Could ancient astronomers really predict eclipses?

Factual Questions
1

There’s a scene in Mel Gibson’s Apocalypto where a group of Mayan rulers are sitting on top of a pyramid, overseeing a slaughter to appease the corn god or whatever, and at the climax of the ceremony a solar eclipse begins. The populace below wigs out, and the rulers give one another this knowing look and then proclaim that their sacrifice has appeased the gods and they will now bring the sun back.

Would it really have been possible for ancient astronomers to make such a prediction? I don’t doubt they had adequate math skills, but how could they figure out the correct calculations without knowing anything about the orbit of the moon? Could they derive it somehow from historical observation?

2

Not knowing anything about the orbit of the moon? It seems that any people with a lunar calendar had a priesthood who spent their entire lives observing and extrapolating the position of the moon in the sky. (A heliocentric model might have made the math easier, but the moon’s orbit is certainly geocentric.)

3

Sure, although perhaps not in the Americas.

4

In the book Stonehenge Decoded, Hawkins tells you how to use Stonehenge to predict eclipses. You could build your own mini-=Stonehenge at home and do it. It’s fun!

5

Solar eclipse is pretty much not going to be on. Easy to work out when there might be one, impossible for them to have had enough accuracy to work out if they would be in the path of totality. The necessary mathematical model wasn’t worked out until 150 years ago. But harder would have been working out the parameters to the model with enough accuracy.
What they critically lacked was accurate time keeping and accurate angular measurements. Without very accurate parameters the numerical model will simply give garbage results. A history of eclipse observations for the entire planet over hundreds of years might have provided enough information, but that would not have been available to them.

Worse, close enough to zero chance that totality would actually touch them. The path of totality of a solar eclipse is typically of the order of a few tens of miles wide. With only two possible eclipses each year, half of which are annular, and not total, you have a path a few tens of miles wide crossing a section of the globe on average once a year. Pick a random spot of the globe. Civilisations will rise and fall before there is a better than even chance of a single total solar eclipse visiting that spot.

Lunar eclipses on the other hand are pretty easy. They are visible by one half of the planet, and occur roughly twice a year - with an easily determined period that allows trivial determination of the exact time, and thus if it will be visible from where you are.

6

The ancient Babylonians could predict solar eclipses, but probably only to within about a year.They did not have much in the way of astronomical theory, but they had centuries worth of detailed astronomical records, and could see the patterns in them. The Greek philosopher (indeed, the first philosopher) Thales is said to have predicted a solar eclipse which occurred in 585 BC. Almost certainly, he was relying on Babylonian data to do this, and his prediction probably amounted to saying that there would be eclipse that year. In this context, one should probably not understand “eclipse” to mean “total eclipse”. For the reasons Francis Vaughan points out, those are very rare at any particular location, and very difficult to predict with rough-and-ready methods, but partial eclipses can be spectacular enough.

Mayans may have been able to mange something similar, but nothing much better.

Later on, from the first century AD, Ptolemaic astronomical theory, which involves some quite sophisticated geometry, allowed for much more accurate predictions. Using the Ptolemaic system you could probably have predicted an eclipse of the Sun to within a few weeks, or maybe a few days if you were lucky. Certainly Ptolemaic astronomers could do this for events such as planetary conjunctions.

7

But a solar eclipse can happen only with a new moon, which is an easily predictable event. So an eclipse prediction within a few weeks yields a couple of possible dates, and within a few days just one.

8

You’re too focused on total eclipses. I have never seen a total eclipse, but I’ve seen a couple of partial ones. The effect is still VERY dramatic even with only 30% occlusion. While the sun looks the same to the naked eye, I was very aware of the decreased amount of light. My first thought was that a cloud passed in front of the sun, but I couldn’t see any clouds. (Actually, my second partial eclipse was here in WA. I couldn’t see anything but clouds thanks to the weather, but the decreased light was still obvious).

In the OP’s example of a theatrical priesthood using this to impress the peons, that scenario would be more than effective. You don’t need a total eclipse.

You also don’t need perfect (or even very good) accuracy. Just look at the end of the world types who are wrong time and again. Their followers don’t seem bothered by this. If an ancient priest could pull it off even once in a lifetime, that’d be impressive.

9

In case anybody is still unclear, ancient eclipse prediction did not involve knowledge or measurement of the moon’s orbit. It involved extrapolation from records of previous eclipses, and teasing out the patterns within them. Indeed, it worked in reverse–studying eclipse records gave insight into the moon’s orbit, not the opposite.

The gold standard for solar eclipse prediction is the triple-Saros cycle. If a fat partial or total solar eclipse occurs at your location, you can be damned sure another fat partial or total eclipse will occur 223 new moons later. (54 years + 31 days in the Gregorian calendar.) For example, I remember a fat partial eclipse when I was in college in February 1979. Without knowing anything else, I know there will be another, visible in Illinois, in March 2033. And indeed, a quick Google confirms this to be the case.

However, the triple-Saros is difficult to discover. You need an accurate and stable calendar and good records over a long period of time, and a lot of patience to peruse them and tease out the 223-month pattern. There is some controversy as to who discovered it first. It appears that the Mayans did not know of it.

On shorter scales, there are other cycles that will give you decent probabilities of an eclipse, but nothing as sure as the triple-Saros. Over very short time frames, you will discover the time between lunar and solar eclipses to be one less than a multiple of six (for example, 17 or 23 months). This eliminates some months as eclipse candidates, and raises the probability in others.

Over longer periods of 2-20 years there are other cycles with greater or lesser predictive power. From fragmentary records it can be difficult to determine which cycle or cycles an ancient civilization was using. So, it’s hard to be sure about their predictive accuracy.

So, could the Mayans have expected with confidence a total eclipse? No, no way. Could they have sensed a greater than usual probability of a fat partial or total eclipse? Yes, definitely. How much greater than usual? Can’t say.

10

Correction to my post: A single Saros cycle = 223 synodic months. The triple Saros, optimal for predicting solar eclipses from a given location, = 669 months.

11

This was addressing the OP’s movie scenario. OTOH, I have witnessed two total solar eclipses, and I can tell you that if you thought 30% occlusion was dramatic, you have no idea how dramatic a total is. There is no photo you have seen that even begins to convey it. The onset of totality is sudden, and the drop in light from 99.9% occlusion to totality jawdropping. Then the appearance of a black disk surrounded by the solar corona in the sky so utterly outside normal experience that there is little doubt people would imagine the end of the world was upon them. The experience is worth travelling across the planet for. (I’m lucky the two I have seen were less than a day’s drive away each. A mate of mine tries to see as many as he can, and had travelled to Libya and China as well as to those that have happened here in Oz.) Other strange thing happen. All the wildlife goes into dusk mode, birds especially. It is quite dark. Partial eclipses, and even annular eclipses just don’t even begin to touch the drama of a total.

12

This. I saw the total eclipse of 1999 in France. Awesome (in both senses of the word), jawdropping, amazing, and humbling.

13

Good point. I was basing my “few weeks” estimate on what I have heard was the Ptolemaic systems level of accuracy in predicting planetary conjunctions, but we know new moons come around very regularly quite regardless of any fancy Ptolemaic math, so yes, put those sources of information together and you might do a lot better for actual eclipses.

14

Yep. I was lucky enough to smack dab in the middle of total solar eclipse many years ago, and it’s like someone throwing a light switch. Dogs went berserk.