Only edition of this detailed Karaite work on the calendar, astronomy, and Karaite law by Joseph ben Moses Sapak. Included with the text are calculations. Apart from the intrinsic value of Geresh Yerahim is its value for its insight into the Karaite view of astronomic approach to lunar calculations in contrast to rabbinic positions.

The calendar was the subject by which the Karaites distinguished themselves from the Rabbanites. It was also the subject of much dispute among the Karaites. In principle the calculation of the Karaite calendar was based on lunar observation, and observation of the barley for the purpose of intercalation. By the middle of the 19^th century the use of mathematical calculation, in addition to visual observation of the new moon, was accepted, following the lead of Isaac ben Solomon , at least by the majority of the Crimean Karaites. Like the Rabbanite calendar, the Karaite calendar is based on the calculation of the new moon. Karaites also recognize the 19-year cycle with seven leap months of 29 days each; determination of the beginning of the month, however, in addition to being based upon the calculation of the moment of the appearance of the new moon (molad) and its location in accordance with special tables, also depends upon direct observation of the new moon. Thus, if direct lunar observation is made on the eve of the 30^th day of the month, the following day becomes the day of the new moon; otherwise, the 31^st day becomes the day of the new moon and the preceding month is determined to have had 30 days. The month of Nisan is regarded as the first month of the calendar year. In practice, however, following the tables of Bashyazi, the calendar is calculated in advance, by approximation (haqrava), as though the new moon was observed. In Israel, in order to emphasize this "approximation," observations are conducted in advance, in the spring, and accordingly the calendar of the following year (starting in the month of Tishri) is printed. Rabbi Samuel Magdi has been trying for several years to introduce mathematical calculation in principle, so far without success. In determining the date of the holy days, Karaites deviate from Rabbanite usage in the following manner: the New Year Festival may begin on any day of the week (contrary to the Rabbanite rule, which provides for the postponement of the day of the New Year in three specific cases); as a result, the Karaite Day of Atonement does not always coincide with the Rabbanite; Passover and Sukkot (Feast of Tabernacles) are observed everywhere in the world for seven days only; the Feast of Weeks (Shavuot) falls on the 50^th day following the Saturday of the Passover week (in accordance with the literal interpretation of Lev. 23:11, which the Talmud interprets in a different manner), and is therefore always on a Sunday; Ḥanukkah is not recognized, but Purim is, although the Fast of Esther is not; the Fast of Gedaliah is observed on the 24^th of Tishri (as it was by the exiles returning from Babylon). Other fast days, with the exception of the Tenth of Tevet, are also observed on dates that differ from the rabbinic fast days (Karaites relate the fast days to the destruction of the First Temple, not the Second Temple). Special rules apply to the sanctification of the Sabbath. Prohibition of work extends, beyond the 39 actions proscribed by Rabbanite Judaism, to any action not forming part of the prayer service or not absolutely necessary for nourishment or the satisfaction of other physical human needs. The earlier Karaite teachers (up to Jeshua b. Judah), like the Samaritans and the Beta Israel , prohibited the kindling of lights on Friday for use on the Sabbath (see Eshkol ha-Kofer, no. 146), and even taught that a light already lit had to be extinguished on the Sabbath; Jeshua b. Judah and his successors, however, taught that light on the Sabbath was permitted as an indispensable need and for the joy of the Sabbath (see Adderet Eliyahu, 1835, 31a). To this day, however, Karaites are either "friends of light" or "enemies of light," depending on whether or not they use artificial light on the Sabbath. Sexual intercourse is also prohibited on the Sabbath, and Karaites also oppose a number of alleviations of Sabbath precepts sanctioned by the rabbis.