Talk:Holidays related to Easter

Whose Easter are we calculating here? Or, more specifically, which calendars are we using? (And, when do we switch from Gregorian to Julian?) According to Wikipedia, depending on the answers to these questions we can have easter in March, April or May. The sample output from the Algol 68 program makes me think that we are supposed to calculate what wikipedia calls the "Western Christianity"'s Easter, but that was just one of the issues which cause different people to assign different dates to easter in the same year. I think a full specification would be appropriate here. (Easter is apparently the first sunday after the 14th of the month where it occurs, but I am confused about how you unambiguously determine which month it occurs in. But I am also having problems reconciling this "first sunday after the 14th of the month" rule with easters which occur in the first week of April.) --Rdm 14:40, 29 June 2010 (UTC)

It's horribly complicated as it's usually the Sunday of the weekend after Passover, but is calculated using different rules to those specified in the Torah. And it's been the subject of a number of major fights in the Christian church (e.g., the Council of Nicaea). So I searched around and ended up using the code at the end of http://www.assa.org.au/edm.html (which I think is based on doing the switch to Gregorian rules using the British date; you should be able to confirm from the tables higher up that page) as that at least gives the same days/months as that deeply impenetrable ALGOL code for the dates in the exact challenge. –Donal Fellows 10:34, 30 June 2010 (UTC)

http://www.assa.org.au/edm.html describes several different (and apparently conflicting) methods for determining when easter is. Although the page itself claims to describe how to do this for dates from 326 onward, the code at the bottom of the page is claimed to be accurate for years 1583..4099. (Many -- but not all --european countries switched from the julian calendar to the gregorian calendar between easter in 1582 and easter in 1583.) Given the historical issues here, I think the task should explicitly specify how they are handled (or, perhaps, specify a date range which excludes the conflicts). As it stands now, implementations which do not specify multiple easters in some years necessarily fail to recognize some easters and this is complicated by the fact that the same "date" in different calendar systems will necessarily represent different physical dates. --Rdm 18:34, 30 June 2010 (UTC)

I just changed brief operators to bold operator for readability. I hope it helps.

Not sure what to do about dates before Gregorian. Esp Sep 1752. We might standardise on Common Era (CE) for the program, however even CE is vague about whether either early Gregorian calendar Or Julian calendar dates should be used.

[OT: Time.... to be the Time Lord and just play with time... I heard [an urban myth(?)] that Julius Caesar changed the first month of the year from March to the next January so as to increase his term in the Senate. Hence to this day Sep/Oct/Nov/December are no longer the 7th/8th/9th/10th months. It seems when you are the Consul/Dictator of the Roman Republic you then have enough authority to bend time! EYHO Albert!  :-) ]

NevilleDNZ 00:33, 1 July 2010 (UTC)

For example: http://www.merlyn.demon.co.uk/estrdate.htm#MEDC -- for the year 2011, easter falls on April 11 and April 24 where in the year 2000, it fell on April 17th, April 23 and April 30. And things are much simpler now than they were in the past... --Rdm 02:20, 5 July 2010 (UTC)

Well, all that I can really say for sure is that the code is probably wrong, but both the Algol and Tcl examples are wrong in the same way. (I know they're wrong because when I put in the day of the week, things are all over the place before the switch to the Gregorian calendar; that's an area where I'm pretty sure that the Tcl time formatting engine is correct; the guy who wrote the modern implementation is a serious time geek.) Given the massive uncertainty over calendars, I suggest not worrying about it. As noted, it's been a point of contention for the church for a long time even without programmers involved. –Donal Fellows 09:33, 5 July 2010 (UTC)

I was thinking of adding a Python calculator for future easters that relied on the random module :-)
--Paddy3118 13:07, 5 July 2010 (UTC)

Suggestion

Take away the uncertainty. Find a routine that is easy to follow and has little controversy in calculating Easter for a small period of time around now. Quote the routine in the task description, and have the task be to calculate a less controversial number of Easters around now. You could point to external arguments about dates of Easter for the interested, but meaanwhile you would get a task that aids programming chrestomathy.