Equinox 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! ===Date=== When [[Julius Caesar]] established the [[Julian calendar]] in 45 BC, he set 25 March as the date of the spring equinox;<ref>{{Cite book |last1=Blackburn |first1=Bonnie J. | last2 = Holford-Strevens | first2 = Leofranc |title=The Oxford companion to the year |date=1999 |isbn=0-19-214231-3 |publisher=Oxford University Press | page = 135 }} Reprinted with corrections 2003.</ref> this was already the starting day of the year in the Persian and Indian calendars. 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 β so that in [[First Council of Nicaea|300 AD]] the spring equinox occurred on about 21 March, and by the 1580s AD it had drifted backwards to 11 March.<ref>{{cite book | last1 = Richards | first1 = E. G. | title = Mapping Time: The Calendar and its History | publisher = Oxford University Press | pages = 250–251 | date = 1998 | isbn = 978-0192862051}}</ref> This drift induced [[Pope Gregory XIII]] to establish the modern [[Gregorian calendar]]. The Pope wanted to continue to conform with the edicts of the [[First Council of Nicaea|Council of Nicaea]] in 325 AD concerning the [[Easter controversy#Second phase|date of Easter]], which means he wanted to move the vernal equinox to the date on which it fell at that time (21 March is the day allocated to it in the Easter table of the Julian calendar), and to maintain it at around that date in the future, which he achieved by reducing the number of leap years from 100 to 97 every 400 years. However, there remained a small residual variation in the date and time of the vernal equinox of about Β±27 hours from its mean position, virtually all because the distribution of 24 hour centurial leap-days causes large jumps (see [[:File:Gregoriancalendarleap solstice.svg|Gregorian calendar leap solstice]]). ====Modern dates==== The dates of the equinoxes change progressively during the leap-year cycle, because the Gregorian calendar year is not commensurate with the period of the Earth's revolution about the Sun. It is only after a complete Gregorian leap-year cycle of 400 years that the seasons commence at approximately the same time. In the 21st century the earliest March equinox will be 19 March 2096, while the latest was 21 March 2003. The earliest September equinox will be 21 September 2096 while the latest was 23 September 2003 ([[Universal Time]]).<ref name="YallopEtAl">{{cite book |last1=Yallop |first1=B.D. |last2=Hohenkerk |first2=C.Y. |last3=Bell |first3=S.A. |chapter=Astronomical Phenomena |editor1-last=Urban |editor1-first=S.E. |editor2-last=Seidelmann |editor2-first=P. K. |year=2013 |title=Explanatory supplement to the astronomical almanac |edition=3rd |location=Mill Valley, CA |publisher=University Science Books |isbn=978-1-891389-85-6 |pages=506β507}}</ref> 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