CHRONOLOGY is the science of the divisions of time. It has two main branches— mathematical C., and historical chronology. Mathematical C. is engaged with such of the units for the measurement of time as begin and end with the period of complete evolution of recurring celestial phenomena. See articles CALENDAR, YEAR, MONTH, DAY, and CYCLE, where the chief points in mathematical C. are explained. Historical 0. uses these units among others to measure the distance in point of time between events, and to fix their dates. As in geography and navigation, longitude is measured from some arbitrary line, such as the meridian through Greenwich, so in historical C., dates are fixed by giving their distance from some arbitrary point of time, usually chosen because of some remarkable occurrence which signalized it. Such a fixed point, or epoch, forms the beginning of an era. It is thus that dates have been aptly said to be to events in history what the latitude and longitude of places are to the places in geography and navigation. The mathematical, or, to speak more properly, the astronomical units of time above referred to have not been, as has been already hinted, the only units used in historical chronology. In early times, the more accurate methods of mathematics were unknown, and such vague periods as " a generation," or the lifetime of leading persons in a nation, such as the priestesses of Juno, or of the kings, were assumed as units in historical chronology. The great variety of eras, too, in ancient times confuses the student of chronology. Thus the era of the Greeks began with the year of the first olym piad, or that in which Corcebus was victor; being the first celebration of the games at i which the victor's name was recorded, and which is calculated to correspond to the year 776 B.C. From this epoch, the Greeks measured time by olympiads or periods of four years. Thus, the 3d year of the 12th olympiad would be the year 729 me. The Roman era was reckoned from the founding of the city, being either 752 or 753 D.c. The Roman practice of dating events from the building of the city, seems to be the first instance of the method of reckoning time from a fixed point by single years. It thus forms one of the great stages in chronology. Of other eras we shall merely mention the .Mohamme clan, which commences with the flight of Mohammed, 622 A.D., and which is called the Ifedjrah (q.v.). The Roman and Greek methods of measuring time continued to be in use long after the birth of Christ; the olympiads, indeed, appear to have been employed in Europe down to the 304th olympiad, or 440 A. D. From 312 A.D., however, the public mode of computation throughout the Roman empire by indictions, which were periods of 15 years, beginning with that (see INDIcTIoy); and this mode was at one time Chronology. 842
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almost universally followed in the west. In France, it was not altogether discontinued till the end of the 15th century. The Christian era is said to have been first proposed in the year 527 A.D., and is now universally used in Christendom Part of the business of C. is to determine the relationships of the different eras, so as to enable one to express, in the language appropriate to one mode of computation, the date of an event recorded in another. Owing to the birth of Christ being a comparatively recent event, the Chris tian era is attended by this inconvenience, that we must count backwards from it for the dates of occurrences prior to it. To obviate this, various comprehensive periods, such as the Julian and Louisian periods have been invented, which have the merit of being applicable to most events lying within the limits of history.
Various systems of C., such as the Chinese, Babylonian. Egyptian, Indian, and Chaldean, are worthy of attention. Accounts of the periods which these nations respectively assign to their histories, will be found under the heads CHINESE ENIPIRE, BABYLON, etc. Of sacred a there have been various systems. In these the epochs are the creation of the world, and the flood; but the chief copies of the Bible do not agree as to the dates of these events. While the Hebrew text reckons 4,000 years from the creation to the birth of Christ, and to the flood 1656 years, the Samaritan makes the former much longer, though it counts from the creation to the flood only 1307 years. The Septuagint version differs from both. It removes the creation of the world to 6,000 years before Christ, and 2,250 years before the flood. These differences have never been reconciled. It is, now, however, universally admitted, that the creation of the world is not to be regarded as having occurred even so recently as 6,000 B.C. The mod em understanding of the first chapter of Genesis leaves the period of the creation quite indefinite, and one scheme of interpretation stretches out the days of creation into periods of indefinite length. Of the Newtonian a, all that can be said here is, that it was an attempt, now generally admitted not to have been very successful, to rectify the obvious blunders of ancient chronologers, by determining certain epochs by means partly of astronomical calculations, and partly of the critical examination of such citron ides as measured time by reigns and generations. By a very fine argument, the sound ness of which has since been doubted, Newton set down the date of the Argonautic expedition as being 43 years after the death of Solomon, or 937 B.C.