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. SEE EPOCH.

1. The Chaldaeans invented the Chaldaic Period, or Period of Eclipses, from observing that, after a certain number of revolutions of the moon around the earth, her eclipses recurred in the same order and of the same magnitude. This period consists of 223 lunations, or 6798.28 days, and corresponds almost exactly to a complete revolution of the moon's node.

2. The Egyptians made use of the Dog-star, Syriacal, or Sothic Period, as it is variously called, to compare their civil year of 365 days with the true or Julian year of 365.25 days. The period consequently consisted of 1460 Julian years, corresponding to 1461 Egyptian years, after the lapse of which the dates in both reckonings coincided. By comparing the solar and lunar years, Meton, an Athenian, invented (B.C. 432) a lunar period of 6940 days, called from him the Metonic Cycle, also the Lunar Cycle. About a century afterwards the cycle of Meton was discovered to be an insufficient approximation to the truth, and as he had made the solar year too long by about death of a day. at the end of 4 Metonic cycles the solar reckoning was in advance of the lunar by about 1 day 6 hours. To remedy this, a new period, called the Calippic Period, was invented by Calippus, and consisted of 4 Metonic 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 111,035 days, or about 304 Julian years, which is an exceedingly close approximation, being only 61 minutes too long, when measured by the tropical year; and too short by an almost inappreciable quantity, when measured by the Synodic Month.

3. 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. 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 of 6, 11, 6, and 5 years successively. However, by choosing a period such as will preserve the leap-years in the same relative position to the other years, and at the 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 regular recurrence of the coincidence between the days of the week and of the month. 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 B.C. 9. In calculating the position of any year in the 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 first year of every fourth century.

4. The Julian Period is a cycle of cycles, and consists of 7980 (= 28 x 19 x 15) years, after the lapse of which the solar cycle, lunar cycle, and the Indiction (q.v.) 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, consequently A.D. 1 was the 4714th.

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