better value of 18 years, equal to 223 months (the so-called Saros period). Geometry Distinction of theorems and problems While Oenopides's innovations as an astronomer mainly concern practical issues, as a geometer he seems to have been rather a theorist and methodologist, who set himself the task to make geometry comply with higher standards of theoretical purity. Thus he introduced the distinction between 'theorems' and 'problems': though both are involved with the solution of an exercise, a theorem is meant to be a theoretical building block to be used as the fundament of further theory, while a problem is only an isolated exercise without further follow-up or importance. Compass and straightedge Oenopides apparently also was the author of the rule that geometrical constructions should use no other means than compass and straightedge. In this context his name adheres to two specific elementary constructions of plane geometry: first, to draw from a given point a straight line perpendicular to a given straight line; and second, on a given straight line and at a given point on it, to construct a rectilineal angle equal to a given rectilineal angle. Miscellaneous opinions attributed to Oenopides Several more opinions in various areas are attributed to Oenopides: He is said to have given an explanation of the flooding of the Nile each summer. On the basis of observations of the temperature of water in deep wells he seems erroneously to have inferred that underground water is in fact cooler in summer than in winter. In winter, when rain fell and seeped into the ground it would soon evaporate again because of the heat in the soil. However, in summer, when water in the ground was supposedly colder, there would be less evaporation. The surplus of moisture would then have to be carried off otherwise, thereby causing the Nile to overflow. To Oenopides was attributed the opinion that formerly the Sun had moved along the Milky Way. However, when it saw how Thyestes, a mythological figure, was served his own son for dinner by his brother Atreus, the Sun was so horrified that it left its course and moved to the zodiac instead. Oenopides was said to have regarded the universe as a living organism, God or the Divine being its soul. He is also said to have considered air and fire as being the first principles of the universe. Notes References István M. Bodnár, Oenopides