That astronomy was successfully cultivated by the Israelites of old, is proved by the perpetual chronological calendar which was formed and brought to perfection in the days of the Talmudical doctors. This calendar is composed both of the lunar and solar revolutions. Though it may not be strictly the province of this chapter to treat upon this subject, the reader will excuse the digression, in order to introduce a short extract of this calculation to show the basis upon which the same is founded, and prove that a knowledge of astronomy existed in those days, by the teachers of the Talmud. According to the Mosaic Law, the Israelites are directed to calculate the year and compute their holy days according to the lunar year. Twelve lunar, _synodical revolutions_, _i.e._ 29 days, 12 hours, 44 minutes and 3 seconds, compose one simple year. Thus we make sometimes 353, 354, 355 days, allowing for fractions. Yet the _Epactem_ of 10 days, 21 hours, 11 minutes and 20 seconds, in which the solar year exceeds the lunar, might be the cause, that the holy days would be removed from their respective seasons,--which would be the case, when calculating by the lunar only. So that in a period of seventeen years the feast of Passover would be in the autumn instead of the spring, and the feast of tabernacle in spring instead of the autumn. On this account it was that the Jewish chronologists took care to remedy this defect, by forming alternately, sometimes to compose the year of thirteen lunar months, as 383, 384, 385 days, for which reason they adopted a period of 19 years, in which they formed seven complete years,--as the 3, 6, 8, 11, 14, 17, 19, complete of thirteen lunar months, and the interval twelve years, _simple_, of twelve lunar months only; and in this periodical calculation of 19 years, according to the above rotation of twelve _simple_ and seven _complete_ years, the _lunar_ and _solar_ years then agree, without any variation whatever.[A] Hence it is that the Jewish calculation is very exactly and astronomically contrived, for it has never failed since its first introduction, now nearly fifteen centuries. This is a sufficient proof that the science of astronomy was known to the ancient Israelites.