PUBLICATIONS of the ASTRONOMICAL SOCIETY of the PACIFIC Vol. XLI San Francisco, California, October, 1929 No. 243 the OLDEST

PUBLICATIONS OF THE

ASTRONOMICAL SOCIETY OF THE PACIFIC

Vol. XLI San Francisco, California, October, 1929 No. 243

THE OLDEST ASTRONOMERS KNOWN

By Professor H. H. Turner

I gladly respond to the suggestion that I should write for the Astronomical Society of the Pacific something about the old astronomers, for, though California has the best possible opportunities for appreciating the most recent advances in astronomy, it may not as yet be so well equipped for learning of the steadily growing knowledge of the past, a growth in the last quarter of a century nearly as remarkable as that provided by the splendid instruments on the Pacific shores. Who were the earliest astronomers of note? It is a question which appeals to a public wider than that of astronomers themselves. The whole world feels an interest in those early heroes who, in loneliness almost certainly, and perhaps in the face of active opposition, laid the sure foundations on which our present-day science was built. And the correct answer to this question was not known, not even suspected, a quarter of a century ago. Dr. J. K. Fotheringham, Reader in Ancient Astronomy and Chronology in the University of Oxford, re- cently collected the main facts in a public lecture with the title "The Indebtedness of Greek to Chaldean Astronomy," printed in the Observatory magazine for October, 1928, and he dates our first inkling of the correct answer from 1906, twenty-three years ago. Before then we should have given the Greeks the credit for being the first serious astronomers, the Chaldeans being regarded merely as astrologers who tried to foretell events by the stars. A single example will suffice to illustrate this attitude. Writing in 1862 a Historical Survey of the Astronomy of the Ancients, Sir George Cornewall Lewis begins with the Greeks ; though in some concluding chapters he refers to the Babylonians (Chaldeans) and Egyptians as having made "rude

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observations unassisted by instruments and doubtless irregu- larly and imperfectly recorded ; it may be reasonably suspected that they were directed particularly to phenomena, such as eclipses, to which a superstitious interest attached." "We cannot," he continues, "consistently with the capacity and tendencies of the Oriental mind, suppose that either of these nations ever rose to the conception of astronomy as a science." He is inclined to ascribe the traditional origin of astronomy in Babylon to the fancies of the Greeks themselves. "The Greeks were so much in the habit of finding a fabulous origin for every useful art and invention that the stories which connected the original cultiva- tion of astronomy with the Babylonians, the Egyptians, and the Phoenicians would not, of themselves, deserve more credit than the legend which made Amphion the inventor of music" (p. 262). Now let us turn to Dr. Fotheringham's lecture, and learn something of Naburiannu and Kidinnu, two Chaldeans whom the Greeks called Naburianos and Cidenas, who "are entitled to a place among the greatest of astronomers," but whose achievements have been hitherto ascribed to others. Until 1906 Cidenas was known only from a passage in Pliny as having made a statement about the planet Mercury, and Naburianos was entirely unknown. Since 1906 they have become known (through Greek texts printed for the first time between 1906 and 1926, and Babylonian texts published from 1900 to 1926), as authors of systems for computing the places and motions of the Sun and Moon. Much of this knowledge is due to the industry of German Jesuit Fathers, Epping, Strassmaier, and Kugler; others who have contributed are Kroll, Schiaparelli, Weidner, and Schnabel. Let us do honor to these names, for they have taught us to know and honor the great names of Naburiannu and Kidinnu, thus removing one of Sir G. C. Lewis's strongest arguments against the existence of serious astronomy in Chaldea, that no great names had come down to us. He writes, "If any of the Chaldaean and Egyptian priests had really possessed the profound and exact knowledge of astronomy which is attributed·to them in a body, we should probably have heard his name. Someone would have gained an

