Could Thales have known the cause of solar eclipses? Aëtius (A.D. 100), the author of an epitome of an older collection of the opinions of philosophers, says that Thales was the first to declare that the sun is eclipsed when the moon comes in a direct line below it, the image of the moon then appearing on the sun’s disc as on a mirror; he also associates Thales with Anaxagoras, Plato, Aristotle, and the Stoics as holding that the moon is eclipsed by reason of its falling into the shadow made by the earth when the earth is between the sun and the moon. But, as regards the eclipse of the moon, Thales could not have given this explanation, because he held that the earth (which he presumably regarded as a flat disc) floated on the water like a log. And if he had given the true explanation of a solar eclipse, it is impossible that all the succeeding Ionian philosophers should have exhausted their imaginations in other fanciful explanations such as we find recorded.
The key to the puzzle may be afforded by the passage of Herodotus according to which the prediction was a rough one, only specifying that the eclipse would occur within a certain year. The prediction was probably one of the same kind as had long been made by the Chaldæans. The Chaldæans, no doubt as the result of observations continued through many centuries, had discovered the period of 223 lunations after which lunar eclipses recur. (This method would very often fail for solar eclipses because no account was taken of parallax; and Assyrian cuneiform inscriptions record failures as well as successful predictions.) Thales, then, probably learnt about the period of 223 lunations either in Egypt or more directly through Lydia, which was an outpost of Assyrio-Babylonian culture. If there happened to be a number of possible solar eclipses in the year which (according to Herodotus) Thales fixed for the eclipse, he was, in using the Chaldæan rule, not taking an undue risk; but it was great luck that the eclipse should have been total. It seems practically certain that the eclipse in question was that of the (Julian) 28th May, 585.
Thales, as we have seen, made the earth a circular or cylindrical disc floating on the water like a log or a cork and, so far as we can judge of his general conception of the universe, he would appear to have regarded it as a mass of water (that on which the earth floats) with the heavens encircling it in the form of a hemisphere and also bounded by the primeval water. This view of the world has been compared with that found in ancient Egyptian papyri. In the beginning existed the Nū, a primordial liquid mass in the limitless depths of which floated the germs of things. When the sun began to shine, the earth was flattened out and the water separated into two masses. The one gave rise to the rivers and the ocean, the other, suspended above, formed the vault of heaven, the waters above, on which the stars and the gods, borne by an eternal current, began to float. The sun, standing upright in his sacred barque which had endured for millions of years, glides slowly, conducted by an army of secondary gods, the planets and the fixed stars. The assumption of an upper and lower ocean is also old Babylonian (cf. the division in Genesis 1. 7 of the waters which were under the firmament from the waters which were above the firmament).
It would follow from Thales’s general view of the universe that the sun, moon and stars did not, between their setting and rising again, continue their circular path below the earth but (as with Anaximenes later) moved laterally round the earth.
Thales’s further contributions to observational astronomy may be shortly stated. He wrote two works On the solstice and On the equinox, and he is said by Eudemus to have discovered that “the period of the sun with respect to the solstices is not always the same,” which probably means that he discovered the inequality of the four astronomical seasons. His division of the year into 365 days he probably learnt from the Egyptians. He said of the Hyades that there are two, one north and the other south. He observed the Little Bear and used it as a means of finding the pole; he advised the Greeks to follow the Phœnician plan of sailing by the Little Bear in preference to their own habit of steering by the Great Bear.
Limited as the certain contributions of Thales to astronomy are, it became the habit of the Greek Doxographi, or retailers of the opinions of philosophers, to attribute to Thales, in common with other astronomers in each case, a number of discoveries which were not made till later. The following is a list, with (in brackets) the names of the astronomers to whom the respective discoveries may with most certainty be assigned: (1) the fact that the moon takes its light from the sun (Anaxagoras), (2) the sphericity of the earth (Pythagoras), (3) the division of the heavenly sphere into five zones (Pythagoras and Parmenides), (4) the obliquity of the ecliptic (Œnopides of Chios), and (5) the estimate of the sun’s apparent diameter as 1/720th of the sun’s circle (Aristarchus of Samos).
ANAXIMANDER.
Anaximander (about 611–547 B.C.), a contemporary and fellow-citizen of Thales, was a remarkably original thinker. He was the first Greek philosopher who ventured to put forward his views in a formal written treatise. This was a work About Nature and was not given to the world till he was about sixty-four years old. His originality is illustrated by his theory of evolution. According to him animals first arose from slime evaporated by the sun; they lived in the sea and had prickly coverings; men at first resembled fishes.
But his astronomical views were not less remarkable. Anaximander boldly maintained that the earth is in the centre of the universe, suspended freely and without support, whereas Thales regarded it as resting on the water and Anaximenes as supported by the air. It remains in its position, said Anaximander, because it is at an equal distance from all the rest of the heavenly bodies. The earth was, according to him, cylinder-shaped, round “like a stone pillar”; one of its two plane faces is that on which we stand; its depth is one-third of its breadth.
Anaximander postulated as his first principle, not water (like Thales) or any of the elements, but the Infinite; this was a substance, not further defined, from which all the heavens and the worlds in them were produced; according to him the worlds themselves were infinite in number, and there were always some worlds coming into being and others passing away ad infinitum. The origin of the stars, and their nature, he explained as follows. “That which is capable of begetting the hot and the cold out of the eternal was separated off during the coming into being of our world, and from the flame thus produced a sort of sphere was made which grew round the air about the earth as the bark round the tree; then this sphere was torn off and became enclosed in certain circles or rings, and thus were formed the sun, the moon and the stars.” “The stars are produced as a circle of fire, separated off from the fire in the universe and enclosed by air. They have as vents certain pipe-shaped passages at which the stars are seen.” “The stars are compressed portions of air, in the shape of wheels filled with fire, and they emit flames at some point from small openings.” “The stars are borne round by the circles in which they are enclosed.” “The sun is a circle twenty-eight times (v. l. 27 times) the size of the earth; it is like a wheel of a chariot the rim of which is hollow and full of fire and lets the fire shine out at a certain point in it through an opening like the tube of a blow-pipe; such is the sun.” “The sun is equal to the earth.” “The eclipses of the sun occur through the opening by which the fire finds vent being shut up.” “The moon is a circle nineteen times the size of the earth; it is similar to a chariot-wheel the rim of which is hollow and full of fire like the circle of the sun, and it is placed obliquely like the other; it has one vent like the tube of a blow-pipe; the eclipses of the moon depend on the turnings of the wheel.” “The moon is eclipsed when the opening in the rim of the wheel is stopped up.” “The moon appears sometimes as waxing, sometimes as waning, to an extent corresponding to the closing or opening of the passages.” “The sun is placed highest of all, after it the moon, and under them the fixed stars and the planets.”