Easter date (millionth time asked, sorry)

I know that, in general, the date of Easter is supposed to be the
first Sunday to follow the first full moon, following March 21.

But aren't lunar cycles 28 days? Meaning that, if our full moon this
year is March 25 (I think it is), in 2006 it will be on March 24 (364
days, or 13 cycles times 28 days/cycle, later), and Easter would then
be, predictably, March 26. And then March 25 in 2007 and so on.

Obviously I'm missing something.

Could someone explain why the date of Easter doesn't simply creep back
one day a year (two days in leap years), until it gets near March 21
(depending on the full moon that year), jump to April 18 or
thereabout, and then start sliding back again for another 20 or so
years?

in wrote:
I know that, in general, the date of Easter is supposed to be the
first Sunday to follow the first full moon, following March 21.

But aren't lunar cycles 28 days? Meaning that, if our full moon this
year is March 25 (I think it is), in 2006 it will be on March 24 (364
days, or 13 cycles times 28 days/cycle, later), and Easter would then
be, predictably, March 26. And then March 25 in 2007 and so on.

Obviously I'm missing something.

Could someone explain why the date of Easter doesn't simply creep back
one day a year (two days in leap years), until it gets near March 21
(depending on the full moon that year), jump to April 18 or
thereabout, and then start sliding back again for another 20 or so
years?

The popular notion that Easter is the first Sunday after the first Full
Moon following the vernal equinox... is not quite right. The date of
Easter is defined to occur on the Sunday following the *ecclesiastical*
Full Moon that falls on or after March 21st. Subtle differences...

Sam Wormley wrote:
in wrote:

I know that, in general, the date of Easter is supposed to be the
first Sunday to follow the first full moon, following March 21.

But aren't lunar cycles 28 days? Meaning that, if our full moon this
year is March 25 (I think it is), in 2006 it will be on March 24 (364
days, or 13 cycles times 28 days/cycle, later), and Easter would then
be, predictably, March 26. And then March 25 in 2007 and so on.

Obviously I'm missing something.

Could someone explain why the date of Easter doesn't simply creep back
one day a year (two days in leap years), until it gets near March 21
(depending on the full moon that year), jump to April 18 or
thereabout, and then start sliding back again for another 20 or so
years?


The popular notion that Easter is the first Sunday after the first Full
Moon following the vernal equinox... is not quite right. The date of
Easter is defined to occur on the Sunday following the *ecclesiastical*
Full Moon that falls on or after March 21st. Subtle differences...


Average length of lunar day (days) 29.53059
Rotational period (days) 27.32166
Orbital period (days) 27.32166

Ref: http://www.solarviews.com/eng/moon.htm

perrm wrote:
I know that, in general, the date of Easter is supposed to be the
first Sunday to follow the first full moon, following March 21.

It is not the first real, or astronomical, Full Moon. It is the first
ecclesiastical Full Moon. Usually, it won't make a difference, but it
occasionally does. Using the ecclesiastical Full Moon involves a
long but straightforward arithmetical formula, which frees the Church
from having to worry about the vagaries of celestial mechanics. (To
be more precise, this is only so for Easter in Western countries. For
those using the Orthodox calendar, Easter is counted according to the
real astronomical Full Moon and the actual moment of the equinox as
reckoned at Jerusalem. Plus they use the Julian calendar instead of
our Gregorian one. But let's ignore that for the moment.)

But aren't lunar cycles 28 days? Meaning that, if our full moon this
year is March 25 (I think it is), in 2006 it will be on March 24 (364
days, or 13 cycles times 28 days/cycle, later), and Easter would then
be, predictably, March 26. And then March 25 in 2007 and so on.

Obviously I'm missing something.

Yes--principally that lunar cycles (presuming you mean the mean synodic
period, the average time from New Moon to New Moon) are longer than 28
days. It is about 29.53 days long, and although that extra 1.53 days
isn't much, it adds up over the course of a year. Plus the synodic
period varies somewhat. All this is why the Western countries avoid
using the actual celestial timing.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

On Sat, 19 Mar 2005 14:41:20 GMT, in wrote:

I know that, in general, the date of Easter is supposed to be the
first Sunday to follow the first full moon, following March 21.

It doesn't use the actual astronomical full moon. The date is from a
calculated full moon, following a couple of different formulas,
depending on whether you are a western Christian (e.g., Roman Catholic
or Protestant) or an eastern Christian (Orthodox and Oriental
Churches).

See this entry on the calndar faq:

http://www.tondering.dk/claus/cal/node3.html#SECTION003130000000000000000

Nick


--
Nick Theodorakis

contact form:
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On Sat, 19 Mar 2005 14:41:20 GMT, in wrote:

I know that, in general, the date of Easter is supposed to be the
first Sunday to follow the first full moon, following March 21.

The date of Easter is based on the Pascal Full Moon, so-called, and
not the astronomical full moon. The time of the Pascal Full Moon is
based on the 19 year Metonic cycle. The reason is to permit the date
of Easter to be calculated in advance using reasonably straightforward
calculations.

JRS: In article , dated
Sat, 19 Mar 2005 22:50:35, seen in news:sci.astro.amateur, William
Hamblen posted :
On Sat, 19 Mar 2005 14:41:20 GMT, in wrote:

I know that, in general, the date of Easter is supposed to be the
first Sunday to follow the first full moon, following March 21.

The date of Easter is based on the Pascal Full Moon, so-called, and
not the astronomical full moon. The time of the Pascal Full Moon is
based on the 19 year Metonic cycle. The reason is to permit the date
of Easter to be calculated in advance using reasonably straightforward
calculations.

Not exactly; it's the Paschal Full Moon, after the name of Easter in
Romance languages of yore. Pascal was a French scholar, after whom a
computer language is named. Also, it's a modified Metonic cycle.

I have a number of versions of the calculation (all agreeing as to
results) on my Web site, including images and translations of Zeller's
papers.

But, while the Papal Bull /Inter Gravissimas/ gives authority to the
Gregorian Leap Year Rules and the Gregorian Easter Rules, it describes
only the former, citing an Annex for the latter. Has anyone found the
latter, or a Vatican copy of the former, on the Web?

--
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Web URL:http://www.merlyn.demon.co.uk/ - w. FAQish topics, links, acronyms
PAS EXE etc : URL:http://www.merlyn.demon.co.uk/programs/ - see 00index.htm
Dates - miscdate.htm moredate.htm js-dates.htm pas-time.htm critdate.htm etc.