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CHRONOLOGY (compounded of XP6V°S> time, and \6yo$, discourse) is the art of measuring time (see Time), distinguishing its several constituent parts, such as centuries, years, &c, by appropriate irarks and characters, and adjusting these parts, in an orderly manner, to past transactions, by means of eras, epochs and cycles, for the illustration of history. The principal means for marking the divisions of time are afforded by the motions of the heavenly bodies, particularly the sun and the moon, which produce the natural division of time into years, months and days. The necessities of life, requiring still smaller and more precise divisions of time (which can be measured only by artificial means), gave rise to hours, minutes and seconds. This division of time is called the artificial. Even in the natural division, however, there is something arbitrary, as it depends solely on the will what point in the motions of the heavenly bodies shall be taken as the point of beginning; for example, in the annual rotation of the earth, whether we shall take the longest day of summer or the shortest day of winter. The first lawgivers, therefore, fixed the civil beginning and end of the month, day and year, and, at the same time also, the smaller divisions of these larger portions of time. From this separation of the natural and artificial or civil division of time, arises a division of chronology into mathematical, astronomical and historical. Astronomical chronology determines the duration of the natural portions of time by the revolutions of the heavenly bodies; historical chronology treats of the civil divisions of time, of the methods of reckoning time among different nations, of ancient periods or remarkable epochs, &c. It is obvious that each of these divisions of chronology requires the assistance of the others. AH historical chronology is grounded on the astronomical, which cannot determine the duration of the periods of time without the aid of the civil division. Mathematicians and astronomers determine the natural periods of time as they are indicated by the motions of the sun and moon. It is left to legislators to determine by law on what day the year shall begin, how many days shall constitute a month, how many a week, &c. This civil regulation is the foundation of the calendar (q. v.) or almanac. Thus far must astronomical chronology be connected with historical; but the latter only can teach us the divisions adopted by different people. Historical chronology explains, 1. the form of the year among different nations, as it is regulated by lawgivers, founders of religions, and other founders of civil society: 2. those events which are selected by different nations as eras, that is, as points from which they begin their reckoning; e. g., the Yuga of the Hindoos, the era of Nabonassar, the era of the Seleucidae, among the Chaldeans Syrians, Persians, Egyptians; the creation of the world, among the Jews; the birth of Christ, among Christians ; the Olympiads, among the Greeks; the building of Rome and the consular era, among the Romans; the Hegira, or flight of Mohammed, among the Mohammedans, &c. As so many different eras render the reckoning of time difficult, it, 3dly, selects a form of the year and an era to which it refers those of other nations, and by which it arranges the history of all nations and times. The European chronologist and historian must refer the eras and years of all people to those used in modern Europe. Mathematical and astronomical chronology is taught in the manuals of astronomy. Among these may be mentioned the Astronomic of Lalande (2d vol. p. 270, 2d ed.) The Manual of Astronomical and Technical Chronology (from the sources) of D. L. Ideler (vol. 1, Berlin, 1825, vol. 2, 1826) is an excellent work. This savant has dore much for the advancement of this science by his extensive researches. (See Epoch and History.)