The (Catholic) Easter date was fixed by the Council of Nicea in 325 AD. The rule is as follows: Easter is the first Sunday after the first full moon of spring.
Taking into account modern astronomical calculations, in 2019,
- spring begins on March 20 at 22:58 (universal time);
- the first full moon that follows takes place on March 21 at 2:43 (universal time).
Therefore, Easter would fall on the first Sunday following, i.e. on March 24, 2019.
However, the Easter date in 2019 is April 21.
The reason why the two dates do not coincide is that the authorities of the Catholic Church do not want to depend on astronomers. For this reason, they have introduced the following simplifications into their calculations:
1) Early spring (in the northern hemisphere):
In astronomy, it coincides with the vernal equinox, the moment when the terrestrial equatorial plane crosses the centre of the Sun and the Earth’s North Pole emerges from its winter night. The equinox date fluctuates in the calendar, in part because the Earth’s period of revolution around the Sun is 365.2422 days instead of 365-366 calendar days (other factors influence the calculation of the spring date, but their influence is too small to be mentioned here). As a result, early spring is between March 19 and 21. The church has decided to set the date from spring to March 21.
2) The dates of the full moon:
It is not easy to determine the exact orbit of the Moon and the corresponding lunar phases. In the 5th century BC, the Greek Meton of Athens found an elegant solution to simplify this problem. After a number of observations he concluded that the Sun and Moon are in the same relative position with respect to the Earth after 235 lunar months, or 19 years. This cycle has become the basis for the calculation of the full moon in the Catholic Church. In 1582, this rule was adapted to the new Gregorian calendar, our current calendar.
The rule for calculating the Easter date therefore remains valid, but we must also take into account the two simplifications mentioned above. In other words,
- spring begins on March 21;
- the first full moon after March 21, 2019 (according to Meton calculations) takes place on April 20, 2019.
Therefore, Easter falls on the Sunday following this full moon, i.e. on April 21, 2019.
In addition to 2019, there are other years when the “ecclesiastical” Easter date does not correspond to the “astronomical” Easter date.
In the thousand years following 1582, the year of the Gregorian calendar reform, there were 84 such occurrences. The previous date was in 1981, the next will be in 2038.
The German mathematician Carl Friedrich Gauss published in 1800 a mathematical algorithm to calculate the Easter date for a given year. He uses different terms of the comput such as the golden number or the epacte. This algorithm is still used today, sometimes in a simplified version.