Astronomy

Astronomical Determination of Passover Date



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Easter is an interesting holiday for astronomical reasons. The Christian Church didn't pick the date out of thin air. It is based on an ancient lunar calendar using the Jewish calculation for Passover, the 14th day of Nisan, the month Spring begins. Jewish months start with a new moon. 14 days later is the full moon. The Nisan full moon is called the "Pascal Full Moon", and begins the Passover celebration. For Christians, both Passover and Easter are associated with Jesus' momentous visit to Jerusalem. Hence the Easter rule became simply, "The first Sunday after Passover". More correctly, using the astronomical definition of Spring in the northern hemisphere, this became "The first Sunday after the first full moon after the vernal equinox."

Note the word "after". Passover can start on any day of the week, whenever the Pascal full moon occurs. Easter, always a Sunday, usually follows Passover by a few days. But, occasionally the full moon falls on Sunday, so Passover and Easter would be on the same date. Another part of the ancient Christian rule was "but not with the Jews", that is not on the same day as the beginning of Passover. Perhaps this was so people could celebrate both holidays. It's certainly handy for mixed-marriages, solving the age-old problem of which in-laws to have holiday dinner with. The simplest way to avoid the coincidence was for Christians to choose the Sunday after the full moon, never on the full moon.

There's another quirk. The "vernal equinox" doesn't always occur on March 21st, sometimes it's on the 20th. So the rule uses an "ecclesiastical" definition of the vernal equinox, rather than the astronomical one, always choosing the 21st. This means the earliest Easter can be is the 22nd of March. This happened back in 1818 and will happen again in 2285. Amazingly enough, this year's Easter is on the second earliest possible date, March 23rd. That won't happen again for 152 years, until 2160.

So when is Passover this year? April 20th. But that's almost a month after Easter. How did that happen? Well 2008 is also a leap year in the Jewish calendar. Because the calendar is moon-based (lunar) Jewish leap years are a big deal. A whole extra month is added this year before Nisan can start. That moves this year's Pascal full moon into April. It doesn't affect the Easter calculation, though. Christians follow Julius Caesar's solar calendar as amended by Pope Gregory, and only have a single February leap day to deal with. One final complexity, Jewish days begin at sundown, so both April 20th and Passover begin on the evening of April 19th.

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