Dear Cecil:

Oops! You goofed. In your first book, “The Straight Dope,” you say that “Easter falls on the first Sunday subsequent to the first full moon after the vernal equinox.” This isn’t necessarily true. The date of Easter is determined by an ecclesiastical formula (see “Astronomical Algorithms” by Jean Meeus,
Willmann-Bell, 1991):

Divide by Quotient Remainder

the year x 19 - a
the year x 100 b c
b 4 d e
b 8 25 f -
b-f 1 3 g -
19a b-d-g 15 30 - h
c 4 i k
32 2e 2i-h-k 7 - l
a 11h 22l 451 m -
h l-7m 114 31 n p

Then n = number of month (3=March, 4=April), and p 1 = day of the month on which Easter falls.
The actual (ecclesiastical) date of Easter does not always fall on the astronomical date of Easter. For example, in 1981 the vernal equinox fell on March 20, the next full Moon was on April 19, and the following Sunday was
April 26. However, Easter Sunday in 1981 actually fell on April 19, as computed by the above formula.
The next time that the astronomical date of Easter will differ from its actual ecclesiastical date will be in 2019, when the astronomical date is March 24, but the ecclesiastical formula gives the actual date of April 21.

David Simpson
Laurel, Maryland
[Note: This message has been edited by Ed Zotti]

SimpsonDG did relay the following:
Dear Cecil: Oops! You goofed. [snip]

  1. Cecil, of course, never “goofs”. Of course, there have been a few, rare, occasions when Ed incorrectly transcribed one of Cecil’s flawless answers.

  2. Unfortunately, your equation for calculating the date of Easter is incomprehensible when displayed using a proportional font. If I’ve done this correctly, below is the equation the way I believe you intended it to be displayed:

Divide by Quotient Remainder
============ === ======== =========
the year x 19 - a
the year x 100 b c
b 4 d e
b+8 25 f -
b-f+1 3 g -
19a+b-d-g+15 30 - h
c 4 i k
32+2e+2i-h-k 7 - l
a+11h+22l 451 m -
h+l-7m+114 31 n p

Then n = number of month (3=March, 4=April), and p+1 = day of the month on which Easter falls.

Well, that looked okay if you are using Internet Explorer, but still looks odd if you are using a BrandX browser. (If this doesn’t work let someone else try–I’m going home.)

<table cellpadding=0 cellspacing=0 border=0>
<tr><td align=left>Divide </td><td align=left>by</td><td align=left>Quotient  </td><td>Remainder</td></tr>
<tr><td align=left>the year x</td><td align=left>19</td><td align=center>-</td><td align=center>a</td></tr>
<tr><td align=left>the year x</td><td align=left>100  </td><td align=center>b</td><td align=center>c</td></tr>
<tr><td align=left>b</td><td align=left>4</td><td align=center>d</td><td align=center>e</td></tr>
<tr><td align=left>b+8</td><td align=left>25</td><td align=center>f</td><td align=center>-</td></tr>
<tr><td align=left>b-f+1</td><td align=left>3</td><td align=center>g</td><td align=center>-</td></tr>
<tr><td align=left>19a+b-d-g+15   </td><td align=left>30</td><td align=center>-</td><td align=center>h</td></tr>
<tr><td align=left>c</td><td align=left>4</td><td align=center>i</td><td align=center>k</td></tr>
<tr><td align=left>32+2e+2i-h-k</td><td align=left>7</td><td align=center>-</td><td align=center>l</td></tr>
<tr><td align=left>a+11h+22l</td><td align=left>451</td><td align=center>m</td><td align=center>-</td></tr>
<tr><td align=left>h+l-7m+114</td><td align=left>31</td><td align=center>n</td><td align=center>p</td></tr>

Then n = number of month (3=March, 4=April), and p+1 = day of the month on which Easter falls