Home > Calendar, Ephemeris Program > 03/25/2016 – Ephemeris – Easter, the reason for our calendar

03/25/2016 – Ephemeris – Easter, the reason for our calendar

March 25, 2016

Ephemeris for Good Friday, Friday, March 25th.  The Sun will rise at 7:35.  It’ll be up for 12 hours and 26 minutes, setting at 8:02.   The Moon, 2 days past full, will rise at 10:17 this evening.

The so-called movable feasts of the church calendar are based on the date Easter falls on.  They span from Ash Wednesday to Pentecost.  And Easter is determined by astronomical events.  In 1582 the fact that the actual vernal equinox had fallen 10 days behind the Julian calendar then in use which was decreed by Julius Caesar in 45 BC.  Pope Gregory XIII in the 1580’s resolved to fix the situation and commissioned some astronomers to work on the problem.  The solution was to fix the 10 day problem by eliminating the days October 5th through 14th of the October 1582 calendar and modifying the leap year rule to keep February 29th in calendars whose years were evenly divisible by 4, except those century years not also divisible by 400.  Thus the year 2000 was a leap year, but 1900 and 2100 was and will not be.

Times are for the Traverse City/Interlochen area of Michigan. They may be different for your location.


The Gregorian Calendar in essence decoupled Easter from Passover by keeping the formula first Sunday after the first full moon after March 21st, as a marker for the vernal equinox, and keeping March 21st on or near the vernal equinox.  Passover this year starts sunset on April 22nd,  while Orthodox Easter will occur on May 1st.  The reason is the relationship between the Jewish lunar calendar and the Julian Calendar I mentioned in yesterday’s post. The difference between the Julian and Gregorian Calendars is now 13 days.

The error in the Gregorian calendar is at most 1 day in 3,300 years, in relation to the seasonal year.  But the Gregorian Calendar makes calculating the date of Easter more complicated.  It introduces something called Epact to the list of chronological cycles in an almanac.  The quantity called Epact is the age of the moon on January 1st, and still has a relationship with the Metonic Cycle and the Golden Number which I discussed yesterday.  This year the value is 21.

As I’ve admitted before, the first paragraph of these posts are generated by a computer program.  Part of that program is a list of holidays, and those designated as movable feasts use the date of Easter as a starting point.  I use the 10 step method from Astronomical Algorithms by Jean Meeus.  Easter for the Julian Calendar is a simpler 6 step method.


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