ABSTRACT
The ordinary Jewish year consists of twelve months— Nisan (corresponding with April), Iyar (May), Sivan (June), Tammuz (July), Ab (August), Ellu (September,) Tishri (October), Cheshvan or Marcheshvan (November), Kislev (December), Tebeth (January), Shebat (February), and Adar (March). In leap year another month, known as the second Adar, is added. The year then consists of thirteen months. The Jewish Calendar, as it now exists, is entirely based upon astronomical principles. The period at which it attained its present scientific form is uncertain. There was a time, however, when scientific calculation played little if any part in the fixing of the Calendar. The early Rabbins were probably acquainted with certain astronomical rules, but they placed their chief reliance upon actual observation of the heavenly bodies, and especially upon the observation of the moon. This practice remained in force for centuries. The keystone of the system was the date of the new moon.
