How Did The Ancients Measure Time/Years So Accurately?

Factual Questions
1

I know their calendars were typically off a bit, but still some of them came pretty close. For example, as recently as the 13th century Roger Bacon was able to estimate that their calendar was off by 1 day in about 125 Years. How did he do that? He certainly didn’t wait 125 years to determine the solstice had moved a day.

Presumably by measuring the length of days and so forth an estimate could be made. However 1d/125y is an accuracy of 1 part in (about) 45,000. Mechanical clocks were just starting to appear at that time, and I doubt they initially had that type of accuracy. Water clocks and sundials couldn’t have been much better.

Going back further other ancients came up with comparable measurements - and perhaps they could have officials make measurements over very long times; but that’s still quite a feat.

So, how did they do it?

Maybe we can find a loophole to the 2012 problem -but hurry - time’s running out!

2

The Church had set up the calendar so that the equinox was supposed to take place on a particular date. After enough time went by, the equinox could be observed to be on a different date. Fine measurements weren’t needed, just observations of when the equinoxes (and the solstices) occurred, compared with when they were expected to occur; this link has Bacon’s writing on the subject Opus Majus of Roger Bacon 1928 - Google Books

3

So, that looks like Bacon had access to when equinoxes and solstices occured way in the past (i.e Ptolemy’s days) and from that knew how many total days had passed and how many days the equinoxes or solsitices had slipped.

Yes?

4

Using reason and logic.

Eratosthenes (~200 BC) and Hipparchus (~150 BC) made commendable estimates of the distance from the Earth to the Sun using basic trigonometry and algebra.

A calendar is relatively simpler to determine and keep after realizing that the Earth revolves around the Sun – nearly 3,000 years ago.

Then came Christianity and plunged Western civilization into 1,500+ years of superstition, fear, ignorance and darkness… much of it is still lingering around in substantial measure.

I’m not familiar with Asian civs so it’s not unlikely that Chinese astronomers may have made reasonably adequate estimates too and had reasonably accurate calendars.

5

Yes

6

:confused: This is a whoosh, right? Hellenistic astronomers in general did not believe that the earth orbited the sun. Many of them were aware that such a heliocentric model had been proposed by Aristarchus (who btw lived about 2300 years ago, not 3000 years ago), but it was never a consensus scientific view, even before Christianity.

Anyway, a simple solar calendar isn’t any easier to determine with a heliocentric model than with a geocentric one. Comparing equinox observations separated by long periods of time, you divide the number of days in the interval (accounting for whatever leap years or intercalary months had been included in the meantime) by the number of years, and the quotient is your standard year length. Whether you consider the underlying physics as involving a revolving sun or a revolving earth is irrelevant: it’s their relative position that determines the solar year.

An approximate equinox observation isn’t that difficult: if the sun is on the due east point of your horizon when it comes up, that’s an equinox occurring at sunrise. Even if you’re off by up to a day in identifying the actual moment of equinox, over long periods of time it won’t contribute much error. E.g., if you estimate that 36526 days passed in between two equinox observations a hundred years apart, that gives you a year-length of 365.26 days, which is a bit too long but not enough to produce serious calendar errors in the short term.

And yes, the Hellenistic astronomers did have access to some quite distant observational data acquired from Babylonian astronomical records, or as they put it, “the Chaldaeans”. A few hundred years’ conscientious recordkeeping is enough to produce sufficiently large period relations to get quite accurate calendar parameters.

7

Let me try to summarize what everyone else has said in a slightly different way, and let’s see if it is clearer. The Church decided that Easter would be celebrated on the Sunday after first full moon after the solar equinox. So far so good. They computed the equinox based on their best solar calenders at the time. Things were pretty good, for a while, in Rome. Nothing seemed amiss for 50 years. Nothing seemed wrong at precisely 125 years. But as centuries passed, Easter was coming later and later, relative to the equinox (if I figured the math right), and the “resurrection of Christ” meant to coincide with Spring and regrowth for an agrarian community, just wasn’t matching up – particularly for more northern climates – which are also newer to Christianity. This cultural–ecclesiastical mismatch was a problem.

And it was simple enough to see. Chart sun-up and sun-down. You don’t need accurate clocks (giving exact time) just precise ones (giving the same “wrong” time each time.) Count the days your calendar is wrong, and the years since you started using the calendar, and divide. And you get approx. 125 years. I say approximately, because Earth’s path around the sun isn’t 365 solar days, or 364 and one quarter days. But some slightly lesser number that only adds up over centuries.

8

The part that amazes me is that the records could be kept for such a long time. Even if they were literally carved in stone, I’m amazed that they didn’t get lost somewhere in the middle, or misunderstood by the later generations.

