Last night was the first night of Passover. According to Wikipedia…
Passover begins on the 15th day of the month of Nisan, which corresponds to the full moon of Nisan, the first month of the Hebrew calendar, in accordance with the Hebrew Bible.
This is also the first full moon after the vernal equinox, and Easter is always the first Sunday after the first full moon after the equinox. Easter, Ash Wednesday, Palm Sunday, and Pentecost are the original “moveable feasts,” so-called because the dates are variable. However, all are calculated from the Paschal Moon.
So, let’s say you don’t have a calendar handy, but need to know the date of Easter for some random year. There are are some algorithms you could use. If you have your copy of “Practical Astronomy with your Calculator” by Peter Duffett-Smith you could use that. Duffett-Smith used an algorithm by Samuel Butcher published in Nature in 1876 (Nature, 1876 April 20, vol. 13, p. 487.) It was described by M. J. Montes as follows:
This algorithm holds for any year in the Gregorian Calendar, which (of course) means years including and after 1583.
In the text below, / represents an integer division neglecting the remainder, while % is division keeping only the remainder. So 30/7=4 , and 30%7=2 .
a=year%19
b=year/100
c=year%100
d=b/4
e=b%4
f=(b+8)/25
g=(b-f+1)/3
h=(19*a+b-d-g+15)%30
i=c/4
k=c%4
l=(32+2*e+2*i-h-k)%7
m=(a+11*h+22*l)/451
Easter Month =(h+l-7*m+114)/31 [3=March, 4=April]
p=(h+l-7*m+114)%31
Easter Date=p+1 (date in Easter Month)
The algorithm uses a series of successive divisions to reach the answer. In case you don’t have your calculator handy, but do have online access, I’ve created a Google Docs spreadsheet that does the calculation for you. All you have to do is enter the year in question. Of course, you could just use this handy form, or refer to this chart.
