But the solar and lunar years diverge by 11 days every year. In the 2nd century a Roman called Hippolytus devised an eight-year cycle.
Scores of formulae have been devised to try to reconcile the two as a method of marking time. A century on, 84-year tables were introduced, which were still in use in parts of the British Isles as late as 931.
Calculating one against another is seriously complicated.
There have been various attempts to reconcile this, including the famous saltus lunae (the moon's jump) whereby one of the 30-day months in the lunar cycle gets arbitrarily shortened to 29 days.
So Christians wanted to have their feast day around the same time as the Jewish festival which was fixed by the first full moon following the vernal equinox – the spring day when night and day are exactly the same length.