A Short History of Astronomy 



[CH. II. 



different places, and that it was recognised that, if a traveller 

 were to go far enough north, he would find the pole to 

 coincide with the zenith, whereas by going south he would 

 reach a region (not very far beyond the limits of actual 

 Greek travel) where the pole would be on the horizon 

 and the equator consequently pass through the zenith ; in 

 regions still farther south the north pole would be per- 

 manently invisible, and the south pole would appear above 

 the horizon. 



Further, if in the figure H E K w represents the horizon, 

 meeting the equator Q E R w in the east and west points E w, 

 and the meridian H Q z p K in the south and north points 



H and K, z being the zenith 

 and P the pole, then it is 

 easily seen that Q z is equal 

 to P K, the height of the 

 pole above the horizon. 

 Any celestial body, there- 

 fore, the distance of which 

 from the equator towards 

 the north (declination) is 

 less than p K, will cross 

 the meridian to the south 

 of the zenith, whereas if 

 its declination be greater 

 than p K, it will cross to 

 the north of the zenith. 

 Now the greatest distance 



of the sun from the equator is equal to the angle between 

 the ecliptic and the equator, or about 23^. Consequently 

 at places at which the. height of the pole is less than 23! 

 the sun will, during part of the year, cast shadows at midday 

 towards the south. This was known actually to be the case 

 not very far south of Alexandria. It was similarly recog- 

 nised that on the other 'side of the equator there must be 

 a region in which the sun ordinarily cast shadows towards 

 the south, but occasionally towards the north. These two 

 regions are the torrid zones of modern geographers. 



Again, if the distance of the sun from the equator 

 is 23 |, its distance from the pole is 66^; therefore in 

 regions so far north that the height p K of the north pole 



FIG. 15. The equator, the horizon, 

 and the meridian. 



