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This page answers some frequently asked questions about Easter Sunday dates.
Click here for a separate page showing Western Easter dates for all years in a format that allows you to see which prior and future years have the same Easter Sunday date.
Why do Easter Sunday dates only go to 4099?
The Gregorian calendar is accurate to 4099, but will require a one-date adjustment in 4100 or soon after. Until a decision is made about when this adjustment occurs, we cannot accurately use Gregorian dates after 4099. The Gregorian calendar is a vast improvement on the Julian calendar, which had fallen 10 dates out of alignment by 1582.
An extra day called a leap day is added into certain years so that on average, a year measures a full rotation of the earth around the sun. This ensures that seasons and astronomical events align with the calendar and its manthly and daily divisions. The Julian calendar has a leap day in every year evenly divisible by 4. The Gregorian calendar has a leap day in every year evenly divisible by 4, except century years which have a leap day if evenly divisible by 400. Some examples of century years are 1900, 2000, 2100, etc. So the difference is that the Julian calendar has a leap day in every century year, while the Gregorian has a leap day in only 1 of 4 century years.
How often do Western and Orthodox Easters occur on the same date?
The chart below shows 100 year-spans with the green bars showing how many of those 100 years had the same Western and Orthodox Easter Sunday dates. As you can see, these are becoming more rare over time, with the last one occurring on 24th April 2698. Other colour bars show the frequency of how many weeks Orthodox Easter follows Western Easter. You can see that over time, the Orthodox Easter is generally slipping later into the year, further from the March Equinox and further after the Western Easter.
How many different Easter Sunday dates are there?
There are 35 possible Western Easter Sunday dates from March 22 to April 25. As above, Orthodox Easter Sundays are gradually slipping forward in the Gregorian calendar. For years 1583 to 4099, there are 52 possible Orthodox Easter Sunday dates from April 1 to May 22. The chart below shows the distribution of these dates.
Why are Orthodox Easters often more than one full moon after the March Equinox?
Firstly, please remember that Easter dates are not directly related to either the Equinox or the astronomical full moon. See this page for more information and the correct definition of Easter Sunday date.
Orthodox Easter Sunday dates are based on the original calculation method in the Julian calendar, celebrated on the corresponding Gregorian calendar date. The Julian calendar no longer aligns with seasons or other astronomical events such as Equinox dates. The Gregorian calendar was introduced from 1582 to correct the increasing misalignment of the Julian calendar. It is accurate in its current form until at least 4099. Consequently, Orthodox Easter Sunday dates are generally slipping later into the year.
This chart compares the distribution of Orthodox Sunday dates over two different thousand-year periods to show the gradual shift of dates. Easter Sunday dates in late April and into May can occur more than one full moon after the March Equinox.
Of course, what's happening here is that the distribution of dates shifts by one date for each century year that the Julian calendar has a leap day, but the Gregorian calendar does not. For the chart below, these years are 1700, 1800, 1900, 2100, 2200, 2300, 2500 and 2600 (there are no April 8 Easter Sundays from 2583 until after 2600).
Do Easter dates repeat every 11 years?
No. But surprisingly, half of Easter Sunday dates do repeat after 11 years. This applies to both Western and Orthodox Easter Sunday dates from 1583 to 4099. The chart below shows the first repetition of dates for Western and Orthodox Easters.
There are even more 11-year repetitions than charted if we include Easter Sundays that repeat 5 years later then 6 years later. For example, Western Easters increase to 1,257 occurences.
There are also some spans of consecutive Easter Sunday dates that repeat, such as 100 successive Easter dates from 1948 to 2047 that are repeated 152 years later from 2100 to 2199!
Why can't Easter be on the Sunday after the full moon after the Equinox?
This is commonly quoted as the Easter Sunday date and it's wrong. But it might seem like a simpler way to set Easter date. An astronomical full moon occurs at an instant in time, which means that it occurs on 2 different dates depending on which time-zone you're in at that time instant. So a full moon might occur on a Saturday in some parts of the world with Easter Sunday on the next day. In other parts of the world, that same full moon could occur on the Sunday with Easter celebrated one week later, on the following Sunday. Clearly, this is not a good approach. Any simplification would be better based on a worldwide date, such as "the first Sunday in April".
Calendars are man-made inventions that merely give days labels such as day names, numbers and month names. They define a one year period corresponding to a single rotation of the earth around the sun. This cannot be done precisely with an even number of 24-hour days, so leap days are used to correct accumulating errors. Accuracy is important because it also defines four seasonal periods that align with recognisable agricultural phases. Not surprisingly, each new calendar is an improvement on the one it replaces, with the current Gregorian calendar accurate until at least 4099!
Of course, churches are somewhat resistant to change, particularly with principles in use over hundreds of years, so current methods are likely to remain. Orthodox Easter Sundays are based on the Julian calendar which is inaccurate by 13 dates at the time of writing and will continue to become more inaccurate over time. There is some consideration being given to a correction in the distant future, however, these proposals are always controversial and rarely succeed.