Julian calendar Warning: You are not logged in. Your IP address will be publicly visible if you make any edits. If you log in or create an account, your edits will be attributed to your username, along with other benefits.Anti-spam check. Do not fill this in! === Transition history === {{See also|Adoption of the Gregorian calendar|Old Style and New Style dates}} In 1582, [[Pope Gregory XIII]] promulgated the [[Gregorian calendar]]. Reform was required because too many leap days were added with respect to the astronomical seasons under the Julian scheme. On average, the astronomical solstices and the equinoxes advance by 10.8 minutes per year against the Julian year. As a result, 21 March (which is the base date for [[Computus|calculating the date of Easter]]) gradually moved out of alignment with the March equinox. [[File:Julian to Gregorian Date Change.png|thumb|This is a visual example of the official date change from the Julian calendar to the Gregorian.]] While Hipparchus and presumably [[Sosigenes of Alexandria|Sosigenes]] were aware of the discrepancy, although not of its correct value,{{sfn|Richards|1998|page=216}} 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 [[First Council of Nicaea|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 and New Style dates|"Old Style" (O.S.)]] is sometimes used to indicate a date in the Julian calendar, as opposed to [[Old Style and New Style dates|"New Style" (N.S.)]], which either represents the Julian date with the start of the year as 1 January or a full mapping onto the Gregorian calendar. 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 switch to the reformed calendar until 1752, or Russia, which did not switch 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. Summary: Please note that all contributions to Christianpedia may be edited, altered, or removed by other contributors. If you do not want your writing to be edited mercilessly, then do not submit it here. You are also promising us that you wrote this yourself, or copied it from a public domain or similar free resource (see Christianpedia:Copyrights for details). Do not submit copyrighted work without permission! Cancel Editing help (opens in new window) Discuss this page