individual reputation: his writings or his discourses would have become celebrated ; and he would have been distinguished from the mass. Neither Plato nor Aristotle, nor any of the earlier writers, however, name any Babylonian or Egyptian in reference to astronomical observations" (p. 288). It is an impressive argument, but a dangerous one; rather like invoking the testimony of witnesses who did not see the crime committed. Apparently the fault was with the witnesses, especially Aristotle, to whom his nephew, Callisthenes, sent a collection of Babylonian observations by request, an incident which Dr. Fotheringham calls "the great event in the develop- ment of exact astronomy in Greece." But Aristotle apparently forgot to leave on record the names of the observers to whom he was indebted, though we owe to him so much in other ways that we can afford to forgive this omission. It is not surprising to find that there is a certain amount of truth in the views of Sir G. C. Lewis, in spite of their being essentially at variance with this modern view. It was by the study of eclipses that Naburiannu and Kidinnu arrived at their wonderfully exact knowledge; and it may well have been that this study had its origin in the "superstitious interest attaching to them." The important thing was that the study was suffi- ciently protracted and systematic to reveal the recurrence of eclipses at nearly the same place after an interval of fifty-four years and one month. Once this essential fact was grasped (and clearly it could not have been realized without at least a century or two of watching and recording), it is pretty clear that there would be a real interest in verifying it at each successive repetition. But this could not be done without a civilization which could preserve continuity of record over long periods, through priestly traditions or otherwise. Apparently the Chaldeans had such a measure of civilization, so that they accumulated observations of the eclipses of the Sun and Moon over many centuries, from which Naburiannu and Kidinnu deduced such accurate values for the movements of these bodies that in one instance they actually beat the best modern knowledge. "So it seems," writes Dr. Fotheringham, "that Cidenas's (Kidinnu's) Canon of Eclipses actually contained a better value

for this motion than that which is used in our standard modern Canon. You may imagine how much better Cidenas's value was if you compare it, not with Oppolzer's value for his own day, but with Oppolzer's value for Cidenas's day." Let us turn aside for a moment to consider how the study of ancient eclipses, even without telescopes, is capable of giving us results of such surprising accuracy. In the figure let aOA represent a portion of the Sun's apparent path in the heavens, and bOB a portion of the Moon's path. The two paths are inclined at the small angle of five degrees and intersect each other at a point 0, called the node. There are two such nodes,

Fig. 1

the other being always at the opposite point of the heavens ; though both of them change their places slowly. It takes a month for the Moon to go completely round the heavens, a year for the Sun to go completely round, and about 19 years for the node to go completely round. But these are only rough values ; the great question is. What are the accurate values? and "aris- ing out of that" as they say in Parliament, will the values be such that the same positions come round again? We can more readily visualize the second question if we think of three objects that can be actually seen such as three planets. Mars, Jupiter, and Saturn. On a rare occasion we might see them all shining close together, and should naturally inquire when such a coin- cidence or "conjunction" would again occur. Knowing the times of revolution of Mars and Jupiter, we could easily cal- culate when they would come together again, but should probably find that Saturn would not be anywhere in their neigh- borhood. If it happened to be (say) about ninety degrees (or a quarter, of the sky-circuit) away, then we might expect a triple conjunction after four times the interval just calculated. But probably its distance would not be such a simple fraction

of the circuit as one-fourth ; it might be between one-eleventh and one-twelfth, say; and then at the eleventh conjunction of Mars and Jupiter, Saturn would not be near enough ; at the twelfth it would have gone too far. In fact it would be, so to speak, a piece of good luck if the periods happened to fit so that the triple conjunction came round in a reasonable time. Now the period of fifty-four years and one month repre- sents just such a piece of good luck, for the Sun, the Moon, and the node. We cannot see the node ; we can only infer its posi- tion from the way in which the Moon covers up part of the Sun. It is seen in the figure how if the Moon (dark) passes the Sun (bright) at A/", which is far from the node O, it will only cover up a little of it; if at P, which is nearer the node, it will cover more of it. At the node itself the Moon would just fit the Sun and cover it up completely; whereas at the dis- tant positions M and Q it will just graze the Sun, and beyond them it will miss it altogether. How much of the Sun is eclipsed depends on the distance of the node when the Moon passes the Sun (or vice versa; but for simplicity we consider only solar eclipses). And by a piece of good luck, the node comes round again very nearly to the same distance after the lapse of this period of fifty-four years and one month. The italicized words very nearly remind us that the recurrence is not exact, so that the conditions slowly change. The eclipse recurs at the same place a good many times but is slowly moving either northward or southward and will ultimately cease to recur at that place. Moreover, it changes slightly in character (duration, time of day, et cetera) at each repetition. Enough has perhaps been said to show how the Chaldean astronomers, once they had realized the reasons for these re- currences, were able to infer such accurate results. But it was no small thing to realize the existence of the node and its movements in relation to those of the Sun and Moon. It made them astronomers of high rank, and entitled their observations to a leading place among any observations ever made. Let us again quote from Dr. Fotheringham's lecture : "Cidenas had at his disposal the recorded magnitude of every lunar eclipse visible at Babylon for three hundred and

sixty years No modern astronomers have made any such collection of observations It is the rarest thing for a modern observatory to continue a program of observations through thirty years, let alone three hundred and sixty. The only modern observations that can compare in continuity with the Baby- lonian are the Greenwich Meridian observations, which have been maintained since 1750. Against these I have nothing to say, but their analysis is not so simple as it once seemed, and we could do with another Cidenas.''

Oxford University July, 1929