In 1000 BC the summer solstice was on July 2nd [pdf] | Hacker News Reader

In 1000 BC the summer solstice was on July 2nd [pdf]

beda.cz

15 pointsFF0000itor10 comments

It's not just calendars. It's clocks too. GMT is less than 200 YEARS old. Yet even more than that, GMT reflects recent changes to our very concept of time.

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.

The sun didn't change. The way people date the sun did. A Julian calendar prior to October 1582 and from there on a Gregorian calendar

http://en.wikipedia.org/wiki/Gregorian_calendar

And there was no July in 1000 BC as the month is named after some Julius Ceasar who lived quite a bit later. The whole argument is a silly, ahistoric play on calendar definitions.

This posting made me look back to another posting on HN some time ago about 'Falsehoods Programmers believe about Time'.

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

1581 AD 10 Mar 18:01 11 Jun 19:34 13 Sep 06:54 11 Dec 20:06 00:02

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?

http://en.wikipedia.org/wiki/Gregorian_calendar

"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."

what is the significance of this?

I think it demonstrates precession, and the difference between the tropical and sidereal years.

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.

Another way to see the effect of the precession of the Earth's axis is in astrology. What's your sign? It's the constellation in which the sun stands on the day of your birthday. It's easy to see that: go up into the stratosphere in a balloon, and you'll see both the stars and the sun. Identify the constellation in which the sun stands, and there's your sign. So, my birthday is September 7, and my sign is Virgo. But every time I go up to the stratosphere on my birthday, what do I see? The sun is smack dab in the middle of Leo. Now I don't know anything about astrology, and trust me I don't want to, but a possible explanation would be this: the zodiac moves relative to the sun by about one constellation every 2150 years (25800 years for a full rotation), due to the precession of Earth's axis. So my best guess is that they made up the astrological signs about 2000 years ago and forgot all about precession.

That's correct. The zodiac has precessed relative to the seasons. This is the definition of the "Age of Aquarius": that the sun is currently in Aquarius at the time of each vernal equinox, even though by historical definition the vernal equinox should occur at the border from Pisces to Aries. The sun has shifted by 1.5 constellations since the ancient originations of astrology.

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