The article says 2015 autumn equinox will be at UT 8:20. But the referenced source says 8:21, and various other sources say x:22. Why the discrepancy/variation? Is this event not one precise instant for the whole Earth? What is the true exact time, to at least the second? What is the most reliable source?-220.127.116.11 (talk) 14:41, 21 September 2015 (UTC)
2015-Sep-23 08:20:32.876 68.182392 179.9999887 0.0002366
2015-Sep-23 08:20:33.876 68.182392 180.0000000 0.0002366
2015-Sep-23 08:20:34.876 68.182392 180.0000113 0.0002366
The dynamical Coordinate Time scale is used internally. It is equivalent to the current IAU definition of "TDB".
Giving a final result of:
2015-Sep-23 08:20:33.876 UTC (+68.182392 = CT/TDB)
In conclusion, 8:20:34 and 8:21 UTC is the proper rounded time, 8:20 is understandable -- but 8:22 seems somewhat wrong, for any general usage.-18.104.22.168 (talk) 21:49, 21 September 2015 (UTC)
The Horizons output indicates UT; it does not say if that is UT1 or UTC. According to the US Naval Observatory the prediction is that UT1-UTC = 0.24414 second on 23 Sept. Can you indicate where it is stated whether Horizons considers UT to mean UT1 or UTC? Jc3s5h (talk) 15:47, 22 September 2015 (UTC)
is Universal Time This can mean one of two non-uniform time-scales based on the rotation of the Earth. For this program, prior to 1962, UT means UT1. After 1962, UT means UTC or "Coordinated Universal Time". Future UTC leap-seconds are not known yet, so the closest known leap-second correction is used over future time-spans.
In discussing the determination of the date of the equinox, Equinox § Date observes that "the leap year intervals in [Pope Gregory's] calendar were not smooth (400 is not an exact multiple of 97)", but does not explain the significance of these numbers. The Gregorian leap-year cycle is 400 years long and contains 97 leap years (every fourth year, but not in century years xx00 unless xx is a multiple of 4). Needs clear explanation, which the foregoing is not very. --Thnidu (talk) 08:29, 22 September 2015 (UTC)