The full Moon occurs when the Sun and the Moon are on opposite sides of our world. We see the full face of the Moon illuminated by the Sun because as we turn up our eyes to the Moon, the Sun is shining from below our feet, lighting up the far side of the Earth. This month we were treated to the most spectacular sort of full Moon. The Sun and the Moon were not just roughly in opposite directions, they were in exactly opposite directions. The Moon passed through the Earth’s shadow and grew darker and darker until it turned sunset red. That moment when the entire Moon seems to turn to blood is the time when it is most obvious that the moment of the full Moon has been reached.
The full Moon this month is also special because it is the one that defines when Easter is celebrated and it is problems with the calculation of Easter that gave us the modern (Gregorian) calendar. Every genealogist and historian who has researched in a time and place before the arrival of the modern calendar or in a culture where it is only one of the calendars in use, needs to learn to navigate the transitions from one way of looking at time to another.
So, how does the Moon help determine Easter’s date. Easter is defined as the first Sunday after the first full moon on or after the vernal equinox. Well, that is what is often said and it is approximately true but we need to make a few corrections.
Equinox
The first correction, and it is perhaps a bit of nit picking is that “vernal” means spring and anyone living south of the equator might be justified in complaining that Easter takes place in the fall, not the spring. Either March equinox or Northern vernal equinox might be a better term.
Then what about the word “equinox”? The term means when day and night are equal. “Equinox” comes from Latin words meaning “equal night.” More accurately though, the equinox occurs at the particular moment when the sun is directly above a point on the Earth’s equator. The date that this moment occurs in any given year depends on where you are on the globe and can be one day different depending on your position. If that one day shift mattered, then people in some places might be celebrating Easter more than a month earlier than people in other locations. In the end though, that turns out not to happen. For purposes of computation the equinox isn’t actually used. Instead of the true equinox, which can occur on different dates in different years and on different dates depending on where you are on the Earth, the date March 21 is used. No need to observe the Sun, just use a calendar.
That calendar based solution works well as long as March 21 always occurs about when the equinox occurs. If your calendar drifts, you eventually have problems. The old Julian calendar drifted three days every 400 years. That might not seem like much but eventually Easter began to occur later and later in the year. At different times in different places, Pope Gregory’s calendar replaced the calendar of Julius Caesar. Days were dropped from years to bring the calendar back into synchronization with the Sun.
Full Moon
The second group of corrections has to do with the Moon. We think of the full Moon occurring on a specific day but of course, as we realized when thinking about lunar eclipses, the Moon is actually full at one precise moment. The problems with the full Moon are the same as the problems with the equinox. For the purpose of setting the date of Easter, the date of the full Moon is set by mathematical tables. The moment when the Moon is truly full may happen on the day given in the tables or a few days before or after.
Does it Make a Difference?
It certainly matters to the day you celebrate Easter. This year (2014) is special because Western churches (using updated calculations) and Orthodox churches (using original calculations) celebrate Easter on the same day. That last happened in 2011 and won’t happen again until 2017. As the Julian and Gregorian calendars drift apart it will happen less and less. April 24, 2698 will be the last time that the same date is used by both groups. Already now, the dates of Easter can be as much as 5 weeks apart.
To the genealogist and the historian, it makes a difference. The calendar is our map of time. Use the wrong map and we are lost, though we will think we know where, or rather when, we are. In different places, different cultures and different times, the calendar has been and still is different from what we expect. If you have ancestors from different Christian denominations, then what records with dates based on Easter, or any holy day related to Easter, can mean very different dates. In 1584, the second year after the new calendar began to be adopted, Catholics celebrated Easter on a date four weeks different than other groups. In some areas of Europe the changes made to the lunar tables were accepted decades after the change to the calendar, leading to still more dates for Easter.
There was no one map of time. There was, in fact, a giant atlas.