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What does Easter have to do with the moon anyway?

Traditionally, Easter is observed on the Sunday after the first full moon of spring in the northern hemisphere.

By Joe RaoSPACE.com / April 6, 2012

Skywatcher Maxim Senin caught the full moon during the moon's conjunction with Mars in March, in Long Beach, Calif.

Maxim Senin

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Friday (April 6) brings us the first full moon of the new spring season. 

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The official moment that the moon turns full is 19:19 UT, or 3:19 p.m. EDT.

Traditionally, the April full moon is known as "the Pink Moon," supposedly as a tribute to the grass pink or wild ground phlox, considered one of the earliest widespread flowers of the spring. Other monikers include the Full Sprouting Grass Moon, the Egg Moon and, among coastal Native American tribes, the Full Fish Moon, for when the shad came upstream to spawn.

(Traditional names for the full moons of the year are found in some publications, such as the Farmers' Almanac. We also published the complete list of full moon names here on SPACE.com. The origins of these names have been traced back to Native America, though they may also have evolved from old England or, as Guy Ottewell, editor of the annual publication Astronomical Calendar, suggests, "writer's fancy.")

The first full moon of spring is usually designated as the Paschal Full Moon or the Paschal Term.  Traditionally, Easter is observed on the Sunday after the Paschal Full Moon. If the Paschal Moon occurs on a Sunday, Easter is the following Sunday. [Photos: Full Moon Captivates Skywatchers in February 2012]

Following these rules, we find that the date of Easter can fall as early as March 22 and as late as April 25. Pope Gregory XIII decreed this in 1582 as part of the Gregorian calendar. So according to the current ecclesiastical rules, Easter Sunday in 2012 is to be celebrated April 8.

Interestingly, these rules also state that the vernal equinox is fixed on March 21, despite the fact that from the years 2008 through 2101, at European longitudes it actually will occur no later than March 20. 

Adding additional confusion is that there is also an "ecclesiastical" full moon, determined from ecclesiastical tables, whose date does not necessarily coincide with the "astronomical" full moon, which is based solely on astronomical calculations. In 1981, for example, the full moon occurred on Sunday, April 19, so Easter should have occurred on the following Sunday, April 26. But based on the ecclesiastical full moon, it occurred on the same day of the astronomical full moon, April 19!

Hence, there can sometimes be discrepancies between the ecclesiastical and astronomical versions for dating Easter. In the year 2038, for instance, the equinox will fall on March 20, with a full moon the next day, so astronomically speaking, Easter should fall on March 28 of that year. In reality, however, as mandated by the rules of the church, Easter 2038 will be observed as late as it can possibly come, on April 25.

So in practice, the date of Easter is determined not from astronomical computations but rather from other formulae such Golden Numbers.

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