CHRONOLOGY, from xcovoc time, and Aoyo; doctrine, is that science which treats of the natural and artificial di xisions of time, and refers to the points which it thus marks out, the various events recorded in history, Ste.
In order to convey to our readers as full and distinct an account of this useful subject as our limits will per mit, we shall divide it into the following Chapters : 1. MATHEMATICAL CHRONOLOGY ; II. HISTORICAL CHRONOLOGY ; HI. COMPARATIVE CHRONOLOGY; and, IV. TABULAR CHRONOLOGY.
A day is tl At portion of time which elapses while the earth performs a complete revolution about its axis ; and its length is measured by observing the precise instant when the centre of a star passes the meridian on two consecutive clays. A day thus measured is called the sidereal day, and consists of 23 hours, 56' A solar or natural day, is that portion of time which elapses be tween the arrival of the sun at the meridian on two con secutive days. The mean length of the solar day is 24 hours ; but, owing to the inclination of the earth's axis to the ecliptic, and the unequal motion of the earth in its orbit, the solar (lays are of unequal length, some times exceeding, and sometimes falling short of 24 hours. The hour hand of a well-regulated clock performs two complete revolutions on the dial plate in the course of a mean solar day, and twice 365 or 730 revolutions in the course of a year ; whereas the shadow of the stile or gnomon of a sun dial, sometimes perbrins more : nd sometimes less than one revolution during a mean solar day, but always completes 365 revolutions in the course of one year. The difference between the mean solar time, as shown by a well-regulated clock, and the appa rent time, as shown by a sun dial, is called the equation of time, or tile equation of natural days, and has oeen fully explained in the article Asintozzoniv, Vol. II. Pp.
612, 751.
Although the inhabitants of every country have agreed in adopting the mean solar day as the unit of their scale for measuring time, yet they have differed from each other in fixing its commencement. The following Table will show distinctly the practice of various ancient and modern nations.
performs a complete revolution round the heavens, and is either periodical or synodical. The periodical month is the time in which the moon moves from one point of the heavens to the same point again, and is equal to 27d 7h 43 4".7 ; and the synodical month, or lunation,
as it is sometimes called, is that portion of time which elapses between two successive new moons, or between two successive conjunctions of the moon with the sun, and is equal to 29d 12b 44' 11"'. The solar month is that portion of time in which the sun moves through one sign of the ecliptic, and is equal at an average to 30d 10z. 29' 5". The civil month, which is formed for the purposes of civil life, is one which consists of a certain number of whole days, and approaches as nearly as possible to the astronomical or to the solar month.
When the civil month approaches to the lunar month, it is called the civil lunar month, and generally contains 29 and 30 days alternately, with some corrections. This kind of month was used by the Jews, Greeks, and Ro mans, till the time of Julius Cxsar, who introduced the civil solar month, ,consisting of 30 and 31 days alter nately.
In the following Tables, we have shewn the method of dividing the year into months, adopted by the Egyptians, Jews, Greeks, Romans, Arabs and Turks, and the French.
The subdivision of the day into particular parts has likewise been different among different nations. The Chaldeans, Syrians, Persians, Indians, Jews and Romans, divided both the day and night into four parts. The Greeks divided the natural day into 12 hours, in imita tion of the Babylonians, and this practice was afterwards adopted by the Romans. Among almost all modern na tions, the day is divided into 24 hours, reckoned twice from 1 to 12, except among the Italians, Bohemians, and Poles, who reckoned them from 1 to 24. The Arau canians divide the natural day into 12 parts, each of which has a particular name. The Turks follow the practice of the Jews ; and the Chinese divide the mean solar day into 12 hours, each of which is of course equal to two of ours. In the decimal system adopted by the French, the day is divided into 10 hours.
The various methods which have been employed for measuring the subdivisions of the mean solar day by Clepsydra, Dials, Clocks, and IVatches, will be dis cussed under the articles DIALLING, IloitoLoGy, HY