PERIOD, a term used in chronology in the same sense as cycle, to denote an interval of time after which the astro nomical phenomena to which it refers re cur in the same order. It is also employed to signify a cycle of cycles. The Chal dmans invented the Chaldaic period, or period of eclipses, 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. The Egyptians made use of the dog-star, Siriacal, or Sothric period, as it is variously called, to com pare their civil year of 365 days with the true or Julian year of days. The period consequently consisted of 1,460 Julian years, corresponding to 1,461 Egyptian years, after the lapse on which the dates in both reckonings coincided. By comparing the solar and lunar years Meton, an Athenian, invented (432 B. C.) a lunar period of 6,940 days, called from him the Metonic cycle, also the lunar cycle. The Calippic period was invented by Calippus, and consisted of four Me tonic cycles less by one day, or 27,759 days. But as this period still gave a difference of six hours between the solar and lunar reckonings, it was improved by Hipparachus, who invented the Hip parchic period of four Calippic periods, less by one day, or 111,035 days, or about 304 Julian years. The period of the he
liacal or solar cycle, after which the same day of the month falls upon the same day of the week, consists of 28 Julian years. The solar cycle is supposed to have been invented about the time of the Council of Nice (A. D. 325), but it is ar ranged so that the first year of the first cycle corresponds to 9 B. c. In calculat ing the position of any year in solar cy cle care must be taken to allow for the omission of the intercalary day at the beginning of each century, and its inser tion in the last year of every fourth cen tury. The Julian period is a cycle of cy cles, and consists of 7,980 (7=28 X19 x 15) years, after the lapse of which the solar cycle, lunar cycle, and the indiction commence together. The period of its commencement has been arranged so that it will expire at the same time as the other three periods, from which it has been derived. The year 4713 B. C. is taken as the first year of the first period.