This is a good page about Easter calcs and common Myths found on the internet.
http://www.gmarts.org/index.php?go=412
Easter is based upon Paschal Full Moon (PFM) dates and
NOT Astronomical Full Moons dates, and each (PFM) is the particular Ecclesiastical Full Moon (EFM) date after March 20,
Provided it does not coincide with Passover.
Yes, it is complicated!
At the bottom of this linked webpage is a list of differing calculation methods.