April 6, 2012

When is Easter Sunday?

With the approach of Easter I began thinking about the arcane method for calculating the date of Easter Sunday, which I think equates to the Sunday following the first full moon on or after March 21.  What does this lead to in terms of the distribution of dates on which Easter Sunday falls?

I plotted a histogram of the Easter Sunday dates for the years 325AD to 3000AD.  The chart is shown below and the interactive version (modern browser required) allows you to determine the date for a specific year.

Histogram of Easter Sunday dates 325AD - 3000AD

The dates range from 325AD as this was the year the First Council of Nicaea fixed the date of Easter Sunday.  The distribution of dates is not evenly distributed nor is it unimodal as one might expect.  The multiple peaks must follow from the distribution of dates of the full moon.

The visualization is shared using a Creative Commons license, and the source-code is available on GitHub.  I used the method given here for calculating the date of Easter Sunday.