Why does Easter change its date every year, sometimes not on the same day?
Turn the same year into two calendars and see how the rules select Sunday.
When planning spring activities, you discover that Easter falls on a different date each year, and different traditions sometimes fall on the same day. This is not necessarily because someone calculated it wrong, but because the calendar and the calculation rules used are different.
Guess first
If two traditions choose different dates for Easter, does one of them must have miscalculated the full moon in the sky?
Show answer and explanation
Not necessarily, the calendar rules used are different. — The prescribed church calendar is used here and cannot simply be equated with the real full moon moment.
Then connect experience and science
Select the year and calendar rules first
The date of each year is not fixed. Both calendars have been replaced with Gregorian calendar date display to avoid directly subtracting different calendar labels.
Enter year + Gregorian or Julian calendar algorithm
This is not "find the first Sunday after the real full moon"
The Easter algorithm uses the full moon calculated by the church calendar and the fixed vernal equinox, and cannot be directly replaced by the full moon moment in modern astronomy.
Church Calendar Rules ≠ Actual Full Moon Observation
Check whether the same day is selected by both rules
The Gregorian calendar algorithm and the Julian calendar algorithm may result in different Sundays, or they may happen to be the same. The conversion between the original calendar and the Gregorian calendar is always indicated on the right.
Calculate the calendar date first, then use JD to convert to the Gregorian calendar
Calendar arithmetic must use integer arithmetic
Small errors in remainders and rounding can put the date off by a day. Test results should be on a Sunday and between March 22 and April 25 of the respective calendar.
Integer Quotient, Remainder and Day of the Week Test
First select the Gregorian or Julian calendar rules, then do the integer quotient and remainder operations, and finally get the month and day and convert the date. This is the Church's Easter calendar, and the observed full moon cannot be substituted for the intermediate rules.
give it a try
Compare 2024 and look at 2025. The orange calendar is the current choice; the "big dates" of both calendars have been changed to the Gregorian calendar.
No need to memorize symbols
- JD
- Used to change the same day from one calendar label to another
- mod
- Remainder of integer division
- Gregorian/Julian calendar
- Two calendars, not two time zones
Algorithm, accuracy and scope of application
Based on Chapter 8 of the second edition, printed pages 67–70, including Easter calculations for the Gregorian and Julian calendars. The program is limited to 1600-2399, and the core algorithm can test the range 1583-4099. There is no Gaussian special year exception patch to the Gregorian calendar rules; both rounding and remainder are integer operations. The Julian calendar result is first converted to JD and then the Gregorian calendar is displayed, and 13 days are not always added. Examples from the original book 1818, 1954, 1991–1993, 2000, etc. are used for verification. Not simulating the sect's full historical policies or precise moon phases.
my understanding check
Which calendar does the big date display in?
Gregorian calendar — Before you can directly compare two dates, you must first unify them to the Gregorian calendar.
Is the full moon in the Easter rules the real full moon calculated by the precise ephemeris?
no — The book clearly states that it is a full moon in the church calendar.
What day of the week should the calculation result fall on?
Sunday — Both algorithms in this chapter calculate Easter Sunday.
First confirm which set of calendar rules is used before comparing dates; the moon in the calendar does not have to be the same as the actual moon in the sky.
Jean Meeus · Astronomical Algorithms, 2nd ed. · No. 8 chapter