Why Do Orthodox Christians Sometimes Celebrate Easter on a Different Day?

This has long been confusing to people.  Some years the Orthodox celebrate Easter (which we call Pascha) on the same day as the west, and some years it is later.  It is never earlier.  Why the difference?

The first council of Nicea established Easter as the first Sunday after the first full moon that falls on or after the Vernal Equinox. But all Orthodox Christians use the old calendar to calculate the date of the Equinox.  (See my post on the Old Calendar.) This means that the Old Calendar and New Calendar Orthodox are united for the Paschal season (from the start of Great Lent through Pentecost), even though they are 13 days off for the liturgical calendar and the rest of the great feasts.

During 2010 and 2011 Easter and Pascha fell on the same day.  This year and for the next few years the dates are different.  Here are the dates:

Year	Western/Gregorian	Eastern/Julian
2012	April 8	April 15
2013	March 31	May 5
2014	April 20	April 20
2015	April 5	April 12