Why does the amount of days in an year on average of the Gregorian calendar only have 4 decimal places (365.2425)?

Question

Alfonsine tables available at the time of the Gregorian reform provided enough information (however inaccurate) for the calendar to have been designed such that it expressed more precision regarding the amount of days in a year on average (e.g., 365.2425463 instead of 365.2425).

Why isn't the Gregorian calendar as precise as it could have been, with regards to its designation of leap years? Did Pope Gregory XIII just get lazy and think that three rules (every 400 inclusive-or (every 4, but not every 100) years are leap years) were already far too complicated of an algorithm for the average person to handle? Was there some sort of a Christian taboo on writing numbers with more than 4 decimal places? Or perhaps did he think, "We're not really sure if 365.2425463 days in a year is accurate anyway, so we may as well truncate it to 365.2425. Just to be safe."?

Answer 1

Why? Because there was no point.

First, according to more modern astronomical measurements, the current length of the year is closer to about 365.2422 days, so they would've been relatively less accurate had they used a more precise value of 365.2425463 days per year.

Which leads to a very important point about math: you need to be very mindful about how much precision you actually have in your measurements.

Also, let's look at what that very modern document from NASA says:

Before contemplating further corrections to the Gregorian Calendar we must consider how exact the value of 365.2422 is. The length of the average tropical year is now more precisely 365.24219 days but it varies somewhat from year to year and does not track the seasons precisely. Also, because of tiny orbital effects the average tropical year varies by about .00005 days per 1,000 years. Thus correcting any error of this magnitude is probably a waste of time.

That is not a new attitude. Most of the rest of this answer is based on this document. In it, we read about Copernicus:

Copernicus did not believe it was possible to have a perfect calendar, as the solar year was too variable.

So even back then, there was a belief that the calendar would drift in a variable way making too detailed corrections pointless.

Now look at the some measurements that were taken in that era from page 19:

• 1252 Alfonsine 365.24254630
• 1543 Copernicus 365.24269676
• 1551 Prutenic 365.24719907
• 1574-75 Ignazio Danti 365.24166667

We can see pretty clearly that there wasn't a lot of agreement beyond a couple decimal points. As such, it is easy to see why someone might not bother to pay attention to that 0.0000630. They would see it as not a true reflection of reality, just an artifact of the imprecise math and in modern terms, well within the error bars.

It appears that the person ultimately responsible for the calendar was one Aloysius Lilius. From page 20, we see he came up with:

365 +1/4 – 1/100 + 1/400 + 4/100,000

which corresponds to the errors:

• minus 1 day every 4 years;
• plus 1 day every 100 years;
• minus 1 day every 400 years;
• minus 4 days every 100,000 years (that means minus 1 day every 25,000 years).

This was then the basis of the calendar with the last part dropped. We can easily understand why, though. It would not require any change from the Gregorian calendar in another 23,418 years!

At the time, the general view of the age of the Earth was in the thousands of years. In fact, the Alfonsine tables referenced in the question put it at 6984 BC. In addition, the general Catholic belief in the second coming of Christ gave a general expectation that there was an end date, and that it was at most hundreds or thousands of years away. If your worldview has the Earth lasting on the order of 10,000 years, why worry about 25,000?

So in summary:

1. Their measurements weren't good enough to get that kind of precision required for a more accurate calendar
2. They had reason to believe that the year was variable enough to make more precision impossible
3. If the "best guess" was right, it would be trivial to fix on the year 25,000
4. They had good reason (in their view) that the year 25,000 would not happen

Answer 2

The Gregorian Calendar was introduced (to the Catholic World) in 1582, the result of preparation over the preceding five or so years.

However the popularization of decimal fractions would wait another three years until the publication of La Thiende [The Tenth] in 1585 by the Flemish mathematician Simon Stevin. Though not the inventor of a decimal representation of fractions, the publication in 1585 of both La Thiende and La Disme [The Decimal]) popularized them and explained their use.

An important consideration in the preparation and adoption of the Gregorian Calendar was that it be easily understood by those not of great mathematical sophistication. Using a description reliant on an unfamiliar mathematical notation would not have aided the cause.

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Note that our entire concept of "number" has changed dramatically over recorded history. A striking observation from Euclid's Elements is how the portion dealing with what we now term Number Theory is discussed entirely in terms of "length of a line segment or arc". That was "number" to Euclid and contemporaries.

The very acceptance (in Europe) of negative numbers dates only to the 13th century (and may comingle with the simultaneous development of double-entry bookkeeping) - so is barely 300 years old at the time of the Gregorian calendar reforms. One must take great care in interpreting historical concept of "number" to not overlay our modern understanding and interpretation.

Answer 3

The existing answers are good, but I would add one additional detail, building on your concept of "future-proofing":

The exercise of reforming a calendar was intended to make a calendar more accurate, but was not intended to eliminate the need for any theoretical future adjustment.

The Gregorian reform was inspired by the Julian reform, and the Julian reform of the Roman calendar was merely the last of many "consular" modifications of the calendar. Under the Roman calendar system, it was known and accepted that imperfections in the length of a year would accumulate over time and need to be corrected for; the Julian reform was meant to make those corrections necessary much less frequently, but did not pretend that it would make them unnecessary forever.

The Gregorian reform was a further improvement - but if it too was imperfect, and would require someone far in the future to add an intercalary day to a year to bring the calendar back into alignment with the true year...well, that was some future Pope's problem.

Answer 4

The other answers have touched some very good points, but there is a mathematical point I would like to make as well: The way the leap year rules are designed is fundamentally incompatible with the decimal expansion. Specifically, rules of the kind "leap/no leap year every X years" don't really care about the number of digits, they are more of a form of continued fractions.

In particular the fact that the decimal expansion has only finitely many digits is both a happy accident in the first place and an unintended consequence of a deliberate choice made afterwards:

1. The error made when setting the length of a year to 365 days is very close to 1/4 of a day, so for the Julian calendar the choice was made to include a leap year every four years, leading to 365.25 days per year on average. If it had been close to 1/3 instead, they would have chosen a leap year every three years, leading to 365.333333333...., an expansion with infinitely many digits. The same for any other factor that has prime divisors other than 2 or 5. In fact you can have a look at the Hebrew calendar, which is based on a complicated 19 year cycle for a real world example of this.

2. When the Julian calendar then was updated to the Gregorian calendar, a deliberate choice was made to keep the rules simple to calculate for a given year. Dropping a leap year every 132 years, would have had almost the same effect as the current rule of dropping one every 100 and adding 1 every 400, with an average day of 365.2424242424...., again with infinitely many digits. However such a rule would have made it incredible tedious to calculate if a given year is a leap year or not, while the current rule only involves calculations that can be made mentally within seconds. But those calculations again are easy because the numbers involved only have prime factors 2 and 5, which again result in a finite number of digits. (Another happy accident here by the way, something like 1 in 300 would have lead to infinitely many digits again.)