SOLAR CYCLE. - This is the improperly given name for a period of 28 solar years (Latin circulus solis, French cycle solaire, German Sonnenzyklus), after which the succession of the days of the week again corresponds exactly to the dominical letters A-G, conventionally assigned to the progressive order of the days of the week that begin the year.
If, for example, a given year begins on a Sunday, with 1 Jan. called A, 2 Jan. B, 3 Jan. C, and so on, the series A-G (Sunday-Saturday) will again correspond to the days 1-7 Jan. after 28 years. The solar cycle, which in fact bears no relation to the course of the sun, was widely used in the Middle Ages for the direct determination of the “dominical letter” (v.) and for the computation of Easter.
BIBL.: E. Brinckmeier, Praktisches Handbuch der historischen Chronologie... des Mittelalters, Berlin 1882, pp. 68-69; A. Giry, Manuel de diplomatique, Paris 1894, pp. 134-37; H. Grotefend, Abriss der Chronologie des deutschen Mittelalters und der Neuzeit (Grundriss der Geschichtswissenschaft, ed. by A. Meister, I, III), 2nd ed., Leipzig-Berlin 1912, pp. 5-7. Alessandro Pratesi