You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
90 lines
2.6 KiB
Python
90 lines
2.6 KiB
Python
# -*- coding: utf-8 -*-
|
|
"""
|
|
This module offers a generic easter computing method for any given year, using
|
|
Western, Orthodox or Julian algorithms.
|
|
"""
|
|
|
|
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:
|
|
|
|
`GM Arts: Easter Algorithms <http://www.gmarts.org/index.php?go=415>`_
|
|
|
|
and
|
|
|
|
`The Calendar FAQ: Easter <https://www.tondering.dk/claus/cal/easter.php>`_
|
|
|
|
"""
|
|
|
|
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))
|