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.
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.