Posts Tagged ‘Full Moon’

04/14/2014 – Ephemeris – Why does Easter occur on a different Sunday every year?

April 14, 2017 Comments off

The answer is astronomical!

Ephemeris for Good Friday, Friday, April 14th.  The Sun will rise at 6:59.  It’ll be up for 13 hours and 26 minutes, setting at 8:26.  The Moon, 3 days past full, will rise at 11:48 this evening.

Easter will be celebrated by western and eastern christian churches this Sunday.  Easter is a movable feast in that it falls 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 where the year was 365.25 days long.  Passover started at sunset this past Monday night.  The western churches eventually adopted the Gregorian calendar to keep in sync with the seasons.  The Eastern churches did not, however Easter is late enough this year so they both fall on the same date.

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


The seasonal, or officially the Tropical Year, from vernal equinox to vernal equinox is approximately 365.24220 days long, about 11 1/2 minutes shorter than the Julian (after Julius Caesar) Calendar year.  The Julian Calendar kept up with the year by having three 365 ordinary years and one leap year of 366 days.  It over corrects.  To make the calculation for Easter easier in the various dioceses of the far-flung church, the vernal equinox, the day the Sun crosses the celestial equator, heading northward was defined as March 21st.  The actual vernal equinox was falling behind the Julian Calendar by 0.8 days every century.

By 1582 the Roman Catholic Church under Pope Gregory XIII decided to correct the problem.  By then the real vernal equinox occurred on March 11th.  Easter is supposed to be a spring feast, and using March 21st as the vernal equinox would eventually push Easter into summer.  The Pope instituted a commission to look into the problem.  This commission headed by Christophorus Clavius* came up with what we know as the Gregorian Calendar.  First, eliminate 10 days from the calendar.  This was done in October 1582 between October 4th and 15th.  Then to keep the calendar in sync with the actual year it was decreed that leap years would continued for years divisible by 4; except that century years, those divisible by 100 be ordinary years, except those by also divisible by 400.  Thus the year 1900 was an ordinary year, but the year 2000 was a leap year, and the year 2100 will be an ordinary year.  Adoption of this as a civil calendar took 400 years to be universal.

The Greek Orthodox and other eastern churches kept the Julian Calendar, so on occasion their Easter is sometimes celebrated in May.  The Jewish Calendar is, as I alluded to in the program transcript, a lunar calendar.  It has a relationship to the Julian Calendar in that 19 Julian Years equals 235 lunar months almost exactly. This is called the Metonic Cycle.  Those 235 months equal 12 lunar years of 12 and 13 months.  So without correction Passover too will slowly head into summer in millennia to come.

* Clavius was honored by having a large, rather spectacular crater on the Moon named for him.  Search these posts for Clavius to find it.





08/18/2016 – Ephemeris – Viewing the full Moon tonight

August 18, 2016 Comments off

Ephemeris for Thursday, August 18th.  The Sun rises at 6:49.  It’ll be up for 13 hours and 53 minutes, setting at 8:42.  The Moon, at full today, will rise at 8:54 this evening.

The full moon, contrary to what you’d think is a poor time to observe it.  The moon is essentially gray on gray.  And at full moon we are looking at the moon from about the same perspective as the sun, so there are no shadows to delineate its fine features.   Since the actual instant of full moon occurred at 5:27 this morning, some shadows will be creeping in on the moon’s upper right face as it is seen in the evening.  Full moon is the best time to see the maria or lunar seas, the dark areas that make up the man in the moon.  In binoculars can be seen the bright rays* emanating from the crater Tycho near the south end of the moon.  Other craters have rays too, but none so long and distinctive. Night by night for the next two weeks the moon’s illuminated landscape will wane.

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

* Rays are caused by the ejecta from the impact that created the crater.  They are thought to be small craters themselves which show up best at full moon because they have no shadows in them.


High contrast full Moow

The full Moon taken last night, 7 hours before to was officially full. The contrast was greatly enhanced to bring out Tycho’s ray system. Credit Bob Moler.

10/27/2015 – Ephemeris – Hunter’s Moon

October 27, 2015 Comments off

Ephemeris for Tuesday, October 27th.  The Sun will rise at 8:13.  It’ll be up for 10 hours and 25 minutes, setting at 6:39.   The Moon, at full today, will rise at 7:09 this evening.

Tonight’s full moon is known as the Hunter’s Moon because it is the full moon after the harvest moon.   The minute the Moon will be full will be at 7:05 this morning, about an hour before it sets. This was the time Native Americans and Europeans went out to secure the meat for the winter.  And it also coincides with the time of year of our hunting seasons.  So in that regard it fits nicely.  The names of the full moons throughout the year generally mesh with the activities or weather conditions of that month.  For instance December’s full moon is the Cold Moon or the Long Nights Moon.  December has the longest nights.  This list is taken from, the website of the Old Framers Almanac, from which I’ve been getting folklore  tidbits for years.

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


Other sources for full moon names: