IDEAS ABOUT THE UNIVERSE

AS long as ago as 340 B.C.Aristotle, in his book On the Heavens, was able to put forward two good arguement for believing that the Earth was a round ball rather than flat plate. First, he realised that eclipses of the moon weren caused by the Earth coming between the sun and the moon. The Earth's shadow on the moon was always round , which would be true only if the Earth was would have been elongated and elliptical, unless the eclipses always occurered at a time when the sun was directly above the centre of the disk.

Second, the Greeks knew from their trafels that the Pole Star appeared lower in the sky when viewed in the south than it did in more northerly regions.From the difference in the apparent position of the Pole Star in Egypt and Greece, Aristotle even quoted an estimate that the distance around the Earth was four hundred thousand stadia. It is not known exactly what length a stadium was, but it may have been about two hundred yards. This would make Aristotle's estimate twice the currently accepted figure. The Greeks even had a third arguement that the Earth must be round, for why else does one first see the hull? Aristotle thought that the Earth was stationary and that the sun, the moon, the planets, and the stars moved in circular orbits about the Earth was the centre of the universe and that circular motion was the most perfect. This idea was elaborate by Ptolemy in first centuary A.D. into a complete cosmological model. The Earth stood at the centrew, surrounded by eight spheres, which carried the moon, the sun , the stars, and the five planets known at the time: