Astronomy
Easter
Calendar

Why does Easter's date change?

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December 21, 2015 7:34AM

Easter's date changes because it's based on Jewish Passover, which is based on a lunar calendar.

Easter is the first Sunday after the first full moon after the vernal equinox. Those things don't stay in sync, so the date of Easter moves around.

If that wasn't hard enough, there are all sorts of questions about determining the full moon. For the full story, see the US Naval Observatory Web Site.