... Early priests in Babylonia, Egypt, China and other countries, even among the Maya in America, examined therefore the motions of the Sun, Moons and planets across the sky, and came up with a variety of calendars, some still in use. ... During the year the Sun traces a full circle around the sky, so that if we keep a separate count of "Sirius days" and "Sun days", at the end of the year the numbers will differ by 1. ... Thus the calendar still slips, but at a very slow rate, about one day in 128 years...