The New International Encyclopædia/Period

PERIOD (Lat. periodus, from Gk. περίοδος, a going round, circumference, circuit, cycle, sentence, period, from περί, peri, around + ὁδός, hodos, road). A term used in chronology in the same sense as cycle, to denote an interval of time after which the astronomical phenomena to which it refers recur in the same order. It is also employed to signify a cycle of cycles. The Chaldeans invented the Chaldaic period or Saros, from observing that, after a certain number of revolutions of the moon round the earth, her eclipses recurred in the same order and of the same magnitude. This period consists of 223 lunations, or 6585.32 days, and corresponds almost exactly to 19 ‘eclipse years.’ The eclipse year is the time required for the sun, in his apparent motion among the stars, to complete a circuit from one of the nodes (q.v.) of the lunar orbit back again to the same node. On account of the motion of the lunar nodes, the eclipse year contains only 346.62 days, and 19 such years contain 6585.78 days. The error of the Saros is thus only 0.46 day (about 11 hours) in 223 lunations, or 19 eclipse years.

Various important periods or cycles are used in the calendar (q.v.) for predicting the dates of new and full moon. These phases recur on the same dates every nineteen years (except that leap years may change the dates one day), which fact was discovered by Meton, an Athenian, who invented (B.C. 432) a lunar period of 6940 days, or 19 years, called the Metonic cycle (q.v.), also the lunar cycle. The calippic period consists of 76 years, or four Metonic periods, and is thus able to take account of leap years. The period of the heliacal or solar cycle, after which the same day of the month falls upon the same day of the week, consists of 28 Julian years of 365¼ days each. If the year had regularly consisted of 365 days—that is one day more than an exact number of weeks—it is evident that at the end of seven years the days of the month and week would again correspond; but the introduction of an intercalary day into every fourth year causes this coincidence to recur at irregular periods. (To ascertain when the same days of the week and month will recur in the Gregorian calendar, see Calendar, section on Perpetual Calendar.) The Julian period is a cycle of cycles, and consists of 7980 (28⋅19⋅15) years, after the lapse of which the solar cycle, lunar cycle, and the indiction (q.v.) commence together. The time of its commencement was arranged so that it will expire at the same time as the other three periods from which it is derived. The year B.C. 4713 is taken as the first year of the period, consequently A.D. 1 is the 4714th year of it. (See Chronology; Calendar; Cycle.) Astronomers also use the term period to designate the quantity of time required by a planet or other celestial body to complete a revolution in its orbit. (See Elements.) In this sense there may be different periods for the same body, according to the point selected as the beginning and end of the periodic orbital motion. The location of the position from which the motion is supposed to be viewed may also change the period materially.