03/24/2016 – Ephemeris – Why is this Sunday Easter?

Ephemeris for Thursday, March 24th.  The Sun will rise at 7:37.  It’ll be up for 12 hours and 23 minutes, setting at 8:01.   The Moon, 1 day past full, will rise at 9:19 this evening.

This Sunday is Easter, only 5 days later than the earliest Easter can ever be.  Yesterday’s full moon or the tabular date for it is called the Paschal Full Moon, an attempt for the Christian Church to match the solar Roman calendar to the Jewish lunar calendar in regards to the date of Passover.  It doesn’t always work, especially when Easter turns out to be early as it is this year.  The simple formula for western churches is the first Sunday after the first full moon after the vernal equinox which is defined as March 21st, no matter the date spring actually started, which was the 20th, this year.  All this started to be counted using the Julian Calendar, which is 11 minutes longer than the seasonal or tropical year.  We’ll see how that was corrected for tomorrow.

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

Addendum

The Jewish calendar does not have a relationship with the Gregorian Calendar, so Passover will drift later and later into spring over the years.  The Jewish calendar does have a relation to the Julian Calendar in that 19 years equals 235 lunar months.  This was probably discovered by the Babylonians but was popularized by the Athenian Menton in the 5th century BCE.  It’s a way to relate the lunar calendar to the solar or seasonal calendar.  We call it the Metonic cycle.

In  a lunar calendar the months alternate between 29 and 30 days because the lunar month is 29.53 days.  Also a 365.25 day year is 12.37 lunar months.  The solution for all this is quite complex, with 12 common or 12 month years and 7 13 month great years to fit the 19 year cycle.   It also means that the phases of the moon repeat on or near the same date at 19 year intervals.  If you see a quantity called the Golden Number in almanacs, which happens to be 3 this year, that’s where we are (1-19) in the Metonic cycle.  The Gregorian Calendar breaks this relationship.  We’ll see how tomorrow.

03/29/2013 – Ephemeris – How we know this Sunday is Easter

Ephemeris for Good Friday, Friday, March 29th.  The sun will rise at 7:28.  It’ll be up for 12 hours and 38 minutes, setting at 8:06.   The moon, 2 days past full, will rise at 11:14 this evening.

Wednesday was the day of the Paschal full moon, the full moon after the first day of spring, which is defined by Christian churches as March 21st.  For the date of Easter the actual full moon isn’t used, but a rather complicated formula is used to find the new moon.  There’s a complicated correction to then find the date of the full moon, which 2 days ago.  This year that formula works out correctly.  It can be a day off either way.  The next Sunday then is Easter.  Orthodox churches use the Julian calendar and a different formula but they’ll celebrate Easter on May 5th.  The formula was set up to approximate the Jewish lunar calendar.  It works out close this year because Passover started at sunset last Sunday night.

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

03/30/2012 – Ephemeris – April Preview

Ephemeris for Friday, March 30th.  The sun will rise at 7:26.  It’ll be up for 12 hours and 41 minutes, setting at 8:08.   The moon, at first quarter today, will set at 3:36 tomorrow morning.

The 4th month of the year begins Sunday with Palm Sunday.  Daylight hours in the Interlochen/Traverse City area and will increase from 12 hours and 48 minutes Sunday to 14 hours 13 minutes on April 30th.  The altitude, or angle, of the sun above the southern horizon at local noon will be 50 degrees Sunday and will ascend to 60 degrees on April 30th.  The altitude of the sun in the Straits area will be a degree lower.  The actual time of local apparent noon this month for the Interlochen/Traverse City area, when the sun passes due south, will be about 1:43 p.m.  The full moon next Friday is the Pascal full moon meaning the western churches will celebrate Easter the following Sunday.  Passover starts that Friday night, while Orthodox Easter is the 15th.

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

04/18/11 – Ephemeris – Paschal Full Moon

Monday, April 18th.  The sun rises at 6:53.  It’ll be up for 13 hours and 37 minutes, setting at 8:30.   The moon, 1 day past full, will rise at 9:49 this evening.

Last night our time the moon was full.  It was the Paschal full moon,  the first full moon of spring, or officially after March 21st.  Western Christian churches celebrate Easter at the Sunday following.  That would be the 24th, just one day short of the latest possible date of Easter.  Eastern churches use a slightly different calendar and a different calculation method, but this year they celebrate Easter the same date.  The calculation methods were chosen to make Easter roughly coincide with Jewish Passover.  Christian churches use a calendar based on the earth’s orbit of the sun, or actually the seasons, while the Jewish calendar is based on the lunar cycle.  That’s also pretty close this year because Passover begins tonight at sundown.

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