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The full moon used to calculate the date of Easter. This full moon does not correspond to any
astronomical event, but is a historical artifact determined from tables. An algorithm due to J. H. Conway which is valid
for years Y between 1900 and 2200 calculates the Paschal full moon as
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(1) |
where
 denotes a congruence  , i.e., the remainder when n is divided by m, and
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(2) |
is the golden number, except that when this gives April 19, it is changed to April 18, and when it
gives April 18 and  , then it is changed to April 17. The following table gives the dates
of the Paschal full moons for 1980-2009.
| 1980 |
March 31 |
1990 |
April 10 |
2000 |
April 18 |
| 1981 |
April 18 |
1991 |
March 30 |
2001 |
April 8 |
| 1982 |
April 8 |
1992 |
April 17 |
2002 |
March 28 |
| 1983 |
March 28 |
1993 |
April 7 |
2003 |
April 16 |
| 1984 |
April 16 |
1994 |
March 27 |
2004 |
April 5 |
| 1985 |
April 5 |
1995 |
April 14 |
2005 |
March 25 |
| 1986 |
March 25 |
1996 |
April 3 |
2006 |
April 13 |
| 1987 |
April 13 |
1997 |
March 23 |
2007 |
April 2 |
| 1988 |
April 2 |
1998 |
April 11 |
2008 |
March 22 |
| 1989 |
March 22 |
1999 |
March 31 |
2009 |
April 10 |
A modification of (1) to
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(3) |
where
 is the floor function and
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(4) |
extends the range of valid years from  to  in the Gregorian calendar.
For the Julian calendar, the correct equation is
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(5) |
 Easter, Golden Number, Full Moon
© 1996-2006 Eric W. Weisstein
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