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Period (Term used in chronology)

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 Chaldæans 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 compare their civil year of 365 days with the true or Julian year of 3654 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 Metonic cycles cycles less by one day, or 27,759 days. But as this period still gave a PERIODICALS PERIODICALS difference of six hours between the solar lished as the organ of the Roman Cathand lunar reckonings, it was improved olic party. To meet the demand for by Hipparachus, who invented the Hip- critical literature at shorter intervals parchic period of four Calippic periods, than three months, there was published less by one day, or 111,035 days, or about in 1865 the "Fortnightly Review," which 304 Julian 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. The solar cycle is supposed to have been invented about the time of the Council of Nice (A. D. 325), but it is arranged so that the first year of the first cycle corresponds to 9 B. c. In calculating the position of any year in solar cycle care must be taken to allow for the omission of the intercalary day at the beginning of each century, and its insertion in the last year of every fourth century. The Julian period is a cycle of cycles, and consists of 7,980 (=28×19×15) 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.