Re: Why was Pope Gregory's adjustment 10 days not 8 days?

Dear Moongazer and Calendar People

The year of the council of Nicea was 325 rather than 532, so making the
reckoned drift equal to 9 or 10 days.

If the Gregorian calendar were extrapolated back, it'd match the Julian
calendar in the 200s century and be one day ahead in the 300s century
when the council of Nicea took place. So an adjustment of 9 days would
be needed for a perfect match at the Council of Nicea.

However the equinox normally occurs on the 20 March Gregorian rather
than 21 March and 11 day adjustment would have been needed to put it on
21 March normally.

10 days seems to be a compromise between the 9 and 11 days argued here.

Why the difference between the 9 and 11 days? This is because an
inaccurate equinox date of 21 March Julian Calendar (from Ptolemy) was
used at the Council of Nicea.

Karl

10(08(06 till noon

-----Original Message-----
From: East Carolina University Calendar discussion List
[mailto:CALNDR-L@...] On Behalf Of Moongazer
Sent: 01 May 2009 06:22
To: CALNDR-L@...
Subject: Why was Pope Gregory's adjustment 10 days not 8 days?

I wrote an article recently on a rare calendric event of the Jewish
Calendar.
The article, " http://geocities.com/calendar.luchot Myths and Maths of
the
Blessing of Sun " discusses some aspects of the Julian calendar and the
Gregorian reform in the following terms:

Because the mean year-length of the Julian calendar is longer than a
tropical year, each season begins on progressively earlier dates in the
calendar. Their start dates regress in the calendar at the rate of 7.8
days
per 1000 years.

This was a problem for the Church, because it caused Easter to drift
ever
closer toward summer. ... Easter is a northern spring festival and must
occur shortly after the equinox. In 532, the council of Nicaea had
irreversibly linked Easter not to the equinox itself, but to its
presumed
date, March 20. However by 1582 it was occurring on March 10. To correct
this, Pope Gregory 13th reformed the calendar. As a one-off adjustment
he
dropped 10 days from that year, and ...

However, on checking the maths, I find that it doesn't quite add up. The
above was based on an estimate of the mean
http://en.wikipedia.org/wiki/Tropical_year tropical year as 365.24219
days .

Accordingly, the actual regression of the seasons in the Julian calendar
is
7.81 days per 1000 years. The calculation is: 365.25 - 365.24219 =
0.00781
days/year. From 532 (the year of the council of Nicaea) to 1582 (the
year of
Pope Gregory's reform) is 1,050 years, and 1050 years x 0.00781
days/year =
8.2005 days. So why did Gregory drop 10 days rather than 8 days?

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