Easter is considered the most important date on the Christian calendar — sorry Christmas — but unlike its counterpart, the holiday changes dates every year. Whether these communities follow the Gregorian calendar or still use the Julian calendar, the holiday still adheres to direct astronomical observations.
The death of Jesus is believed to have occurred around Jewish Passover, which is traditionally held on first full moon following the vernal (spring) equinox. However, Easter’s history has seen several councils and attempts at reform, not to mention different calendars, all in an attempt to set a consistent date. Despite many realignments, the moon’s influence hasn’t subsided in dictating when the holiday is celebrated.
In 325 C.E., Roman Emperor Constantine organized the First Council of Nicaea, where Church leaders sparred over official canon, its understanding of the trinity, and when Easter should be celebrated. Consensus was finally reached that Easter would be held after the first full moon occurring on or after the vernal equinox. But as Constantine’s empire expanded, the shifting date presented new challenges in new communities. Since the full moon can vary in each time zone, the Church would later use the 14th day of the lunar month — the paschal full moon — and host Easter on the following Sunday.
This gets even more complicated after the Great Schism in 1054, when the Eastern Orthodox Church and the Catholic Church went their separate ways, and in 1582, when the Gregorian calendar was adopted as a shorter alternative to the Julian calendar. In the Julian calendar, still used in some Orthodox churches, the equinox date usually falls on earlier time slots in March.
Essentially, Easter’s date depends on how early or late a full Moon occurs after the spring equinox. Its astronomical significance is one that scientists continue to point out. Since Easter can never occur before a full Moon, it’s impossible for Easter to coincide with a new Moon, which is separated from a full Moon by 14 days. Thus, if a new Moon can never occur on Easter, as Neil deGrasse Tyson pointed out on Sunday, then an eclipse can never occur on Easter, either.
Attempts at standardizing Easter’s date haven’t subsided. The World Council of Churches proposed a new Easter calculation to replace the calendar-based observations with direct astronomical observation. This could have united the adherents of the Gregorian and Julian calendars, and while the reform was adopted in 2001, it was never implemented. Each calendar continues to deal with the lunar phases in its own way.