Rav Nachshon Gaon רב נחשון גאון

Rav Nachshon bar Tzadok was Gaon of Sura about the years 4631–4641. His name rests on one observation that entered every calendar table: thirteen nineteen-year cycles—two hundred and forty-seven years—return the Jewish calendar almost to its starting position. The "iggul of Rav Nachshon", the cycle of Rav Nachshon, made it possible to set out the years a century ahead by copying the old table.

Almost is not exactly: the discrepancy accumulates at somewhat over an hour per cycle, and after several cycles the table drifts. The Rishonim knew this and gave warning; but for the practice of a community living a generation or two, the cycle worked.

His gaonate fell in years when Sura contended with Pumbedita for primacy, and the Jewish calendar was just becoming one for the whole diaspora: what looks today like a technicality was then the question of whether the whole people keeps the festival on one day.

The years are taken from the Epistle of Rav Sherira Gaon—the letter of 4747 (987), the only source in which the age of the Savoraim is set out by name and year. There is no other testimony, and that should be kept in mind.