In astronomy Eudoxus is famous for the beautiful theory of concentric spheres which he invented to explain the apparent motions of the planets and, particularly, their apparent stationary points and retrogradations. The theory applied also to the sun and moon, for each of which Eudoxus employed three spheres. He represented the motion of each planet as produced by the rotations of four spheres concentric with the earth, one within the other, and connected in the following way. Each of the inner spheres revolves about a diameter the ends of which (poles) are fixed on the next sphere enclosing it. The outermost sphere represents the daily rotation, the second a motion along the zodiac circle; the poles of the third sphere are fixed on the latter circle; the poles of the fourth sphere (carrying the planet fixed on its equator) are so fixed on the third sphere, and the speeds and directions of rotation so arranged, that the planet describes on the second sphere a curve called the hippopede (horse-fetter), or a figure of eight, lying along and longitudinally bisected by the zodiac circle. The whole arrangement is a marvel of geometrical ingenuity.
Heraclides of Pontus (about 388-315 B. C.), a pupil of Plato, made a great step forward in astronomy by his declaration that the earth rotates on its own axis once in 24 hours, and by his discovery that Mercury and Venus revolve about the sun like satellites.
Menaechmus, a pupil of Eudoxus, was the discoverer of the conic sections, two of which, the parabola and the hyperbola, he used for solving the problem of the two mean proportionals. If a:x=x:y=y:b, then x²=ay, y²=bx and xy=ab. These equations represent, in Cartesian co-ordinates, and with rectangular axes, the conics by the intersection of which two and two Menaechmus solved the problem; in the case of the rectangular hyperbola it was the asymptote-property which he used.
We pass to Euclid’s times. A little older than Euclid, Autolycus of Pitane wrote two books, On the Moving Sphere, a work on Sphaeric for use in astronomy, and On Risings and Settings. The former work is the earliest Greek textbook which has reached us intact. It was before Euclid when he wrote his Phaenomena, and there are many points of contact between the two books.
Euclid flourished about 300 B. C. or a little earlier. His great work, the Elements in thirteen Books, is too well known to need description. No work presumably, except the Bible, has had such a reign; and future generations will come back to it again and again as they tire of the variegated substitutes for it and the confusion resulting from their bewildering multiplicity. After what has been said above of the growth of the Elements, we can appreciate the remark of Proclus about Euclid, ‘who put together the Elements, collecting many of Eudoxus’s theorems, perfecting many of Theaetetus’s and also bringing to irrefragable demonstration the things which were only somewhat loosely proved by his predecessors’. Though a large portion of the subject-matter had been investigated by those predecessors, everything goes to show that the whole arrangement was Euclid’s own; it is certain that he made great changes in the order of propositions and in the proofs, and that his innovations began at the very beginning of Book I.
Euclid wrote other books on both elementary and higher geometry, and on the other mathematical subjects known in his day. The elementary geometrical works include the Data and On Divisions (of figures), the first of which survives in Greek and the second in Arabic only; also the Pseudaria, now lost, which was a sort of guide to fallacies in geometrical reasoning. The treatises on higher geometry are all lost; they include (1) the Conics in four Books, which covered almost the same ground as the first three Books of Apollonius’s Conics, although no doubt, for Euclid, the conics were still, as with his predecessors, sections of a right-angled, an obtuse-angled, and an acute-angled cone respectively made by a plane perpendiular to a generator in each case; (2) the Porisms in three Books, the importance and difficulty of which can be inferred from Pappus’s account of it and the lemmas which he gives for use with it; (3) the Surface-Loci, to which again Pappus furnishes lemmas; one of these implies that Euclid assumed as known the focus-directrix property of the three conics, which is absent from Apollonius’s Conics.
In applied mathematics Euclid wrote (1) the Phaenomena, a work on spherical astronomy in which ὁ ὁριζων (without κυκλος or any qualifying words) appears for the first time in the sense of horizon; (2) the Optics, a kind of elementary treatise on perspective: these two treatises are extant in Greek; (3) a work on the Elements of Music. The Sectio Canonis, which has come down under the name of Euclid, can, however, hardly be his in its present form.
In the period between Euclid and Archimedes comes Aristarchus of Samos (about 310-230 B. C.), famous for having anticipated Copernicus. Accepting Heraclides’s view that the earth rotates about its own axis, Aristarchus went further and put forward the hypothesis that the sun itself is at rest, and that the earth, as well as Mercury, Venus, and the other planets, revolve in circles about the sun. We have this on the unquestionable authority of Archimedes, who was only some twenty-five years later, and who must have seen the book containing the hypothesis in question. We are told too that Cleanthes the Stoic thought that Aristarchus ought to be indicted on the charge of impiety for setting the Hearth of the Universe in motion.
One work of Aristarchus, On the sizes and distances of the Sun and Moon, which is extant in Greek, is highly interesting in itself, though it contains no word of the heliocentric hypothesis. Thoroughly classical in form and style, it lays down certain hypotheses and then deduces therefrom, by rigorous geometry, the sizes and distances of the sun and moon. If the hypotheses had been exact, the results would have been correct too; but Aristarchus in fact assumed a certain angle to be 87° which is really 89° 50', and the angle subtended at the centre of the earth by the diameter of either the sun or the moon to be 2°, whereas we know from Archimedes that Aristarchus himself discovered that the latter angle is only ½°. The effect of Aristarchus’s geometry is to find arithmetical limits to the values of what are really trigonometrical ratios of certain small angles, namely
1/18 > sin 3° > 1/20, 1/45 > sin 1° > 1/60, 1 > cos 1° > 89/90.