Easter is actually celebrated on the first Sunday after the first full moon of spring. But in 2019 the rule of thumb is not correct, Easter this year falls on 21 April. Reason for it is age-old church formulas.

Next Wednesday is the beginning of spring this year. The next day is a full moon, the first in spring. According to the old rule of thumb for calculating the Easter date, the following Sunday (24 March) could be Easter – the Sunday after the first full moon in spring. But Easter 2019 is only on 21 April – after the second full moon of spring on 19 April. So the common rule of thumb is not true this year. Experts call this the “Easter paradox”. The reason for this is ancient church formulas that are not based on current data but follow fixed rules.

Easter is a mobile calendar festival. It commutes year after year between the 22nd of March as the earliest and the 25th of April as the latest date. For the determination of the valid date in each case already in the year, 325 on the council of Nicäa three criteria were fixed: the spring, the full moon and in each case the next Sunday. The beginning of spring was firmly defined as 21 March – independent of the current equinox of astronomy, according to which spring, for example, already begins on 20 March this year.

The Meton cycle determines the “Easter Border
The situation with the full moon is more complicated: the actual appearance in heaven was not and is not the basis for this. Rather data are used, which result from theoretical realizations over the always same movement of the moon around the earth. Astronomers speak of the 19-year Meton cycle, which was already known in antiquity and functions reliably.

It is named after the Greek astronomer Meton of Athens, who discovered it in the 5th century BC. Accordingly, the moon phases lie every 19 years again on the same calendar days – this knowledge made precise predictions possible over the centuries.

However, there may be deviations from the actual full moon of astronomy of one day – as in this year. The “church full moon” – also called “Easter border” – derived from the Meton cycle already takes place on March 20 this year. Thus it lies however still outside the spring, whose beginning was fixed on 21 March on the part of the church.

But by 2038 it should work again with the rule of thumb.
The first full moon of spring is, therefore, the full moon of 19 April – and the following Sunday (21 April) is accordingly Easter. By the way, the full moon on April 19 is the actual astronomical full moon. The full moon relevant for Easter from the metonic cycle is also the day before on April 18th – but this does not change anything for the next following Easter Sunday.

The earliest possible Easter date occurs when the church full moon falls on 21 March and this day is a Saturday. Then Easter is celebrated on 22 March. That is rare and happened last in the year 1818. The next time Easter will fall only 2285 again on this date.

The latest Easter date is April 25th. It occurs when the church full moon falls on 18 April and this day is also a Sunday – a special case of the Easter paradox. This was last the case in 1943 and will only be repeated in 2038. The Easter paradox as in this year was last in 1974 – and will also occur again in 2038.