When the sun’s yearly course in the heavens had been determined, it was found that it was restricted to that band of stars called the Zodiac, Fig. 2; the sun’s position in the zodiac at any one time of the year being found by the midnight culmination of the stars opposite the sun; this and the apparent and heliacal risings and settings were alone the subjects of observation. It is obvious, then, that when observations of this nature had gone on for some time, men would be anxious to map the stars, to make a chart of the field of heaven; and such a work was produced by Autolycus three and a half centuries before Christ. We also owe to Autolycus and Euclid, who flourished about the same time (300 B.C.), the first geometrical conceptions connected with the apparent motions of the stars. In the theorems of Autolycus there is a particular reference to the twelve parts of the zodiac, as denoted by constellations. The following are the most important propositions which he lays down:— 1. “The zodiacal sign occupied by the sun neither rises nor sets, but is either concealed by the earth or lost in the sun’s rays. The opposite sign neither rises nor sets, _i.e._, visibly, _i.e._, after sundown, but it is visible during the whole night. 2. “Of the twelve signs of the zodiac, that which precedes the sign occupied by the sun rises visibly in the morning; that which succeeds the same sign sets visibly in the evening. 3. “Eleven signs of the zodiac are seen every night. Six signs are visible, and the five others, not occupied by the sun, afterwards rise. 4. “Every star has an interval of five months between its morning and its evening rising, during which time it is visible. It has an interval of at least thirty days—between its evening setting, and its morning rising—during which time it is invisible.” (That is, the space passed over by the sun in its annual path is such that a star which you see on one side of the sun, when the sun rises at one time, would be seen a month afterwards on the other side of the sun.) Autolycus makes no mention of the planets. Their irregular movements rendered them unsuited to the practical object which he had in view. He is, however, stated by Simplicius, as quoted by Sir G. C. Lewis to have proposed some hypothesis for explaining their anomalous motions, and to have failed in his attempt. Euclid carries the results arrived at in this early pre-telescopic age much further; in a little-known treatise, the _Phenomena_,[3] he thus sums up the knowledge then acquired:—