The Paschal full moon of 2021 rises tonight to make way for Easter

Palm Sunday, March 28 brings us the first full moon of the new spring season: the Paschal full moon. The official moment that the moon will turn full is 2:48 p.m. EDT (1848 GMT).

Traditional names for the full moons of the year are found in some publications such as The Farmers' Almanac. We also published the full list of full moon names here on Space.com earlier this year. The origins of these names date back a few hundred years to Native American tribes, though they may also have evolved from old England or, as astronomy author Guy Ottewell, suggests, "writer's fancy." 

Traditionally, the March full moon is known as "Worm Moon," supposedly because when the ground softens the earthworm casts reappear, inviting the return of the robins. Other lunar monikers included "Crow Moon" (when the cawing of crows signals the end of winter), "Crust Moon" (because the snow cover becomes crusted from thawing by day and freezing at night) and "Sap Moon" (marking the time of tapping maple trees).

The first full moon of spring is also designated as the Paschal Full Moon or the Paschal Term — 14 or 15 Nisan on the Jewish Calendar, which is also marks Pesach, or Passover. Easter is observed on the Sunday after the Paschal Full Moon. If the Paschal Moon occurs on a Sunday, Easter is the following Sunday. 

March full moon 2021: Catch the big 'Worm Moon' (and 1st 'supermoon' of the year) on Sunday

So according to the current ecclesiastical rules Easter is to be celebrated on Sunday, on April 4. Interestingly, 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, by these standards, Easter is coming a little earlier than usual in the calendar this year. 

 Dating discontinuities  

Interestingly however, these rules also state that the vernal equinox is fixed on March 21, even though at European longitudes from the years 2008 through 2101 it occurs no later than March 20. 

Hence, there can sometimes be discrepancies between the ecclesiastical and astronomical versions for dating Easter. Adding additional confusion is that there is also an "ecclesiastical" full moon, determined from ecclesiastical tables and whose date does not necessarily coincide with the "astronomical" full moon, which is based solely on astronomical calculations. 

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

In the Dutch journal Hemel en Dampkring, Vol. 71, No. 4 (April 1973), Steven Verhezen investigated the years 1583 to 2582 and found 78 cases where the date of astronomical Easter differs from that of ecclesiastical Easter: an average of about once every 12.82 years. 

In 1981, for example, the full moon occurred on Palm Sunday, April 19, so Easter should have occurred on the following Sunday, April 26. But based on the "ecclesiastical" full moon (which occurred a day earlier) Easter occurred on the same day of the full moon, April 19.

And in the year 2038, the equinox falls on March 20 with a full moon the next day, so astronomically speaking, Easter should fall on March 28 of that year. However, as mandated by the rules of the Church, Easter in 2038 will be observed as late as it can possibly come, on April 25! 

Since the beginning of the 20th century, a proposal to change Easter to a fixed holiday rather than a movable one has been widely circulated, and in 1963 the Second Vatican Council agreed, but only provided that a consensus can be reached among Christian churches. The second Sunday in April has been suggested as the most likely date.

To this day, we still are awaiting that consensus.

 The other side of the lunar coin 

The Paschal full moon of spring is also the mirror-image of the full Harvest Moon of autumn. What sets the Harvest Moon apart from the others is that instead of rising at its normal average of 50 minutes later each day, it seems to rise at nearly the same time for several nights. 

In direct contrast to the Harvest Moon, the Paschal Full Moon appears to rise considerably later each night. Below we've provided some examples for ten North American cities. 

The local moonrise times for March 27, 28 and 29 are provided, the middle date being that of the Paschal full moon. 

 Although normally the moon rises about 50 minutes later each night, over this three-night interval for our relatively small sampling, we can see that the rising of the moon comes, on the average, just over 75 minutes later each night. A quick study of the table shows that the night-to-night difference is greatest for the more northerly locations (Edmonton, located at latitude 53.6 degrees north latitude, sees moonrise come an average of 90 minutes later). Meanwhile, the difference is less at southerly locations (at Miami, Florida located at latitude 26 degrees north, the average difference is about 65 minutes). 

The reason for this seasonal circumstance is that the moon appears to move along the ecliptic and at this time of year when rising, the ecliptic makes its largest angle with respect to the horizon for those living in the Northern Hemisphere. 

In contrast, for those living in the Southern Hemisphere, the ecliptic at this time of year appears to stand at a more oblique angle to the eastern horizon. As such, the difference for the time of moonrise is noticeably less than the average of 50 minutes per night. At Christchurch, New Zealand for instance, the night-to-night difference amounts to just 27 minutes.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Follow us on Twitter @Spacedotcom and on Facebook.