Easter

[Roger|Dyalog]

What's the date of this year's Easter?

      )copy dfns easter
C:\Program Files\Dyalog\Dyalog APL-64 17.1 Unicode\ws\dfns.dws 
                                           saved Tue Feb  5 20:01:46 2019
      easter 2020
20200412

The function was written by John Scholes, our late great Genius, Gentleman, and Mischievous Schoolboy (and Master). The documentation for the "easter" function begins as follows (http://dfns.dyalog.com/n_easter.htm):

In 325 A.D. the Council of Nicaea decreed that Easter was to fall upon the first Sunday after the first full moon on or after the Vernal Equinox.

Suggested by John R. Clark, this algorithm is taken from D.E.Knuth's "The Art of Computer Programming - Fundamental Algorithms" Vol 1, Ch 1.3.2 Ex14.

Knuth says: "The algorithm, due to the Neapolitan astronomer Aloysius Lilius and the German Jesuit mathematician Christopher Clavius in the late 16th century is used by most Western churches to determine the date of Easter Sunday for any year after 1582. The first systematic algorithm for calculating the date of Easter was the "canon paschalis" due to Victorius of Aquitania (457 A.D.). There are many indications that the sole important application of arithmetic in Europe during the Middle Ages was the calculation of Easter date, and so such algorithms are historically significant."

For more on the life and works of John, see:

• John M. Scholes https://en.wikipedia.org/wiki/John_M._Scholes
• John Scholes, 1948-2019 https://johnscholes.rip/
• Dfns (direct functions) https://en.wikipedia.org/wiki/Direct_function
• D-Function workspace https://dfns.dyalog.com/