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In the article, as it is now (January 12 2015), it is claimed that the Gregorian calendar reform left the superflous leap day AD 300 unaltered, but the Gregorian calendar reform removed ten, not nine superflous days in 1582 (reforms occuring after February 24 1700 eleven days). Will someone please explain this more clearly.220.127.116.11 (talk) 02:53, 12 January 2015 (UTC)
Good question. According to the table in this article, the Julian and (proleptic) Gregorian calendars do not match up in AD 325, they are one day off. So apparently they jumped ahead one too many days in the conversion. Don't know if this was a simple mistake, or if there was some rationale behind it. --Lasunncty (talk) 08:51, 12 January 2015 (UTC)
It seems unlikely that one of the best astronomers of his time made a mistake even a secondary school pupil would be able to spot. I have only a guess about a solution, and no text sources for this, but is it possible that the Equinox Tables used in Alexandria in AD 325 could have been composed before February 24 AD 300, thus giving correct information about the vernal equinox in late third century, rather than early fourth century? If the intentions in 1582 were to synchronise calendar dates and equinoxes as given in these Alexandrian Tables, this could be a solution. Any other possible answer to this riddle?18.104.22.168 (talk) 13:29, 12 January 2015 (UTC)
The article says "An equinox occurs when the plane of Earth's Equator passes the center of the Sun"; I'm not an expert, but that doesn't seem right. The earth's equator plane is at all times tilted 23.5 degrees with respect to the earth's orbit around the sun. The earth's equator plane would never, ever, "pass through the center of the Sun". Could a true expert educate me if I'm wrong? Jssprojects (talk) 21:46, 14 January 2015 (UTC)
In astronomy the spring equinox is defined as the moment when the apparent geocentric longitude of the Sun measured along the ecliptic is 0 degrees. Many people claim that the apparent declination (angle from the celestial equator) is then also zero but although this is very nearly true an exact computation will show that there always is a small difference. AstroLynx (talk) 10:24, 15 January 2015 (UTC)
Jssprojects - The earth's tilt causes the subsolar point to travel between 23.5° north of the equator and 23.5° south of the equator. So it does indeed pass through the center of the sun at those two times per year — once heading north, and once heading south.
AstroLynx - Maybe I'm wrong, but it seems that the longitude would be defined by the equinox, not the other way around.