PERIOD, a term used in chronology in the same sense as cycle (q.v.), 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. Various periods have been invented by astronomers, but we can only notice a few of the most important.
The Chaldeans 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. This period consists of 223 inflations, or 6,798.28 days, and corresponds almost exactly to a complete revolution of the moon's node. 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 3651 days. The period consequently consisted of 1460 Julian years, correspond ing to 1461 Egyptian years, after time lapse of which time dates in both reckonings Coincided. By comparing the solar and lunar years, Melon, an Athenian, invented (432 mu.) a lunar period of 6,940 days, called from him time Metonic Cycle (q.v.). also the Lunar Cycle. About a century afterwards, the cycle of Meton was discovered to be an insuffi dent approximation to the truth, as he had made the solar year too long by about of a day, at the end of 4 Metonie cycles the solar reckoning was in advance of the lunar by about 1 day 6 hours. To remedy this, a new period, called the Calippia Period, was invented by Calippus, and consisted of 4 MetonL cycles less by 1 day, or 27,759 days. But as this period still gave a difference of 6 hours between the solar and lunar reckonings, it was improved by Hipparchus, who invented the Hipparchic Period of 4 Calippic periods less by 1 day, or 111,033 days, or about 204 Julian years. which is an exceedingly close approximation, being only 61. minutes too long, when measured by the tropical year; and too short but by an almost inappreciable quantity, when measured by the Synodic Month (see .MoN•ir). The period of the Heliaeal or Solar
Cycle, after which the same day of the mouth falls upon the same day of the week, consists of 28 Julian years. 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 su intercalary day into every fourth year causes this coincidence to recur at irregu :ar periods of 6, 11, 6, and 5 years successively. However, by closing a period such as will preserve the leap-years in the saute relative position to the other years, and at Vie same time consist of an exact number of weeks (both of which objects are effected by using the number 28, which is the least common multiple of 4 and 7), we insure the ..-egular recurrence of the coincidence between the days of the week and of the mouth. The solar cycle is supposed to have been invented about the time of the council of Nice '325 A.D.), but it is arranged so that the first year of the first cycle corresponds to 9 n.e. In calculating the position of any year in the solar cycle, care must be taken to allow for the omission of the interealary day at the beginning of each century, and its insertion in the first year of every fourth century. See LEAP-YEAR. The year 1880 is the 13th of the solar cycle. The Julian Period is a cycle of cycles, and consists of 7,980 (=28X 19X 15) years, after the lapse of which the solar cycle, lunar cycle, and the indiction (q.v.) C0111111CIICC 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 n.e. is taken as the first year of the first period, consequently, 1 A.D. was t.1,9 4714th of it, and the year 1875 was the 6588th.