ARISTOTLE’S PROOFS OF THE SPHERICITY OF THE EARTH Text of a paper, read at the Aristotle 2400 Years World Conference in Thessaloniki, May 24, 2016 There must have been a heated debate among the ancient Greek cosmologists as to the shape of the earth, some of which we can still follow in the reports of the doxographers. Aristotle, who was acquainted with the ins and outs of the debate, intended to terminate it by offering a number of proofs for the sphericity of the earth, but also by arguing against alleged proofs for a flat earth. Since that time, no serious philosopher or astronomer has dared to doubt that the earth is a sphere. Stephen Hawking praised Aristotle for delivering “two good arguments for believing that the earth was a round sphere rather than a flat plane”. 1 Recently, Daniel Graham wrote: “Aristotle defends and indeed proves the sphericity of the earth in the On the Heavens with adequate scientific arguments”. 2 Aristotle’s arguments for the sphericity of the earth can be divided into two groups: empirical and theoretical. 3 In this paper I will evaluate them, to see to what measure they are convincing and also whether they meet reasonable standards for sound argument. Sometimes I will place them against a broader historical background, without any pretention of presenting a complete historical survey. It will prove to be also interesting to see which available arguments Aristotle did not use. 1 Hawking 1988, 2-3. 2 Graham 2013, 95. 3 Simplicius counts five arguments, but the last one has to do with the measurement of the earth’s circumference and thus presupposes the sphericity of the earth (Simplicius, in Mueller 2005, 88-97).