Advanced societies knew when an eclipse was coming because they had detailed tracking info for the Sun and moon. That tracking info could let them know about an eclipse years in advance. But if someone didn’t have all that tracking info and was just looking at the positions of the Sun and moon in the sky, would they be able to tell an eclipse was coming in a few days? Is there something about the paths of the Sun and moon in the preceding days that an observant person could notice to indicate that are about to overlap?
In the days leading up to a solar eclipse, the moon wouldn’t even be visible, since that would correspondend to a new moon: the visible disk of the moon is its completely shadowed side (“the dark side of the moon”) and it’s in the sky in the full daylight hemisphere of the Earth, so completely swamped by sunlight. And all of that would be true of any new moon phase.
Even a day off, you’d be able to see a slight sliver of the Moon. The real problem would be landmarks to compare to: You’d definitely see the Moon and Sun getting closer together, but you wouldn’t know the direction of their relative movement precisely enough without something to compare to. Stars would be the obvious choice, but you can’t see those at the same time as the Sun.
By the time a society got to the point they could carve wood carefully, even if they have no mathematics, they could make something akin to an altazimuth mount that would let them trace the Sun’s path today, and therefore make a pretty close prediction of the Sun’s path the next few days.
By leaving the settings for the Sun’s path but aiming that contraption at the Moon’s azimuth over the few days leading up to the new moon each month they could pretty readily determine whether an eclipse was impossible, unlikely, possible, or quite likely within the tolerances of their instrument. Just by comparing the Moon’s altitude and altitude trend with the setting derived from the Sun.
Even given near perfect tolerances that did predict a close sun/moon pass that would not tell them whether the eclipse would be total or annular, nor whether it would be visible from their location. That would need detailed timing and distance info that instrument alone could not provide.
Did any ancient civilization do that? I have no idea. But fairly simple tools can give a decent approximation if they knew to build them. Which requires at least a qualitative theory of how the Sun and Moon move relative to one another.
That’s a little more advanced than I was thinking. I was wondering more about people who wouldn’t necessarily think about tracking the Sun with improvised devices. People who work outside a lot notice things about the environment which indicate a future event, such as a green sky preceding a tornado or ocean water receding preceding a tsunami. I was wondering about things like that. Someone who is used to looking at the sky, Sun and moon as part of their normal day might look up and notice something unusual about the positions or paths of the Sun and moon that they hadn’t seen before. Even if they didn’t know it would lead to an eclipse at that moment, they would probably make the connection afterwards.
Observing the azimuth of sunrise and moonrise (or setting) would give a clue. If the two are pretty much the same the moon is passing through the ecliptic (guess how it got that name.) So the moon is travelling though the same arc in the sky as the sun. As it drops back, there is a good chance there will be an eclipse.
A lunar eclipse is visible everywhere that the moon is visible, so you have a 50/50 chance of seeing it. And so are pretty common. Predicting one of those would become fairly easy even for a very basic skywatcher with no tools. Just observing the locations on the horizon of the sun and moon as they rise and set, and also observing the gap between the two close to new or full moon would get you there once you have a few years of observing experience.
Solar eclipses are the same problem, at least for partial eclipses. Basic observing would get you into the ballpark of being able to predict that the moon might graze the sun. But the geometry of a total solar eclipse is so tight that you would need at least basic instruments, and a theory of the motions. The subtle difference in angular size of the moon though its elliptical orbit would be imperceptible without careful measurement - and thus the ability to predict annular from total also impossible without those measurements.
So, if you observed the full moon rise closely followed by the sun at exactly the same location on the horizon, there is a very good chance of some sort of solar eclipse. But predicting a total eclipse versus a partial would be near impossible.
If people could see the positions, they could see they were aiming right for each other and closing fast. An eclipse is just a collision of the positions in the sky. Not the three dimensional positions in space, mind you – just the positions as they would appear in the sky (if visible).
Herodotus (the Father of History, and the Father of Lies) maintained that the 585 BC eclipse was predicted by Thales.
There is some doubt about that, but it is possible and plausible.
No, absolutely not. There are no such signs. Ancient civilizations predicted eclipses in the only way possible before accurate instruments and knowledge of celestial mechanics–by keeping records of earlier eclipses and teasing out the patterns within them.
The gold standard in eclipse patterns is the triple-Saros cycle–per which, if an eclipse of a given magnitude occurs at a given location on a given day, another eclipse of similar magnitude will occur at that same location exactly 669 new moons (54+ years) later. Obviously, you need good records over a substantial period of time to discover that cycle, but the ancient Babylonians and Greeks (and possibly others) did it.
There are other, shorter cycles with less predictive power, which can tell you that an eclipse is more likely or less likely, but not guaranteed.
That’s a bit harsh. Did you read the thread before posting?
There certainly are indications a couple weeks in advance that the moon is about to cross the ecliptic quite near the Sun. If “quite near” turns out to be “partly overlapping” or “dead on”, that’s a partial or total eclipse. It’s not a precise thing, and can’t practically be used to predict distant future eclipses. As you say, that takes long records and diligent thought.
But a week out from New Moon every month one can confidently say “No eclipse for sure”, “Might happen”, “Will probably happen though we may not see it here” just from visual observation of the Sun & Moon with zero recordkeeping.
Assuming very little change in declination between the Sun and Moon in a couple of days, if I trace a path from the center of the crescent moon arc along the line of latitude of declination (a curve from the perspective on the surface of the Earth), would it pass through (within tolerances) the center of the Sun if and only if a total or annular eclipse will occur?
Not true. The azimuth of an object at time of rising, from a given latitude, depends only on the declination of the object. Declination and ecliptic latitude are too very different things. It’s possible for two objects to be at the same declination and at very different distances from the ecliptic.
I never noticed that until now! Duh.
Everything that I posted was completely correct and on point to the OP.
Can you tell just by looking at the moon how close it is to the ascending or descending node? If so you are a more skilled observer than I.
Darn, quite right. I think I was getting a trifle over enthusiastic.
@Freddy_the_Pig: I too was overly enthusiastic. Oops on me.
Thanks for setting me (and the record) straight.