A Julian year is the length of an average year in the Julian calendar, 365.25 days. The Julian calendar was introduced by Julius Caesar in 46 BC, taking force in 45 BC or 709 ab urbe condita. ...
Astronomers still use the Julian year as a fundamental unit for ephemeris work, since it provides a quick and simple conversion to Julian dates. An ephemeris (plural: ephemerides) (from the Greek word ephemeros= daily) was, traditionally, a table providing the positions (given in a Cartesian coordinate system, or in right ascension and declination or, for astrologers, in longitude along the zodiacal ecliptic), of the Sun, the Moon, and the planets in the sky at... The Julian day or Julian day number (JDN) is the number of days that have elapsed since 12 noon Greenwich Mean Time (UT or TT) on Monday, January 1, 4713 BC (in the proleptic Julian calendar; or November 24, 4714 BC in the proleptic Gregorian calendar). ...
Note that a Julian year is not the same as a Gregorian calendar year, a tropical year or a sidereal year. The Gregorian calendar is the calendar widely used in the Western world. ... A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ... The sidereal year is the time for the Sun to return to the same position in respect to the stars of the celestial sphere. ...
The indiction caused the Byzantine year to begin on 1 September, which is still used in the Eastern Orthodox Church for the beginning of the liturgical year.
Years were numbered from the supposed date of the incarnation or annunciation of Jesus on 25 March, although this soon changed to Christmas, then back to Annunciation Day in Britain, and the numbered year even began on Easter in France.
A revised Julian calendar was proposed during a synod in Constantinople in May of 1923, consisting of a solar part which was and will be identical to the Gregorian calendar until the year 2800, and a lunar part which calculated Easter astronomically at Jerusalem.
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