Projecting The Pattern Forwards

How far back do the eclipse records of Babylon go? Solar eclipse notations that may be unambiguously interpreted and dated start from 700 B.C., but most postdate 350 B.C. On that basis, assuming that at least a century of records would be needed to decipher the saros, it would seem unlikely that eclipse prediction based on those records would have been possible much before 250 B.C.

Who, then, was first to predict a total solar eclipse correctly? This is a question over which historians of astronomy have argued a great deal, because there is an apparent prediction from much earlier than that.

Herodotus (484—425 B.C.) was a Greek historian who wrote most of the surviving accounts of his era and earlier. He claimed that Thales of Miletus (see Figure 3-3) predicted the solar eclipse in 585 B.C. that occurred during a battle between the Medes and the Lydians. (Lydia was the western end of Asia Minor, where the city of Miletus was located.)

Thales does seem to have understood the rudiments of solar eclipses, recognizing that they are due to the Moon passing in front of the Sun, although in his day the nature of orbits was unsuspected. Thales thought of the Earth as a flat disk floating on a great sea, the Sun and Moon being other disks moving above it, and sometimes they happened to align. The suggestion of the Earth circuiting the Sun remained some time off. Aristarchus of Samos proposed the concept in the third century B.C., but it was not until after the Copernican revolution in the sixteenth century that the idea gained wider acceptance, in the face of ecclesiastical opposition.

The 585 B.C. eclipse certainly seems to have caused the Medes and the Lydians to reconsider their hostile intent and agree to a peace treaty after five years of war, each seeing it as an omen; however, it is not clear that Thales predicted its date and circumstances. We are able to back-calculate to show that the path of totality on the afternoon of May 28 swept along the Mediterranean and fairly centrally from west to east across Asia Minor, where

FIGURE 3-3. The pioneering Greek mathematician Thales is often credited with making the first prediction of a total solar eclipse, although historians of science now doubt whether he did more than suggest that such an eclipse would occur in a certain year. Thales was from Miletus, a town on the western coast of what is now Turkey. The eclipse track passed across that area in 585 B.C., bringing to an end a long-standing war between rival peoples.

the armed dispute was taking place and the Sun was blanked out for over six minutes.

It was a very unusual event, but Herodotus wrote only that Thales gave the year, so one might wonder whether it was a true prediction or just a lucky guess. Predicting that a partial solar eclipse will occur is fine, but getting a total solar eclipse right is another thing entirely. On balance it seems that Thales and his contemporaries did not know how to foresee eclipses by any means other than the short-term relations like the ten-day shifts from one year to the next. Hipparchus used eclipse data, and the saronic cycle, to ascertain accurate values for the year and the lunar months, but did not make forward eclipse predictions.

The eclipse knowledge gathered by the Babylonians lay dormant for many centuries. Hipparchus and others knew that the year was not exactly 365.25 days long, and yet the Julian calendar leap-year cycle based upon that length persisted until the sixteenth century. In the same way, the detailed cycles making eclipse prediction possible were not to be used for a long time.

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