Eclipses Were Predicted Long Before Copernicus

“If the sun-centered model is wrong, how do astronomers predict eclipses to the minute?” It is one of the most common questions put to anyone who doubts the textbook picture, and it sounds decisive. It isn’t. Eclipses were being forecast for more than two thousand years before Copernicus was born, and the oldest method of all needed no model of the heavens’ shape. It needed only patient eyes and careful counting.

The objection

The question assumes that predicting an eclipse proves the theory behind the prediction. But an eclipse is a repeating event. Watch the sky long enough, write down when the eclipses come, and a pattern appears. Count the months between them, carry the count forward, and you can say when the next ones are due. That is exactly what the oldest astronomers did. As we will see, a working forecast shows that the heavens are regular. It does not, by itself, prove any particular picture of how they are arranged.

“For signs, and for seasons”

Scripture tells us why the heavens can be counted at all. God made the lights to be read by people standing on the ground:

And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years:

He appointed the moon for seasons: the sun knoweth his going down.

Thus saith the LORD, which giveth the sun for a light by day, and the ordinances of the moon and of the stars for a light by night, which divideth the sea when the waves thereof roar; The LORD of hosts is his name:

The moon keeps “ordinances,” and it stands “as a faithful witness in heaven” (Psalm 89:37). The Bible does not teach an eclipse cycle. But it does teach that the lights follow appointed laws, faithfully enough that a careful watcher can count on them. Predictability comes from the Creator’s appointment, not from any one geometric model.

Babylon’s counting method

The priests and scribes of Babylon kept records of the sky for centuries. Their eclipse records reach back to 747 BC, and by about 575 BC they were using a fixed scheme to forecast eclipses. Its heart was a cycle of 223 months, a little over eighteen years, after which eclipses repeat in nearly the same order. Within each cycle they marked thirty-eight months when an eclipse was possible: most six months apart, a few five months apart. Two modern historians of Babylonian astronomy describe it this way:

Throughout most, perhaps all, of the Late Babylonian period, the basic method of predicting eclipses of the sun and moon in Babylonia was based upon the so-called Saros cycle (SC) of 223 synodic months.

Notice what this method did not need. It did not need a moving earth, a stationary earth, a round earth or a flat one. It was arithmetic. The historian of ancient astronomy Alexander Jones draws the contrast:

Greek astronomy was geometrical, whereas Babylonian astronomy was arithmetical. That is to say, the Babylonian astronomers of the last five centuries B.C. devised elaborate combinations of arithmetical sequences to predict the apparent motions of the heavenly bodies, while the Greeks persistently tried to explain the same phenomena by hypothesizing kinematic models compounded out of circular motions.

Honesty requires a limit here. The Babylonians predicted when an eclipse was possible and roughly when it would come, and many of their “possibilities” passed without being seen. They also could not predict whether a solar eclipse would be visible in a particular city. Their forecasts were real but coarse. Even so, they answer the objection completely: eclipses were forecast, from the ground, with no model of the heavens at all.

Where the name “Saros” came from

Today the 223-month cycle is called the Saros, and you will often read that this was the Babylonians’ word for it. It was not. The English astronomer Edmond Halley gave it that name in 1691, borrowing the word from a Byzantine dictionary, the Suda, whose entry is garbled: it describes a period of 222 months, not 223. The cycle is genuinely Babylonian; the name is a seventeenth-century label. It is a small point, but it is the kind of detail that both sides get wrong, so we set it straight.

Thales, Antikythera and Ptolemy

The Greek historian Herodotus tells of a battle between the Lydians and the Medes that stopped when day turned to night. Modern astronomers date that eclipse to 28 May 585 BC. Herodotus adds:

Thales of Miletus had foretold this loss of daylight to the Ionians, fixing it within the year in which the change did indeed happen.

Many scholars doubt that Thales could really have done this, since no method of the time could tell where a solar eclipse would be seen. So we say only that Herodotus reports it. The stronger evidence comes later. The Antikythera mechanism, a bronze geared device made more than two thousand years ago and found in a shipwreck, has a spiral dial of exactly 223 months, marked with the months when eclipses were expected. And around AD 150, the astronomer Ptolemy wrote the Almagest, which taught how to compute eclipses with tables, and which already credited the eighteen-year period to astronomers “even more ancient” than himself.

The tables, and Columbus in 1504

In the thirteenth century, astronomers working for King Alfonso X of Castile produced the Alfonsine Tables, computed by Ptolemy’s earth-centered methods. In 1474 the German astronomer Regiomontanus published daily tables of the sun, moon and planets for the years 1475 to 1506, built on the Alfonsine Tables. These were the almanacs that sailors carried.

In 1504 Christopher Columbus was stranded on Jamaica, and the local people had stopped bringing his men food. His son Ferdinand, who was there, tells what happened next:

The Admiral recalled that in three days’ time, at midnight, there would be a total eclipse of the moon.

Columbus told the chiefs that the moon would rise “inflamed with wrath.” On the night of 29 February 1504, Ferdinand writes, “at the rising of the moon the eclipse began, and the higher it rose the more complete the eclipse became.” The food came back. Ferdinand does not name the almanac his father used. Most historians identify it as Regiomontanus’s tables. Either way, the forecast came from tables computed on an earth-centered model, thirty-nine years before Copernicus published his book.

What modern catalogs do

The standard modern reference is the Five Millennium Canon of Solar Eclipses, published in 2006 by Fred Espenak and Jean Meeus for NASA. It lists nearly twelve thousand eclipses, and it groups them by Saros series, a numbering scheme introduced in 1955. You will sometimes hear that “NASA still uses the Saros to predict eclipses.” That is not quite right. The catalog is organized by Saros series, but its predictions are computed from modern tables of the sun’s and moon’s motions, and they are far more precise than any ancient method: exact paths across the ground, timed to the minute.

Both things are true. The ancient cycles really worked, and modern computation works much better. But better tables are still tables of observed regular motion. Precision shows how faithfully the lights keep their ordinances. It does not settle, on its own, what the heavens are shaped like.

The selenelion

One curiosity has puzzled observers since ancient times. Pliny the Elder recorded that once a lunar eclipse was seen “in the west, while both the luminaries have been above the horizon,” that is, the eclipsed moon was visible while the sun was still up. This is called a selenelion, and it is real; it has been photographed many times. On the textbook account, the sun and moon should be on exactly opposite sides of the earth during a lunar eclipse, so both should never be visible together.

The mainstream explanation is refraction: near the horizon the air bends light enough to lift both the sun and the moon about half a degree, so both can be seen when, by straight-line geometry, they are just below the horizon. That effect is real and can be measured. So we present the selenelion as a genuine observation worth wondering at, not as a knock-down proof. Pliny himself found it marvelous but did not let it overturn his view.

The honest bottom line

“You can’t predict eclipses without the sun-centered globe” is simply false. The Babylonians predicted them by counting months, with no model of the heavens at all. The Greeks and medieval astronomers predicted them with an earth-centered model. Columbus used an earth-centered almanac to forecast one in 1504. All of it happened before Copernicus published a word.

At the same time, we should not claim more than the evidence gives. Most of those predictors believed in a round, stationary earth, and modern methods are far more precise than ancient ones. What eclipse prediction proves is that the lights keep faithful, countable laws. Scripture said so first. For where the lights are set, see The Firmament; for the experiments that tried to measure the earth’s motion directly, see The Experiments That Couldn’t Find the Earth Moving.

It shall be established for ever as the moon, and as a faithful witness in heaven. Selah.

Further reading