NASA’s archivists can’t even be relied upon to protect something as iconic as the original tapes of the Apollo 11 moonwalk for a few decades. How did Babylon’s astronomers protect their equinox records so well for centuries?

9

Trivia Question:
Q: What month did Russia’s October Revolution happen in?
A: November

Before Julius Ceasar, the Roman calendar (AFAIK) was pretty slipshod. If they needed to insert an extra day to get the Ides back on track or something, they just did it by decree. Rome, like many other civilizations, realized 365 days was off by a bit, about 1/4 of a day per year… So they decreed the Julian Calendar with fixed months and leap years, which stuck for 1600 years.

When the Roman Empire died, the church took over watching the calendar because feast days and saints’ days were important to the liturgy. With the whole of Europe watching, it was pretty difficult to forget which day was which. Meanwhile, between scientific astronomy and astrology, there was a definite interest in tracking star positions and solar or lunar events.

By the Renaissance, the church realized things were seriously out of whack - key dates like Easter, that depends on the equinox, were way off from where they were predicted. Pope Gregory did the Julius Caesar thing, got some of the best minds together, and came up with the solution to the problem. Over 1600 years the calendar had slipped about 11 days; so 365-1/4 was also not quite right. The current solution is the Gregorian calendar - centuries, except those divisible by 4, are not leap years. 1800, 1900 and 2100 (18, 19, 21) for example are not; 2000 (20) was. Fortunately, someone had kept track for 1600 years to give a good number as to how badly off a Julian year was.

Of course, by the 1600’s and 1700’s, a lot of countries were not going to go along with the Pope’s decree on many things. Things “popish” were probably regarded in protestant countries about the way we thought about Communist ideas during the Cold War. England, IIRC, held out until about 1720’s before decreeing that everyone set their clocks back 11 days. Predictably, people who got paid by the day and paid rent by the month were a little ticked…

One of the last holdouts for the “new calendar” was the staunchly Eastern Orthodox Russian Empire - they wouldn’t take the pope’s word for anything. Even though the revolution that threw out the government and installed the communists happened on November 10th real time, it was still the October Revolution for them.

10

Accurate record keeping isn’t that new. We have lots of records from classical times and enormous amounts of records from medieval times. The problem we have today is that fantastically huge amounts of records are expected to be kept. This is why lots of movies from the 1890’s through the 1920’s are lost. By the time that movies were regularly kept in good shape, lots of TV shows from the 1940’s through the 1960’s were lost. Undoubtedly there are records of things that we think worthless and throw out about which people in the future will say, “Why didn’t people in 2011 keep those records? Those were very important. What stupid, primitive people they were back in 2011.”

11

Exactly. And it didn’t require much in the way of long-term record keeping to notice (though the fact that long-term records existed and exist is still very nice); all you have to do is keep to the calendar that you’ve thought was right, which doesn’t take any record-keeping besides keeping track of the date, which every landlord, renter, banker, and debtor in society is going to be doing anyway.

12

It’s even easier than that. Around the equinox, the sunrise point on the horizon moves nearly a degree a day. If you measure that sunrise on one day is a quarter of a degree north of due east, and sunrise the next day is 3/4 of a degree south of due east, then you know that the moment of equinox occurred a quarter of a day after the first of those two sunrises. I could probably get that much precision with homemade instruments improvised from twigs and tree branches. With actual astronomical instruments, made of something like brass or bronze by an actual craftsman, you could easily get the solstice time down to an hour. That’s good enough that you could see a change of 1 day in 125 years over a span of only about 5 years. Clocks, meanwhile, would not be required at all.

13

[moderator note]
Naxos, keep the religious jabs out of the GQ forum.
[/moderator note]

14

Government bureaucracy, basically. The astronomical records weren’t carved in stone but baked in clay, which holds up almost as well with minimal protection from damage. Read up on the so-called “Babylonian Astronomical Diaries”, which were observational (and economic) data compilations maintained at major temples from at least the seventh century BCE to somewhere around the first century CE. Since divination and maintaining the luni-solar calendar were both highly important parts of temple scribes’ duties, and celestial omens (eclipses, conjunctions, etc.) were a very important form of divination, the diaries were valued as significant data sources and not just “museum fodder”.

We don’t have the complete corpus of all such records, of course, but there were enough of them kicking around in the Hellenistic world to provide crucial data for Greek astronomers trying to calibrate their geometric models.

Very good point; I was trying to keep the description of equinox observation as simple as possible, but you’re quite right that even pretty precise observations are not all that difficult and don’t require a lot of special knowledge or equipment.