April 2, 2015

When Is Easter Sunday?

Easter Sunday is the Sunday following the first full moon after March 21. A few years ago I published this histogram of Easter Sunday dates. A friend then asked whether it was possible to visualize Easter Sunday dates from one year to the next. I've finally gotten around to producing such a visualization.

The time-series chart below shows 1000 years of Easter Sunday dates. You can find an interactive version here (modern browser required). Use the mini-chart to focus the top chart on a specific range of years.

This chart allows us to see repeating patterns in the sequence of Easter Sunday dates. The patterns don't always repeat exactly but the human visual system is good at spotting similar patterns. For example, I was quickly able to match the sequence of Easter Sunday dates starting in 1943 and 2038 (April 25, 21, 17, 13, ...). The sequences are identical for almost 30 years. If you spot any others please let us know.

Implementing this Easter Sundays chart was really just an excuse for me to experiment with the NVD3 library, a collection of reusable charts built atop the excellent d3.js library. The beauty of NVD3 is that it allows you to create fairly complex, interactive charts with only a few lines of code. The code I used is available here.

Happy Easter!