Julian calendar
Transition history

In 1582, Pope Gregory XIII promulgated the Gregorian calendar. Reform was required as the Julian calendar year, with an average length of 365.25 days, was longer than the natural tropical year. On average, the astronomical solstices and the equinoxes advance by 10.8 minutes per year against the Julian calendar year. As a result, 21 March (which is the base date for calculating the date of Easter) gradually moved out of alignment with the March equinox.

While Hipparchus and presumably Sosigenes were aware of the discrepancy, although not of its correct value, it was evidently felt to be of little importance at the time of the Julian reform (46 BC). However, it accumulated significantly over time: the Julian calendar gained a day every 128 years. By 1582, 21 March was ten days out of alignment with the March equinox, the date where it was reckoned to have been in 325, the year of the Council of Nicaea.

Since the Julian and Gregorian calendars were long used simultaneously, although in different places, calendar dates in the transition period are often ambiguous, unless it is specified which calendar was being used. In some circumstances, double dates might be used, one in each calendar. The notation "Old Style" (O.S.) is sometimes used to indicate a date in the Julian calendar, as opposed to "New Style" (N.S.), which either represents the Gregorian date or the Julian date with the start of the year as 1 January. This notation is used to clarify dates from countries that continued to use the Julian calendar after the Gregorian reform, such as Great Britain, which did not adopt the reformed calendar until 1752, or Russia, which did not do so until 1918 (see Soviet calendar). This is why the Russian Revolution of 7 November 1917 N.S. is known as the October Revolution, because it began on 25 October O.S.
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