THE FLYING EARTH 113 



At any given moment of time the horizon of the earth divides 

 the sphere of the heavens exactly in two, and this is true, not 

 merely through each twenty-four hours, but throughout the 

 year. It follows, therefore, that neither the great mass of the 

 earth nor its distance from the middle point of the world occasions 

 the slightest difference in this continuous bisection of the 

 heavens. Did either the one or the other enter into considera- 

 tion, we should see always less than half the sky. Test it 

 any moment and you will see that this is never the case. Long 

 observation had taught the ancients to fix, with a precision 

 still of great value, the exact position of the different constella- 

 tions of the zodiac. They knew, for example, the beginning 

 of the sign of Capricorn was precisely opposite, in the zodiacal 

 circle, to that of the Crab. Take now one of their old diopters 

 or horoscopes, or sight across a water-level, and you see that 

 at the moment the Crab " rises," Capricorn " sets " ; a line 

 from one to the other is a diameter of the zodiacal circle, or, 

 as we say, of the plane of the ecliptic. Now when the heavens 

 have turned about so that we just see Capricorn rising, the 

 Crab is just slipping under the opposite horizon. You may 

 draw a circle, if you like, just as did Ptolemy or the unnamed 

 genius who first worked it out, like this : 



FIG. ii. 



You have now two diameters which cross a common centre, 

 and it is easy to show that this centre is likewise the centre 

 of the earth. Lines from the surface of the earth and from 

 its centre to a common point cannot be the same ; neverthe- 

 less, so far as the observation of the stars reveals, they are not 

 only parallel, but, on account of the vast distance of the stars, 

 one and the same line. The half diameter of the earth, there- 

 fore, is, with respect to the distance, an entirely negligible 

 quantity. Not merely this, but if you believe, as Aristarchus 

 did, that the earth moves round the sun as a centre, and that 



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