solar calendar

solar calendar, any dating system based on the seasonal year of approximately 365 1/4 days, the time it takes the Earth to revolve once around the Sun. The Egyptians appear to have been the first to develop a solar calendar, using as a fixed point the annual sunrise reappearance of the Dog Star—Sirius, or Sothis—in the eastern sky, which coincided with the annual flooding of the Nile River. They constructed a calendar of 365 days,...