Julian Calendar

From Turtledove

The Julian Calendar was a calendar designed by the Alexandrian astronomer Sosigenes in 46 BC and enforced throughout the Roman Empire by Julius Caesar in 45 BC. It remained in use throughout the Western world until 1582, when inaccuracies in the Julian calendar were found to have created a lag of ten full days in the Julian calendar; for instance, the vernal equinox was marked ten days after the day which had twelve hours of sunlight and twelve hours of darkness. A revised calendar correcting this problem, the Gregorian Calendar, was proposed to and decreed by Pope Gregory XIII. The Julian Calendar was abandoned in Catholic countries but remained in use in Protestant countries.