Re: But hang on...
"So the christian easter always corresponds to the Jewish calendar for Passover, which is defined as being somewhere close to the Spring equinox."
The calculation of the date of Easter has a very complicated history. Apparently it was detached from the Jewish calendar after the 3rd century due to a perceived unreliability.
The date can still differ between the Orthodox Church and the Catholic traditions. One depends on the Gregorian calendar and the other the Julian calendar. Added to which is whether the Vernal Equinox is taken as a fixed date of 21st March - or by astronomical observations.
Here is an apparently comprehensive explanation.
The calculation of Easter was apparently so important that the Catholic Church relaxed some of its proscriptions on scientific research. The book "The Calendar" by David Ewing Duncan attempts to cover some of the politics.
Biting the hand that feeds IT
