‘ous to us as the earth orbiting the sun—or perhaps as ridiculous as ‘tower of tortoises. Only time (whatever that may be) wil tell ‘As long ago as 340 Rc the Greek philosopher Aristotle, in his book On the Heavens, was able to put forward two good argu- ments or bieving that he earth fat plate by the earth coming between the sun and the moon, The earth's ‘shadow on the moon was always round, which would be tre only it the earth was spherical. If the earth had been a flat dik, the ‘shadow would have been elongated and cliptica, unless the eclipse always occured at time when the sun was diretly under the ‘centr ofthe disk. Second, the Greeks knew from their travels that the North Star appeared lower inthe sky when viewed inthe south than it did in more northerly regions. (Since the North Star tes ‘over the North Poe it appears to be det above an observer the North Fole, but to someone looking from the equator, it ‘appears to lie just at the horizon) From ehe difference in the apparent postion ofthe North Sa in Egypt and Greece, Aristotle ‘even quoted an estimate thatthe distance around the earth was 400,000 stadia tis not known exactly what length a stadium was, ‘but it may have been about 200 yards, which would make Aristo. {e's estimate about twice the cuttnty accepted figure. The Greeks ‘ven had a third angument thatthe earth must be round, for why «lsc does one firs se the sails ofa ship coming over the horizon, ‘and only later see the hull? Aristotle thought thatthe earth was stationary and that the ‘un, the moon, the planes, and the stars moved in ccular orbits about the earth, He believed this because he felt for mystical reasons that the earth was the center of the universe, and that reular motion was the most perfect. This idea was elaborated by lemy in the second century AD. into 4 complete cosmological ‘model The earth stod atthe centr, surtounded by ight spheres that carted the moon, the wun, the star and che five planets Known at the time, Mercury, Venus, Mats Jupiter, and Saturn (Fig, 1.1). The planes themselves moved on smaller circles at- tached to their respective sphetes in order to account for thee rather complicated observed path in the sky, The outermost sphere ri