Give us our eleven days (Canterbury) Mystery Cache

Cache 5 in a series on the calendar.

The Julian calendar (the subject of Veni, vidi, kalendri) ticked along just fine for many centuries following its introduction on January 1st. 45 BCE. With a leap year every 4th year, the Julian calendar's year is 365 days and 6 hours. The solar year is a little less than the Julian year (about 11 minutes and 14 seconds less). The error is small (about 0.002%), but as the centuries went by it steadily built up. Because the Julian year was too long, every 130 years or so the solar year would get a day earlier in the Julian Calendar. When the Julian Calendar was introduced the winter solstice was on December 25th (solstice is is the reason for the season!), but as the years went by, the winter solstice came earlier and earlier in December.

On the face of it, the error in the Julian calendar was small enough that it didn't need fixing. Over a person's lifetime, the change in solstice / equinox dates would be less than a day. However, the accumulated error was of major concern to the Catholic church because of the way the date of Easter was calculated. Jesus was crucified on (or just before) the start of passover (which is Nisan 15 in the Jewish calendar). In the first couple of centuries after the crucification, many Christians decided when to celebrate Easter based on the date of Nisan 14 (one faction marked Easter on that date; another on the Sunday following Nisan 14). Other groups used other approaches. The Alexandrians marked Easter on the first Sunday following the full moon that followed the northern spring equinox. Although this method makes little sense in our solar-based calendar, it gave the Alexandrians a way of calculating an easter date that was close to the Jewish passover date, and that could be calculated independently (without relying on help from the Jewish community). The Jewish calendar is synchronized to the sun and the moon, and its months move around by several weeks with respect to the solar year. For example, the dates of Nisan 1 for the year 2017 to 2021 are March 28, March 17, April 6, March 26 and March 14.

In hindsight, given we operate a solar calendar, it would have made life a lot easier to make Easter Saturday (say) the second Saturday in April (an idea that has some support).

At the council of Nicea (325CE) it was decided that Easter would be:

1. celebrated on a Sunday (the day of the week of the crucification), and
2. that the method used to calculate the date of Easter would be independent of the Jewish Calendar.

Eventually, the Alexandrian method won out and, later still, a (complex) method for calculating Easter was determined. That method didn't always result in the "right" date (sometimes the Sunday calculated wasn't the first Sunday following the full moon that followed the northern spring equinox), but the alternative of using astronomical observations each year was impractical. For example, Ash Wednesday (the start of Lent) is 46 days before Easter (can be anywhere from February 4 to March 10), so if you were relying on astronomical observations on the equinox (March 21, 22 or 23) to determine when Easter occurs, you wouldn't know when to start Lent until well after the date it should have started.

The formula for calculating easter presumed that the equinox happened on March 21. When the Julian calendar was instituted, the equinox was on March 25, but after a few centuries of extra leap days the equinox was happening a few days earlier, and March 21 was about when the equinox fell when the formula for calculating Easter was devised.

The formula for calculating easter assumed that the length of the Julian year was accurate. Because of the error in the length of a Julian year, as the centuries went by the calculation was "wrong" more and more often. Eventually, in the 16th century, Pope Gregory decided enough was enough. It was time to:

1. Remove 10 days to move the northern spring equinox back to March 21st (ish).
2. Update the leap year rule to reduce the number of leap years
3. Update the easter formula to reflect the changes in the calendar.

The leap year rule was adjusted so that century years were no longer leap years, except those that are divisible by 400. So, 1600 and 2000 would still be leap years, but 1700, 1800, and 1900 would not. In a 400 year period the Julian calendar has 100 leap year; the new Gregorian calendar has 97. The Gregorian calendar is a lot more accurate (the error is down to about 1 day in 3000 years).

One way of removing the excess 10 days would be to just cancel the next 10 leap years. That would have been quite a painless way of doing it. However it would have taken 40 years, and it was decided just to get it over and done with, and move directly to the new formula for Easter.

It was decided to introduce the Gregorian Calendar by removing 10 days from October 1582. Around five European Catholic countries switched to the Gregorian calendar in October 1582, with the rest of Europe (and many other countries outside Europe) remaining on the Julian calendar. Having two calendars operating in Europe was confusing (both at the time, and for historians tracking events in Europe in the period where both calendars were in use). The terms new style (NS) and old style (OS) were often used for Gregorian and Julian dates. Official documents often carried NS and OS dates.

Over the next few centuries, the Gregorian calendar gradually displaced the Julian calendar (no doubt driven in part by the desire to eliminate the confusion of having two calendars operating in Europe, and having to distinguish between NS and OS dates). It was about 170 years before Britain and its colonies moved to the Greogorian calendar. By then it was necessary to remove 11 days from the calendar (the Julian and Gregorian calendars moved to 11 days apart in 1700, which was a leap year in the Julian calendar but not in the Gregorian calendar).

There are reports of a number of issues arising in introducing the Gregorian calendar (I'm not sure how reliable the reports are). Some believed that fate had pre-determing the date they would die, and that their lives were shortened by days being removed from the celendar. For the month from which days were removed, there were reports of landlords wanting a full month's rent, but employers only wanting to pay for the number of days worked. The William Hogarth painting "An Election Entertainment" includes an election campaign banner saying "Give us our eleven days". The election was a couple of years after the calendar change, and the (opposition) tories raised the calendar reform issue in the campaign.

The changes made by Pope Gregory are often referred to as the Gregorian Correction to the Julian Calendar. That makes sense as the calendar in use today is virtually identical to the Julian calendar. All Pope Gregory did was make a small change to the leap year rule, and remove 10 days from October 1582. The Gregorian Correction resulted in the calendar we use today. Further calendar reform seems highly unlikely, given that it would required vast amounts of reprogramming of computer hardware and software. On a related note, it is fortunate that the first century year of the computer age was 2000 (a multiple of 400, and therefore a leap year), and not 2100 (not a leap year, but will be considered a leap year by any software that presumes all years divisible by four are leap years). It will be interesting to see how big the "Y2.1K problem" is in about 80 years time!

Answer the following questions to work out the coordinates of the cache:

• The Julian and Gregorian calendars are currently AB days apart (note: anybody solving this puzzle after Feb 28, 2100 needs to keep in mind that the puzzle was created in 2018).
• Japan adopted the Gregorian Calendar in 1CDE.
• The Duchy of Prussia adopted the Gregorian Calendar in 1FGH.
• Greece adopted the Gregorian Calendar in 1JKL.
• Great Britain adopted the Gregorian Calendar in 1PQR.

The cache is located at S 43 28.DFB E 172 34.PHJ. It is a pill container in a camo bag behind stones at the base of a fencepost (the one where the style of fence changes). Please re-hide well.

This is the fifth cache of a series on history of the calendar. The caches in the series are:

1. A week of it!
2. Beware the Ides (and the Nones and the Kalends)
3. Veni, vidi, kalendri
4. Monk-eying with the Calendar
5. Give us our eleven days