Posts Tagged ‘Lent’

02/12/2013 – Ephemeris – Astronomical basis for Lent and Easter

February 12, 2013 Comments off

Ephemeris for Fat Tuesday, Tuesday, February 12th.  The sun will rise at 7:46.  It’ll be up for 10 hours and 20 minutes, setting at 6:07.   The moon, 2 days past new, will set at 9:17 this evening.

The western ecclesiastical season of Lent starts tomorrow.  It lasts 40 days excluding Sundays ending with the celebration of Easter on March 31st this year.  It is a movable feast in that it’s on a different date each year following the first full moon of spring.  It’s an attempt to follow the Jewish Passover, which starts on the 15th of the month of Nisan.  Being a lunar calendar the 15th the generally the night of the full moon.  And since the Last Supper was a Seder, the Christian church wanted to follow Passover as closely as possible using the Roman Solar based calendar.  The western churches eventually adopted the Gregorian calendar to keep in sync with the seasons.  The Eastern churches did not.  Their Easter this year is on May 5th.

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


The first day of spring is set by the churches as March 21st.  In Michigan’s time zone spring now it always starts on March 20th, and will occasionally start on the 19th until 2103 after the Gregorian correction takes place in 2100.  The Julian Calendar, instituted by Julius Caesar, was the official calendar for all the Christian world until 1582.  The problem with the Julian Calendar is that the leap year every 4 years overcompensated for the seasonal or tropical year.  The average year of 365 1/4 days turned out to be 11 minutes 14 seconds too long.  By 1582 the actual vernal equinox or first day of spring had slipped back to March 10th.

Astronomers under Pope Gregory XIII proposed a change that was adopted by the Roman Catholic world.  First remove 10 days from the calendar between October 4 and 15 1582.  Then modify the leap year formula to keep the leap year on years divided by 4,  except century years.  Century years were ordinary 365 day years unless divisible by 400.  This 1900 was an ordinary year, but 2000 was a leap year.   The western protestant countries came around eventually.  However the eastern churches still use the Julian calendar to set their feasts, thus Eastern Orthodox rarely coincides with western churches.

Note that the dating for Excel spreadsheets, starts the day count on January 1, 1900, except they thought that 1900 was a leap year.