Western Christian churches decide when Easter should occur according to the phases of the moon. It is always the first Sunday after the first full moon after the vernal, or spring, equinox.
Why is it, then, that the first full moon after March 21st fell on Saturday, April 7th, and the following Sunday was Palm Sunday and not Easter Sunday?
Though the TV weather broadcasts may have talked about a full moon last Saturday, to be absolutely astronomically correct the full moon occurred at 3:23 a.m. on Sunday. That three hours and 23 minutes pushed the fall of Easter on by a further week.
Calculations based on the moon and the equinox go back to the Synod of Whitby in 644 when, according to the Venerable Bede: "The Roman monks who had been sent by Gregory the Great to convert the `English' to Christianity found that the missionaries from Ireland observed Easter at a different time from that which had been appointed by the Roman church." The lunar system eliminated the possibility of errors - almost.
A paschal moon is the first full moon after the equinox but is not necessarily equal to an astronomical full moon, which is based on the complexities of lunar motion. Astronomers set the vernal equinox at the moment the sun reaches the celestial equator, an imaginary circle in the sky over the equator.
This can happen on March 21st or 22nd. The ecclesiastical equinox, however, always falls on March 21st.
In 1962 there was a mismatch between the ecclesiastical and the astronomical full moon, a difference that would have made the business of setting Easter a problem.
The astronomical equinox arrived in that year early on March 21st at 1.55 a.m. and was followed six hours later by the astronomical full moon. The ecclesiastical full moon, however, arrived on March 20th, the day before the fixed ecclesiastical equinox on March 21st.
If the astronomers dictated such matters, Easter would have occurred four days after the equinox, on Sunday, March 25th. As things happened, Easter did not arrive until Sunday, April 22nd.