The Month. There can be little doubt, but that this division of time was at first suggested by the phases, or the periodi cal change in the appearances of the moon, and consequently, that in ancient computations the months were invariably lunar. The difficulty, however, of ad justing this month to the annual revolu tion of the earth, led, with the improve ment of astronomy, to the invention of other divisions Hotter this name. Months are now divided into astronomical and civil. The astronomical months, with which chronology is concerned, are mea sured by the revolutions of the moon, and are either periodical or synodical. The periodical lunar month is composed of the time which elapses between the departure of the moon from any part of her orbit, and her return to the same point, which is 27 days, 7 hours, and 43 minutes. The synodical lunar month is reckoned from one conjunction of the sun with the moon to another. This pe riod is not always the same, being sub ject to the variation occasioned by the motion of the sun eastward on the eclip tic: a mean lunation consists of 29 days, 12 hours, and 44 minutes. This was the lunar month mostly in use in ancient times. The civil month is that artificial space of time, by means of which the solar year is divided into twelve parts these months, which were first ordained by Julius Caesar, consist of thirty, or thir ty-one days each, with the exception of February, which commonly contains twenty-eight, and every fourth year twenty-nine, days.
Years. The year may be termed the largest natural division of time. As the diurnal revolittion of the earth would na turally lead to the division into days, and the phases of the moon, with a little at tention, to that into months, so the an nual motion of the earth round the sun, would be marked by the perioili. cal return of certain appearances, sea sons, &c. would in due course lead to the adoption of this larger division. At what time this took place is uncertain, but probably not before considerable ad vances had been made in astronomical science. It was long, however, after its first adoption, before it attained to any thing like an accurate form. The most ancient measure of the year, of which we know, consisted of twelve lu nar months, which, for the facility of computation, being all considered as equal in length, and to contain thirty days each, amounted to 360 days. It is conjec tured that this gave rise to the division of the ecliptic, which still obtains, into 360 equal parts or degrees.
This luni-solar year probably had its rise in Chaldma, or Egypt ; we learn, at least, from the testimony of Herodotus, that it was used in the latter country. Hence, with the diffusion of science, it was carried into other regions, and very gene rally adopted. It was early in use among the Indians, Chinese, the Medes, and Per sians, and the ancient Greeks. Its measure being, however, inaccurate, containing five days and a quarter more than the lu nar, and as much less than the true solar year, and this defect becoming every year more perceptible from the retrocession of the seasons, &c. it was soon consider ed necessary to subject it to some revi sion. The Thebans are supposed to have been the first who undertook its correc tion, by making an annual addition of five days to the luni-solar year. Thales
introduced this improvement into the an cient Grecian year, and it was adopted, with some trifling variations in particular instances, into the Indian, the Chinese, and the Jewish year.
The Roman year, as regulated by Ro mulus, and afterwards reformed by his successor Numa, was reckoned by lunar months, and adjusted to the seasons by a number of intercalary days. It consist ed of ten lunar months, of which Decem ber was the last, -and to these two whole intercalary months were added, but not inserted in the calendar. This year be gan at first in March ; but the Decemviri, who undertook its reformation, changed the order of the months into that in which they now stand, introduced the two in tercalary months, January and February, into the calendar, and made January the first month of the year.
Owing to the ignorance, or the care lessness, of the Pontifices Maximi, to whose care the regulation of the interca lary days was committed, the year was reduced to such disorder in the time of Julius Cmsar, that the winter months had fallen back to the autumn. To restore them to their proper season, Czsar formed a year of 445 days, which has been styled the year of confusion. With the assistance of Sosigenes, a mathemati cian of Alexandria, he afterwards, in the year B. C. 45, instituted a solar year of 365 days 6 hours, which is now known under the name of the Julian year. To adjust this year to the annual revolution of the earth, which is six hours and some minutes more than 365 days, the length of the ordinary year, a day was appointed to be intercalated fourth year in the month of February: this day, from its position in the Roman calendar, was called bissextile, a name which has also been given to the year in which the intercalation takes place.
The Julian year, although it approach es very near the truth, is not, however, perfectly correct. The true time of the annual revolution of the sun in the eclip tic is 365 days, 5 hours, and nearly 49 minutes, which falls short by a few mi nutes of the time assumed in the Julian year. How trifling soever this difference might at first appear, it amounted in a hundred and thirty-one years to a whole day : in consequence of this, the vernal equinox, which Sosigenes, in the first of the Julian correction, observed to fall on the 25th of March, had gone back in A. 1). 325, at the time of the council of Nice, to the 21st, and in A. D. 1582, to the 11th of March. To remedy this growing defect, Pope Gregory XIII. caused the calendar to undergo another correction. In A. D. 1580, he ordered ten days to be cut out of the month of October, so that the fourth was reckoned the 15th day : and to prevent such retro cession in future, in addition to the Ju lian regulation with respect to the bissex tile year, he ordained that the years 1600, 2000, 2400, and every fourth centu tury in succession, should have an inter calation of a day, but that in the other centuries, 1700, 1800, 1900, 2100, &c. the day should be omitted, and those years remain common years. This re gulation comes so near the truth, that the only correction it will require will be the suppression of a day and a half in five thousand years.