The four seasons in the Jewish year are called tekufot. More accurately, it is the beginning of each of the four seasons - according to the common view, the mean beginning - that is named tekufah (literally "circuit," from Pvc related to Pcn, "to go round"), the tekufah of Nisan denoting the mean sun at the vernal equinoctial point, that of Tammuz denoting it at the summer solstitial point, that of Tishri, at the autumnal equinoctial point, and that of Tevet, at the winter solstitial point. The mean length of the seasons, each exactly one quarter of the year, was reckoned by Mar Samuel (c. 165–254, head of the academy at Nehardea in Babylon) at 91d. 7 1/2h. Hence, with his solar year of 365d. 6h., or 52 weeks and 1 1/4 days - identical in length with the Julian year - the tekufot move forward in the week, year after year, by 1 1/4 days. Accordingly, after 28 years the tekufah of Nisan reverts to the same hour on the same day of the week (Tuesday 6 p.m.) as at the beginning: this 28-year cycle is named the great, or solar, cycle (mahazor gadol, or mahazor hammah). This length of the solar year is important in respect of two minor rituals only:

(1) the date of She'elah, the commencement in the Diaspora of the petition for rain inserted in the benediction Birkat ha-Shanim in the Amidah, on December 5 or 6 in the present century;

(2) the Blessing of the Sun on the day of the tekufah of Nisan at the beginning of the 28-year cycle. The frequent occurrence, in the last centuries, of Passover (Nisan 15–21) prior to the day of Mar Samuel's tekufah of Nisan - whereas the purpose of intercalation is to avoid the tekufah of Tevet extending to Nisan 16 (RH 21a) - is held by some scholars to show that in the making of the present Jewish calendar Mar Samuel's value was deliberately departed from, and the length of the solar year was more accurately calculated at 365d. 5h. 55 min. 25 27/27 sec., a calculation associated with the name of Rav Adda (perhaps Rav Adda b. Ahavah, a Babylonian amora of the third century). According to other scholars, this is but the fortuitous result of dividing by 19 the 6939d. 16h. 595p. contained in 235 lunations reckoned at 29d. 12h. 793p. each, the oldest sources knowing no other value for the length of the solar year than 365 1/4d., arising from Mar Samuel's tekufah. Actually clues are traceable in talmudic dicta1, as also in Abraham ibn Ezra's Sefer ha-Ibbur (ed. by S. J. Halberstam, 1874, 8a) and Maimonides' Code2, for values close to the modern estimate of the length of the tropical solar year at 365d. 5h. 48 min. 46 sec. If the average length of the solar year in the present Jewish calendar exceeds this by approximately 6 2/3 min., this discrepancy was left out of account as it was assumed that its cumulative effect would remain negligible over a long period at the end of which the present system was expected to be replaced again by a system based on true values more akin to the earlier Jewish calendar in which New Moons (days of the phasis) and intercalations were proclaimed on the basis of both observation and calculation.