Easter (Computus)
What this is?
A friendly explainer for computus — the old craft of calculating Easter by calendar rules rather than sky‑watching. Type a year to get the date of Easter Sunday in the Western (Gregorian) tradition and the Orthodox (Julian) tradition. Everything runs locally in JavaScript.
We implement two independent formulas for Western Easter and check they agree. For Orthodox Easter, we compute in the Julian calendar then convert to the modern calendar.
Western Easter always falls on a Sunday between March 22 and April 25. Orthodox Easter is usually later.
Use the range to list many years at once.
Answer
Reason why
Western (Gregorian) Easter is the Sunday after the “Paschal full moon” which is defined by fixed tables. Algorithms (like Gauss/Meeus/Butcher) reproduce those tables arithmetically.
Butcher (Gregorian):
a=y mod 19; b=⌊y/100⌋; c=y mod 100; d=⌊b/4⌋; e=b mod 4; f=⌊(b+8)/25⌋; g=⌊(b−f+1)/3⌋;
h=(19a+b−d−g+15) mod 30; i=⌊c/4⌋; k=c mod 4; l=(32+2e+2i−h−k) mod 7;
m=⌊(a+11h+22l)/451⌋;
month=⌊(h+l−7m+114)/31⌋; day=((h+l−7m+114) mod 31)+1.
Meeus (Gregorian, with corrections):
a=y mod 19; b=y mod 4; c=y mod 7; k=⌊y/100⌋; p=⌊(13+8k)/25⌋; q=⌊k/4⌋;
M=(15−p+k−q) mod 30; N=(4+k−q) mod 7; d=(19a+M) mod 30; e=(2b+4c+6d+N) mod 7;
day=22+d+e; adjust: if d=29 & e=6 → Apr 19; if d=28 & e=6 & a>10 → Apr 18.
Orthodox (Julian):
In the Julian calendar compute a,b,c,d,e as customary; convert the resulting Julian date to the modern calendar.