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The First Sunday After… What?

By Daniel Hubbard | May 1, 2011

Last week I finished blogging about the Civil War but Easter occurred last week and I gave a talk about calendars. Here are a few thoughts that lead to why Easter matters so much to our calendar and why you may not really know when an ancestor was born.

We live with three basic celestial rhythms that we use to mark time—the solar year, the lunar month and the day. Ever since people learned to recognize those three rhythms there has been some sort of calendric frustration. The day does not fit evenly into the lunar month and neither fits evenly into the solar year.

Different cultures have found different ways of dealing with the problem. One can define the year not by the cycle of the sun but as a number of cycles of the moon. This is the solution used for the Islamic Calendar with its year of twelve lunations. A calendar of twelve lunations per year is not quite as long as the solar year, so dates reckoned in a lunar calendar will continuously drift through the seasons. A calendar of mixed twelve and thirteen lunation years can be made to stay close to the solar year. “Lunisolar” calendars like the Hebrew and Chinese calendars rely on rules for adding a thirteenth month every few years. In these calendars, a year will either be approximately 354 days or 383 days depending on the number of months in the year.

The official calendars in many other places, beginning as early as the time of the ancient Egyptians, have taken another path. It isn’t hard to imagine that the word “month” is related to the word “moon” but there is no attempt to keep the month in line with the moon in these calendars. We use one such calendar and except for February our months are all a day or two longer than the cycle of the moon’s phases.

Ignoring the moon still leaves the problem that the number of days it takes to complete a year is longer than 365 by a fraction of a day. Leap days came into existence over two thousand years ago to add an extra day every four years. The Julian Calendar, named for Julius Caesar, is the oldest calendar that most genealogists researching in European records are likely to encounter. It adds that one extra day every four years. It gets the average number of days per solar year just about right. If it wasn’t for Easter that certainly would have been good enough.


Why isn’t an average of 365.25 days per year good enough? The real value this tries to mimic, called the mean tropical year, is 365.24219. That’s an error of just under 8 days every thousand years, less than a day in a human lifespan. Personally, I wouldn’t notice that. At eight days per thousand years it would take a long time before Santa would need to exchange his reindeer for chicks and baby bunnies and replace the fur suit with shorts. If you look at other cultures, they have lived and in some cases do live very happily with much larger differences between the calendar year and the solar year. The ancient Egyptians lived with a calendar that drifted with respect to the seasons by one day every four years. The Islamic year drifts with respect to the sun by about 11 days per year. Lunisolar calendars like the Hebrew Calendar track the seasons but have a year that is either about 11 days shorter than the solar year or about 18 days longer than the solar year. Despite those counterexamples, 365.25 days per year was not acceptable. Easter, and the complex way it was defined, is the reason that 8 days per millennium mattered.

The problem begins with a basic decision about how to make a calendar work. One can either observe the phenomena that set the dates on your calendar or you can calculate them. Both have problems. If you decide to determine when the moon is full or when an equinox occurs by observing them, you need certain things. Clear skies, good records, some sort of measuring apparatus and a person or group that everyone agrees are the ones to make the official announcement are all needed. You also need to get the news out fast enough so that your festival can take place. If you decide to calculate you need to know enough to create definitions, tables and formulas that are good enough for your purposes. In the West, calculation was chosen. One of the purposes was to determine the date of Easter. For that purpose, the definitions, tables and formulas were not good enough.

By Definition

Easter is defined as the first Sunday after the first full moon on or after the vernal equinox. A bit complicated but if you know when the moon will be full and when the equinox will occur it should not be a problem. The simplest way to determine the date of the equinox is to use a fixed date in a solar calendar. Calculating the correct full moon gets into a 19-year cycle of lunar months, two sets of one day corrections, one in a 400 year cycle and the other in a 2500 year cycle. Skipping the ephemera, there is a way of calculating the start date of the lunar month that will contain the full moon that falls after the equinox. That full moon is simply defined to be fourteen days after the new moon that starts that month.

It is the fixed date of the equinox that led to the trouble that affects genealogists. Because the average length of the year in Julian Calendar was too long by a few minutes, the vernal equinox shifted by almost exactly 3 days every four hundred years but the Church had defined the equinox to be on March 21. By the time that Gregory XIII reformed the calendar, the equinox was occurring on March 11. With luck, Easter might fall correctly anyway but it could also be off by much more than ten days. Something needed to be done. The Church’s holiest day could not be celebrated at the wrong time.

Starting in the 1580s and continuing into the twentieth century, countries that had used the Julian Calendar converted to the Gregorian. They changed the rule for the occurrence of a leap year from “every year divisible by four” to “every year divisible by four except years divisible by one hundred that are not divisible by four hundred.” So by the Gregorian Calendar 1900 was not a leap year but 2000 was. The new rule isn’t as easy to remember but it keeps Easter where it is intended to be.

Most importantly for genealogists and historians, these countries also picked a date and skipped ahead days—ten in the 1500s and 1600s, eleven in the 1700s, twelve in the 1800s and thirteen in the 1900s. That means odd holes in record books when judging by the dates, no one was born or married or died. It means that subtracting an age at death from a death date won’t give the right birth date if a person was born under one calendar but died according to the other. It means that there have been times and places where crossing a border could change the date by ten or more days. It means that if you aren’t careful, it can appear that event B happened before event A, which looks odd when A caused B. Just as we are amused when after an eight hour flight, local time is only two hours later than when we left, a person crossing a border could arrive at their destination many days before they left on their journey.


Another  bit of calendar confusion occurs when a country makes a change in the official date of New Year’s Day, which many did when they converted from the Julian to the Gregorian Calendar but that change could happen at other times as well.

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Topics: Calendars and Clocks, Records | No Comments »

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