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Because the Julian year is longer than the tropical year by about 11.3 minutes on average (or 1 day in 128 years), the calendar "drifted" with respect to the two equinoxes – so that in 300 AD the spring equinox occurred on about 21 March, and by the 1580s AD it had drifted backwards to 11 March.
In addition, an epoch optionally prefixed by "J" and designated as a year with decimals (2000 + x), where x is either positive or negative and is quoted to 1 or 2 decimal places, has come to mean a date that is an interval of x Julian years of 365.25 days away from the epoch J2000 = JD 2451545.0 (TT), still corresponding (in spite of the use of ...
The (19-year) Metonic cycle is a lunisolar cycle, as is the (76-year) Callippic cycle. [8] An important example of an application of the Metonic cycle in the Julian calendar is the 19-year lunar cycle insofar as provided with a Metonic structure. [9] Meton introduced the 19 year cycle to the Attic calendar in 432 BC.
This calendar denotes "lunar years" as Anno Hegiræ ([since] the year of the Hijra) or AH. This calendar is used in Sunni Islam and related sects. The epoch of the official Iranian calendar is also the Hijra, but it is a solar calendar; each year begins at the Northern spring equinox. This calendar is used in Shia Islam and related sects.
Since the Islamic calendar is a purely lunar calendar of about 354 or 355 days, its year count increases faster than that of solar and lunisolar calendars. S.H. (or SH) [citation needed] is used by the Iranian calendar to denote the number of solar years since the Hijra. The year beginning at the vernal equinox equals the number of the ...
Thus the equinox moved 54° in one direction and then back 54° in the other direction. This cycle took 7200 years to complete at a rate of 54″/year. The equinox coincided with the epoch at the beginning of the Kali Yuga in −3101 and again 3,600 years later in 499. The direction changed from prograde to retrograde midway between these years ...
The use of a syzygy to determine the lunar month was first described in the Tang dynasty Wùyín Yuán Calendar (戊寅元历; 戊寅元曆; 'earth tiger epoch calendar'). The Yuan dynasty (13th/14th century) Shòushí calendar ( 授时历 ; 授时曆 ; 'teaching time calendar') used spherical trigonometry to find the length of the tropical year .
In the Julian calendar, the equinox recedes at the rate of 1 day in 128 years; by the time of the First Council of Nicaea in AD 325 it was falling on 21 March. 22 March is 26 Phamenoth in the Alexandrian calendar. The lunar new year was not allowed to begin before 15 Thoth (12 September, or 13 September if the following February has 29 days ...