Mohammedan Era, or Hegira.— This be gins on 16 July 622. The conversion of the Mohammedan into Christian chronology causes more difficulty and confusion than arises with any of the other modes of reckoning. The Mo hammedan year is purely lunar. It consists of 12 months, and each month commences with the appearance of the new moon. Hence their years have no correspondence with the recur rence of the seasons, and to know the period when a Mohammedan year begins it must be reckoned from the beginning of the era. In chronology, history, etc., they use months of 29 and 30 days alternately, malung the year consist of 354 days. Eleven times in 30 years one day is added to the last month, malting 355 days in the year. The mean length of the year is thus 354 11/30 days, of the month 29 191/360, differ ing from the true lunation by little more than three seconds, or less than a day in 2,260 years. As 33 Mohammedan years amount to only six days (including intercalary days and leap years) more than 32 of our years, by deducting one year from each 33 Mohammedan years, and add ing 621% years, the year of the Christian era will approximately be found. The Hegira is distin guished by the initials A. H.
Persian Era.— The era of Yezdegird III began 16 July 632. The year consisted of 365 days. It was reforrned in 1075 by the addition of a day whenever it was necessary to make the commencement of the year occur on the day of the sun's passing the same degree of the ecliptic. The months have each 30 days, with five or six days intercalated. This era is still used by the Parsees in India.
Indian The best-known eras computed by solar time are the Kaliyuga, which dates from 3,101 years before Christ and the Salivahana from 77 A.D. Both are computed astronomically, losing one day in 60 years by our computation. The era of Vikramaditya, beginning 57 years a.c., is computed by lunar months, with intercalations made according to astronomical observation, and bringing the year up to 365 or 366 days. The Bengali year was formerly identified with the Hegira, but is now reckoned by solar computation.
Chinese The Chinese, like all the nations of northeast Asia, reckon their time by cycles of 60 years. Instead of numbering
them as we do, they give a different name to every year in the cycle. The Chinese months are lunar, of 29 and 30 days each. Their years have ordinarily 12 months, but a 13th is added whenever there are two new moons while the sun is in one of the zodiac. This will occur seven times in 19 years. The boasted knowledge of the Chinese in astronomy has not been suffi cient to enable them to compute their time cor rectly. The first cycle, according to Roman Catholic missionaries, began February 2397 B.C. To find out the Chinese time multiply the elapsed cycle by 60 and arid thc odd years; then if the time be before Christ subtract the sum from 2,398; but if after Christ, subtract 2,397 from it; the remainder will be the year re quired.
Primitive American Chronology.— The na tives of America, previous to its discovery by Europeans, particularly the Peruvians and Mexicans, appear to have had a considerable acquaintance with astronomy and to have reck oned their time with great care. The Mexican year consisted of 365 days, composed of 18 months of 20 days and five added days. At the end of a cycle of 52 years 12 and 13 days were added alternately, making the mean year very near the troth.
In geology, according to United States Geo logical Survey usage, a subdivision of a period of geologic Cane, the period in turn being a subdivision of an era, the largest time unit in use. The rocks laid down during an epoch are usually known as a series, this term being used as a subdivision of a system, the latter being the rocks laid down during a period. The Cambrian (q.v.) period is usually divided into three epochs, Georgian, Acadian and Saratogan, and the Cambrian system into three corresponding series of rocks known by the same names. Other periods are also divided into epochs. The Eocene, Oligocene, Miocene and Pliocene are sometimes given the rank of periods, but are now generally considered to be epochs of the Tertiary period. While periods are divisions the nomenclature of which is fairly well standard ized the world over, epochs are not so constant, and go by various names in various lands. See