Not so; you also need to know the period of regression of the nodes. And the month used with respect to eclipse prediction is the synodic month, not the sidereal month.
Eclipse prediction is not an all-or-nothing science. You can be right part of the time. No doubt the first successful eclipse prediction was made by some hunter-gatherer, speaking on the purest of hunches, saying “I think we’re about due for an eclipse this month”, and getting lucky.
With a little more sophistication, one can note that eclipses tend to be spaced six months apart. Over longer periods of time, they tend to be spaced by numbers one less than a multiple of six, such as 17 or 23 months. You can never have eclipses, either solar or lunar, that are three or nine months apart.
To advance beyond this stage, a civilization needs to develop writing and an accurate calendar, and be stable enough to preserve records over a long period of time. The basic periodicity for solar eclipses is the 57-year triple Saros cycle, so you need to maintain accurate records for at least that long to reliably predict total solar eclipses.
Given the necessary records, discovering the 57-year pattern isn’t all that difficult. The ancient Mayans, Babylonians, Greeks, and Chinese all did it. The ancient Egyptian silence on eclipses is puzzling, but I suspect that they broke the code as well. Herotodus’ story about Thales is the first attribution of a successful eclipse prediction to a specific individual, but that’s more a tribute to Greek history than to Greek astronomy. The sad truth is that the first person to make an really reliable prediction of a total solar eclipse, based on the triple Saros cycle, was probably some faceless palace bureaucrat.
I wasn’t talking about using the lunar sidereal period to compute eclipses–I was talking about observing the path of the moon through the stars over the sidereal period, many periods.
I am fairly certain that the earliest predictions were not a result of accurate eclipse record keeping over hundreds of years, but rather astronomical record keeping in general.
I must differ. The difficulties in constructing an ephemeris accurate enough to predict eclipses without reference to earlier eclipses, simply by extrapolating movement among the background stars, are formidable. I agree with the author of this article:
Even after eclipses could be forecast from ephemerides, it must be borne in mind that the accuracy of the ephemerides themselves were partly a product of eclipse records. Ptolemy stated in the Almagest that his incredibly accurate values for the synodic and nodical months were calculated by comparing eclipses centuries apart. I firmly believe that the “chicken” of eclipse cycles was recorded before the “egg” of accurate ephemerides.
Fred Espanek’s (“Mr. Eclipse”) timeline mentions that in the 6th century BC, “Babylonians (Chaldeans) are said to be able to predict eclipses of the Sun and Moon, supposedly based on cycles such as the saros, but more likely from the positions of the Moon’s nodes.” I’ll have to try it.
More than just predicting the day, it amazes me that astronomers were able to predict the time, duration and path of solar eclipses, without the use of computers.
Anybody know who was the first to correctly predict the time and duration of a total solar eclipse?
That had to await the development of precision astronomical instruments, Arabic numerals, calculus, and an understanding of Kepler’s laws of planetary motion. Sir Edmund Halley published reasonably accurate predictive maps for the eclipse of 1715. Friedrich Bessel all but perfected the science of eclipse prediction in the Nineteenth Century.
It is however worth noting that, precisely because the eventual attainment of accuracy depended on this whole series of improvements, the prediction of solar eclipses was something that had got better over the centuries.
The Babylonians, for instance, had had good methods of predicting lunar eclipses and these could be adapted to produce tentative solar predictions. While in most cases they would predict a solar eclipse and one wouldn’t be seen, they would at least have predicted one when it did happen.
By Ptolemy, methods had reached the stage where he could be reasonably confident in predicting that an eclipse was going to be visible somewhere. Presumably based on this sort of method, there are plenty of cases through late antiquity and the middle ages where individuals are recorded as having predicted particular solar eclipses.
Copernicus was sufficiently confident about making predictions that there’s a chapter in De Revolutionibus explaining how to calculate the duration of a future eclipse (it’s at the end of Book IV). In 1604 the standards were such that Kepler knew that the path of totality for the eclipse in October 1605 was going to cross southern, but not northern, Europe and so alerted colleagues there via his book Astronomiae pars optica, appealing to them to closely observe it.
What’s interesting about 1715 is that that’s the first case where someone - Halley - published a map predicting the path of totality. Furthermore, he published it in the form of a broadsheet, so the prediction was confident, definite and intended for a wide audience.
And it’s little known that, though the irate emperor sentenced them to death, he felt merciful before their execution, and commuted their sentence to labor in the royal mines. In gratitude, the two dimunitive astronomers composed a song which is still sung in America today, and in fact was included in a movie…
