| """ |
| Copyright (c) 2003-2007 Gustavo Niemeyer <gustavo@niemeyer.net> |
| |
| This module offers extensions to the standard python 2.3+ |
| datetime module. |
| """ |
| __author__ = "Gustavo Niemeyer <gustavo@niemeyer.net>" |
| __license__ = "PSF License" |
| |
| import datetime |
| |
| __all__ = ["easter", "EASTER_JULIAN", "EASTER_ORTHODOX", "EASTER_WESTERN"] |
| |
| EASTER_JULIAN = 1 |
| EASTER_ORTHODOX = 2 |
| EASTER_WESTERN = 3 |
| |
| def easter(year, method=EASTER_WESTERN): |
| """ |
| This method was ported from the work done by GM Arts, |
| on top of the algorithm by Claus Tondering, which was |
| based in part on the algorithm of Ouding (1940), as |
| quoted in "Explanatory Supplement to the Astronomical |
| Almanac", P. Kenneth Seidelmann, editor. |
| |
| This algorithm implements three different easter |
| calculation methods: |
| |
| 1 - Original calculation in Julian calendar, valid in |
| dates after 326 AD |
| 2 - Original method, with date converted to Gregorian |
| calendar, valid in years 1583 to 4099 |
| 3 - Revised method, in Gregorian calendar, valid in |
| years 1583 to 4099 as well |
| |
| These methods are represented by the constants: |
| |
| EASTER_JULIAN = 1 |
| EASTER_ORTHODOX = 2 |
| EASTER_WESTERN = 3 |
| |
| The default method is method 3. |
| |
| More about the algorithm may be found at: |
| |
| http://users.chariot.net.au/~gmarts/eastalg.htm |
| |
| and |
| |
| http://www.tondering.dk/claus/calendar.html |
| |
| """ |
| |
| if not (1 <= method <= 3): |
| raise ValueError, "invalid method" |
| |
| # g - Golden year - 1 |
| # c - Century |
| # h - (23 - Epact) mod 30 |
| # i - Number of days from March 21 to Paschal Full Moon |
| # j - Weekday for PFM (0=Sunday, etc) |
| # p - Number of days from March 21 to Sunday on or before PFM |
| # (-6 to 28 methods 1 & 3, to 56 for method 2) |
| # e - Extra days to add for method 2 (converting Julian |
| # date to Gregorian date) |
| |
| y = year |
| g = y % 19 |
| e = 0 |
| if method < 3: |
| # Old method |
| i = (19*g+15)%30 |
| j = (y+y//4+i)%7 |
| if method == 2: |
| # Extra dates to convert Julian to Gregorian date |
| e = 10 |
| if y > 1600: |
| e = e+y//100-16-(y//100-16)//4 |
| else: |
| # New method |
| c = y//100 |
| h = (c-c//4-(8*c+13)//25+19*g+15)%30 |
| i = h-(h//28)*(1-(h//28)*(29//(h+1))*((21-g)//11)) |
| j = (y+y//4+i+2-c+c//4)%7 |
| |
| # p can be from -6 to 56 corresponding to dates 22 March to 23 May |
| # (later dates apply to method 2, although 23 May never actually occurs) |
| p = i-j+e |
| d = 1+(p+27+(p+6)//40)%31 |
| m = 3+(p+26)//30 |
| return datetime.date(int(y),int(m),int(d)) |
| |