A Brief History of the Calendar

The Gregorian calendar in use today is based on a calendar created by the Egyptian astronomer, Sosigenes, at the direction of Julius Caesar. This calendar was placed into effect the year we now call 45 B.C. (Friedman 1989). Sosigenes calculated the length of the tropical year (equinox to equinox) at 365.25 days, and proposed that the normal year have 365 days, and an extra day be inserted every fourth year to make up for the extra one quarter day each year. The actual length of the tropical year is 365.2421987 days calculated with reference to a cesium atomic clock in 1956 by the Harvard University astronomer, Simon Newcomb (Fliegl 1968). The effect of Sosigenes tropical year being shorter than the actual tropical year becomes apparent after several centuries. By the time of Pope Gregory XIII, in the 16th century, the difference between the calendar vernal equinox and the actual vernal equinox amounted to 10 days. Under Pope Gregory, Christopher Clavis developed the rules (based on calculations made by astronomer Aloysius Lilus) for the Gregorian calendar. This is the calendar used by most of the nations of the world today. Each calendar year would be 365 days in length, and an extra day would be inserted every fourth year (any year evenly divisible by 4) except in centennial years (years evenly divisible by 100) unless the centennial year was also evenly divisible by 400. See Table 1 if that was a bit confusing. Since the current calendar date was off by 10 days, Pope Gregory ordered that the final day of the era of the Julian calendar would be Thursday, October 4, 1582 A.D. The following day would be Friday, October 15, 1582 A.D., The first day of the era of the Gregorian calendar (Friedman 1989). This edict took effect almost immediately in the Papal States, but it took many years for other European nations to adopt the new calendar.