1911 Encyclopædia Britannica/Sothic Period
SOTHIC PERIOD, in ancient Egyptian chronology, the period in which the year of 365 days circled in succession through all the seasons. The tropical year, determined as it was in Egypt by the heliacal rising of Sirius (Sothis), was almost exactly the Julian year of precisely 365¼ days (differing from the true solar year, which was 11 minutes less than this). The sothic period was thus 1461 years.
See Egypt, Ancient, § F. “Chronology.”