NYCPHP Meetup

[tedd]

At 11:59 AM -0500 12/7/06, jessica kelly wrote:
>Ted here is a site that has a brief mention of the two calendars.
>http://www.kencollins.com/calendar.htm
>
>As you may/may not know the system was changes somewhere in the ?1600's?
>or so. One October lost several days when the change over occured from
>Julian to Georgian so you need to take that into condideration so
>research well!

Yes, that's a good link and the "system" did change in the 1600's, 
namely October 4, 1582 (a Thursday btw).

Prior to that, the day of the week cannot be calculated easily. In 
order to understand why, we need to look at the calendar history.

Prior to Julius ruling Rome (BC 46) the Roman year was 354 days. When 
Julius came to power, one of his first challenge was to appease the 
farmers who complained because according to their calendar, spring 
time and the time to sow/plant, was actually mid winter. Julius hired 
a mathematician (Sosigenes) to figure out what went wrong.

Sosigenes suggested that the length of the year was wrong. It should 
have been 365.25 days. The extra .25 day after four years became the 
extra day added in February. However, Julius made a mess of 46 BC by 
adding a couple of months making the year 445 days long!

Of course, Sosigenes, not having a calculator, didn't realize that a 
solar year is really 365.2428935. His fix was off by 10 minutes and 
some seconds in a year --not bad for manual computation! This error 
over 1600 years, adds up to about 11 days.

This error was detected by astronomers who noticed that the vernal 
equinox (when light and dark periods are equal length) occurring on 
March 21 was becoming earlier and earlier and in 1582 it was on March 
10. So, Christopher Clarius, an astronomer, went to Pope Gregory XIII 
and addressed the issue.

After seeing all the calculations for 7 months, pope Gregory knocked 
off 11 days on October 4 1582. The next day, he decreed to be October 
15 (so October 5-14 in the year 1582 didn't exist), but to not upset 
the Jews with their Sabbath, the day of the week was NOT changed. Had 
Pope Gregory deleted the days properly (along with the dates), then 
the next day (October 15) would have been a Monday and this would 
have resulted in Jewish citizens working more than ten days before 
being able to observe their Sabbath. This of course would have been 
unacceptable, not even animals nor slaves can be put to work on the 
Lord's Sabbath (Exo 20:9,31:13,Lev 23:3 etc.)

To avoid the leap-year error again, Pope Gregory, decreed that every 
4 years a day will be added. However, on a turn of the century, it 
will only be added if it's divisible by 400. Hence, although year 
2000 was a leap year, while 1900 was not.

So, considering the history of calendars, I again state that I wonder 
how PostgreSQL has the ability to store B.C. dates?

tedd
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