

In 1000 BC the summer solstice was on July 2nd [pdf] - FF0000itor
http://www.beda.cz/~jirkaj/seasons/seasons.pdf

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brudgers
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.

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ronilan
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](http://en.wikipedia.org/wiki/Gregorian_calendar)

~~~
fhars
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.

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evaneykelen
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](https://news.ycombinator.com/item?id=4128208)

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NoCowLevel
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?

~~~
powera
[http://en.wikipedia.org/wiki/Gregorian_calendar](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."

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jkw
what is the significance of this?

~~~
rosser
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.

~~~
thebear
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.

~~~
T-hawk
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.

