Of course, the times were actually calculated from the known rate of precession; we couldn't exactly have said that the summer solstice occurred at 22:00 hours on 2nd July, 1000 BCE. There, you know, wasn't even a July then, given that it was named after a dude who wouldn't be born for another 900 years.
EDIT: As mentioned by other comments, it also captures the shift between the Julian and Gregorian calendars.
1582 AD 10 Mar 23:56 12 Jun 01:30 13 Sep 12:39 22 Dec 01:54 00:02
1583 AD 21 Mar 05:51 22 Jun 07:16 23 Sep 18:24 22 Dec 07:42 00:02
Looks the single most dramatic shift happened in these two years. Would someone with astronomical chops care to explain a bit?
"Four Catholic countries—Spain,[18] Portugal, the Polish–Lithuanian Commonwealth, and most of Italy—implemented the new calendar on the date specified by the bull, with Julian Thursday, 4 October 1582, being followed by Gregorian Friday, 15 October 1582."
GMT was based on noon at the Royal Observatory. But only nominally because GMT was a standard for mechanized time and hence abstracted away the messy variability in Solar noon. The fact that you probably are not going to flame me for describing solar noon as 'messily' variable indicates how deeply anachronistic the table is with respect to the meaning of time.
To carry this further, the calendar is a mechanical abstraction over the messy details of years. The year for Stonehenge users is based on observable natural phenomena just as noon is when the sun is due south. And if your first thought wasn't to flame me for leaving half the world out of the description of noon that's your conceptual blindspot not mine.
My HN search fu is not great, it took me a while to find the posting so I post it here for the convenience of others: https://news.ycombinator.com/item?id=4128208
