rotation, the moon's progression through its phases, and the earth's changes of distance from the sun. As a factor in human life the year is practically traced out by the climatic changes that accompany its progress, but strictly speaking it is the period determined by the recurring configurations of earth and sun. Many periodic phenomena of importance, such as eclipses, transits, occultations, depend like the moon's phases on the configurations of three bodies; and long before Newton's law of gravitation gave the key to the cosmic universe the periods of some of these had been discovered. See CHRONOLOGY (with its various cycles—the metonic of 19 years, the Calippic of 76 years, &c.), CYCLE, DAY, ECLIPSE, YEAR, &c.
When we look into the minute mechanism of nature we find here also the same prominence attached to periodic qualities. Sound and light consist physically of a vibratory or oscillatory motion of some sort; and to the accurate time periodicity of these motions we trace our sensations of harmony in music and colour. In certain respects, however, the periodicity is imperfect, each period not being an exact reproduction of its predecessor. Thus, a tuning-fork or pianoforte-string vibrating freely in air rapidly loses its vibratory character, and its motion steadily decays; and yet, judged by the pitch, the frequency or time periodicity remains the same throughout. In these and similar cases Viscosity (q.v.) ultimately transforms the original vibratory energy into heat (see ENERGY). Heat itself is believed to be some kind of vibratory energy of the molecules; and the spectroscopist demonstrates that intense heat is certainly associated with definite periodic motions, giving rise to rays of corresponding periodicity (see SPECTRUM).
Passing now to the other extreme, we find, chiefly in astronomy, instances of long periods, some of which have not been completed within historic times, but of which the evidence is incontrovertible. The Precession (q.v.) of the equinoxes and the slow changes in the eccentricities and inclinations of planetary orbits may be mentioned by way of illustration. The geologist also has found evidence of periodic changes in the climatic conditions of the earth (see GLACIAL PERIOD, PLEISTOCENE). Generally periodicity involves the idea of time; but we may have periodic qualities depending on position or on grouping. A very good example of this is the periodic law of modern chemistry (see ATOMIC THEORY; also WAVE).
Life is as full of periodic phenomena as inanimate nature; but the increasing complexity of conditions makes the periodicity still less perfect. In the beating of the heart, in the alternation of waking and sleeping, of hunger and satiety we have examples of vital actions with a distinct periodic character.