The 14 day of Nisan is also the historic day of the Jewish Passover, so it was understood that Pascha must follow the Jewish Passover, which it always does in the Eastern Orthodox Church. However, the calculations of the Gregorian Calendar make it possible for the West to celebrate Pascha before the Jewish Passover which violates the spirit of the decrees of the 1st Ecumenical Council.
For purposes of computing Easter, the 14th of the Christian
the "Jewish Passover". Whatever our Jewish neighbors do is of no mathematical significance. The computation has only one Passover, that computed by Christians, not two, one computed by Christians and the other by the Rabbinic calendar.
This year, 2014, both East and West will celebrate Pascha on April 20. The Orthodox Pascha always comes after the Jewish Passover in accordance with the spirit of the decree of the 1st Ecumenical Council. However, in 2005 the West celebrated Pascha on March 27, but the Jews did not celebrate Passover until April 24.
April 24 was Rabbinic 15 Nisan 5765, the first Day of Unleavened Bread. Our pocket calendars call 15 Nisan "Passover", but in the Easter computation "Passover" means 14 Nisan, not 15 Nisan. Rabbinic 14 Nisan 5765 was April 23, 2005. But as already stated, the only 14 Nisan that enters into the Easter computation is the Christian one, not the one from the Rabbinic calendar. In 2005, Gregorian 14 Nisan was March 25th. Since this was after the equinox, it complied with the implicit (though not explicit) Nicene rule of the equinox. Since it was computed independently of any Jewish calendars, it complied with the explicit Nicene rule of independence.
In 2015, the West will celebrate Pascha on April 5, the same date that the Jews will celebrate Passover, something that is strictly forbidden by the Canon 1 of the Council of Antioch in 431.
Again you seem to be using "Passover" in its modern sense of 15 Nisan. In the context of the Easter computation, this is anachronistic. Also, Rabbinic 15 Nisan 5775 will be Saturday, April 4th, not Sunday, April 5th. April 5th will be within the days of Unleavened Bread, of course. If you mean to say that Easter must always fall after the Rabbinic days of Unleavened Bread are over, then you are mistaken. Even your computation puts Easter sometimes within the Rabbinic calendar's days of Unleavened Bread, as it will this year, 2014, when the scriptural seven days of Unleavened Bread in the Rabbinic calendar will fall on 15-21 April in the civil calendar.
Canon 1 of Antioch is obsolete: It refers to a situation that no longer exists, unless there are Karaite groups that sometimes define their Nisan in a way that allows their Passover to fall before the equinox. That is, the canon forbids setting Easter to the 3rd Sunday in Jewish Nisan when Jewish Nisan falls too early.
(Apostolic Canon 7 covers the same ground, more explicitly.) Antioch Canon 1 does not forbid Easter fortuitously to coincide with the third Sunday in Jewish Nisan if Jewish Nisan is computed correctly, nor does it require us to celebrate after the Jewish festival if Jewish Nisan is set too late
from the Christian point of view, as it sometimes is in our time. Our calculations are independent
of the Jewish ones: whatever date our Jewish friends calculate for their festival, we are required by the Nicene decision to take no account of it whatsoever in calculating our festival. Insisting that Easter must fall after Rabbinic 15 Nisan adds a spurious rule to the traditional calculation and is a direct violation of the Nicene decision.
The Gregorian Calendar does not use the actual Spring Equinox, but follows tables written in the 16the century. that at that time were the best scientific guesses on when the Spring Equinox would take place, but now, the date of the Spring Equinox set by the Gregorian Calendar is incorrect. My point is that if we Orthodox are going to make a change on our calculation of the date of Pascha, we should make it according to scientific standards, not just to celebrate Pascha with the West.
The secular drift in the Gregorian calendar is very slow. Overlaid on the secular drift, however, is an oscillation
of the mean equinox about its average position in the Gregorian calendar, as well as an oscillation of the true equinox about its mean. The discrepancy you refer to is mainly a result of these oscillations, not of the much smaller secular drift, as you seem to want to imply. For the next two thousand years or so the average Spring equinox year of around 365.2424 days will remain close to the Gregorian tropical year of 365.2425 days, and so the mean equinox will remain close to its formal Gregorian date of March 21.
Put another way, the Gregorian calendar almost always agrees with the astronomical method. The only year in the next few years in which the astronomical method gives a different date is 2019, when the astronomical method would set Easter to March 24th, while the Gregorian lunar calendar sets it to April 21st. If your party merely converted to the Gregorian calendar, you would be in far better agreement with the astronomical method than you are now.