Easter and the Full Moon

In this festive season, the moon is always in the full phase

Since the dawn of Christendom, the date of Easter, the day on which the resurrection of Christ is celebrated, has been fundamental to the structuring of the entire Christian liturgical calendar.

But the unambiguous determination of the day of Easter so that it could be celebrated on the same calendar day throughout Christendom, irrespective of its geographic location, was a problem that was not normalized until the first ecumenical council held at Nicaea in 325 AD

At that council, convened by the Roman Emperor Constantine, it was determined that Easter Day should be celebrated on the first Sunday after the first Full Moon occurring on or shortly after the spring equinox in the Northern Hemisphere (about March 21). This is the rule since then to determine the day of Easter and, therefore, the Moon will always be in full phase.

But the determination of the equinox, through the calendar then followed, did not guarantee a “coincidence” between the prediction and the reality, due to the imperfection contained in it. The Julian calendar (so named in honor of Julius Caesar) in force at the time of the Council of Nicaea accumulated an inaccuracy of about 11 minutes and 14 seconds in excess each year.

By 1582, the inaccuracy of the Julian calendar resulted in the spring equinox occurring on the 11th instead of the 21st of March as would be expected. This delay introduced errors into the Christian religious calendar and, in practice, Easter was celebrated on different days in different parts of the hemisphere. Something had to be done to reset the official calendar.

Pope Gregory XIII (1502 – 1585) created a commission led by the Jesuit mathematician and astronomer Christoph Clavius ​​(1537-1612) to solve the problem.

in your package Inter Gravity, Pope Gregory XIII consecrates the mathematical work and institutionalizes the calendar that we still follow in the West and that bears his name (Gregorian calendar).

It results from a very satisfactory set of rules for regular adjustments in so-called leap years, which ensures an acceptable compromise in predicting the relative translational motions of the Earth around the Sun and the Moon around the Earth.

It should be added, however, that the determination of the day of the Full Moon, for the determination of Easter Sunday, does not make use of astronomical tables, but rather of what is defined in the Ecclesiastical Tables which, despite not rigorously including the complex movement of the orbit of the Moon, are sufficient to permit a regular and uniform determination of the same moment throughout western Christendom, irrespective of its latitude and longitude.


Author António Piedade is a biochemist and science communicator