Holidays related to Easter: Difference between revisions
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From the year 325 CE on, [[wp:Easter Sunday|Easter Sunday]] has been defined as the first Sunday ''after'' the first full moon ''on or after'' the day of the March equinox. However, the actual astronomical values for the moments of the full moon and equinox are not used. Instead, approximations are used, the first one being that the equinox is assumed to fall on March 21st every year. The tracking of the moon phases is similarly done with relatively straightforward arithmetic (compared to the sort required for astronomical accuracy) which amounts to maintaining a lunisolar calendar in parallel to our standard purely-solar one.
When Pope Gregory reformed the Catholic calendar in 1582 CE, the drifting of Easter with respect to the seasons was the driving motivation, and the rules for determining it were altered to correct that drift. Catholic nations adopted both the new calendar and the new Easter rules right away, while Western Protestant nations adopted them more gradually over the next 350 years or so. Eventually, even nations dominated by the Eastern Orthodox church adopted a similar calendar reform (the Revised Julian calendar), so pretty much the whole world agrees on what day it is for civil purposes. But the Eastern churches never adopted the corresponding Easter rule changes; they still use the original Julian calendar and rules to determine the date of what is known in the West as "Orthodox Easter". Therefore, your output should indicate which computus was used to calculate the dates and, at least for historical dates where the calendar can't be assumed or is location-dependent, which calendar those dates are given in.
You may find algorithms on the [[wp:Computus|Computus]] page. Some of the results:
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