History of Tropical year

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	Revision 29 . . November 15, 2001 8:06 pm by Tompeters [Removed direct link to First Council of Niceae: details relegated to Easter page now.]
	Revision 28 . . (edit) November 14, 2001 5:02 pm by Tompeters [average->mean]
	Revision 27 . . (edit) November 14, 2001 3:13 am by (logged).179.196.xxx [Simon Cassidy. clarified intentions of Gregorian reform.]
	Revision 26 . . November 14, 2001 3:07 am by (logged).179.196.xxx [Simon Cassidy. Details of Easter not decided at Nicaea but after. Mean tropical length should be compared to more constant vernal equinox year-length not vice-versa.]
	Revision 25 . . (edit) November 14, 2001 1:27 am by Tompeters [repaired link to Universal_time]
	Revision 24 . . November 14, 2001 1:24 am by Tompeters [Textual clarifications; complete list of continued fractions]
	Revision 23 . . (edit) November 11, 2001 3:01 am by (logged).179.251.xxx
	Revision 22 . . (edit) November 11, 2001 2:24 am by (logged).179.251.xxx
	Revision 21 . . (edit) November 11, 2001 2:19 am by (logged).179.251.xxx
	Revision 20 . . (edit) November 11, 2001 2:11 am by (logged).179.251.xxx
	Revision 19 . . November 11, 2001 2:09 am by (logged).179.251.xxx
	Revision 18 . . (edit) November 4, 2001 8:31 pm by Tompeters [repeated->continued fractions]
	Revision 17 . . (edit) October 28, 2001 12:20 am by Tompeters [Replaced TDT by TT]
	Revision 16 . . (edit) October 26, 2001 9:27 am by Tompeters [commas]
	Revision 15 . . October 26, 2001 9:19 am by Tompeters [elaborated on variation in year lengths; created sections]
	Revision 14 . . (edit) October 26, 2001 12:01 am by Rmhermen
	Revision 13 . . (edit) October 25, 2001 7:34 pm by Malcolm Farmer [only link first occurrence of each term, and if perihelium->perihelion is correct, then aphelium->aphelion]
	Revision 12 . . October 25, 2001 7:26 pm by Tompeters [added Talk]
	Revision 11 . . October 25, 2001 7:02 pm by Karl Palmen [Perihelion NOT perihelium - Perihelion date PRESENTLY, about Jan 2]
	Revision 10 . . (edit) October 25, 2001 5:22 am by Tompeters
	Revision 9 . . (edit) October 24, 2001 11:13 pm by Tompeters [UT -> universal time; explify Julian year as unit.]
	Revision 8 . . October 24, 2001 9:47 pm by (logged).109.250.xxx [s/consilium of Nicaea/First Council of Nicaea/ -- using more common & accurate name]
	Revision 7 . . (edit) October 24, 2001 8:31 pm by Malcolm Farmer [council of nicaea link]
	Revision 6 . . October 24, 2001 8:22 pm by Tompeters [reformulated (condensed) the digression on [[Delta-T]]; and minor rephrasings.]
	Revision 5 . . October 24, 2001 4:24 pm by Karl Palmen *[The type of day for which the linear term applies is not appropriate for calendars.]**
	Revision 4 . . (edit) October 24, 2001 6:04 am by Victorengel
	Revision 3 . . October 24, 2001 5:35 am by Tompeters [Added source for numerical values]
	Revision 2 . . (edit) October 24, 2001 5:20 am by Tompeters [removed Passover (jewish feast)]
	Revision 1 . . October 24, 2001 5:13 am by Tompeters [Time for the Sun to complete a full circle along the ecliptic; basis for calender year.]

Difference (from prior major revision) (author diff)

Changed: 36c36

This distinction is relevant for calendar studies. The main Christian moving feast has been Easter. After the First Council of Nicaea in 325 it was decided that Easter would be celebrated on the Sunday on or after the first full moon after 21 March, the day of the vernal equinox. The church therefore made it an objective to keep the day of the vernal (spring) equinox on or near 21 March, and the calendar year has to be synchronized with the tropical year as measured by the mean interval between vernal equinoxes. From about 1000 A.D. the mean tropical year has become increasingly shorter than this mean interval between vernal equinoxes.

This distinction is relevant for calendar studies. The main Christian moving feast has been Easter. Several different ways of computing the day of Easter were used in early christian times, but eventually the unified rule was accepted that Easter would be celebrated on the Sunday on or after the first full moon after the day of the vernal equinox, which was established to fall on 21 March. The church therefore made it an objective to keep the day of the vernal (spring) equinox on or near 21 March, and the calendar year has to be synchronized with the tropical year as measured by the mean interval between vernal equinoxes. From about 1000 A.D. the mean tropical year has become increasingly shorter than this mean interval between vernal equinoxes.