6 BROWN : 



For one on the other Tropic, on the same meridian and at 

 the same hour, the sun is 47 degrees north of the zenith. 



The sun is vertical at noon twice a year for all parts. of the 

 Torrid Zone, as will be instantly manifest when we remember 

 that precisely one year elapses between the departure of the 

 sun southward from Cancer and his return northward to the 

 same declination. 



For an observer on the equator the two poles are in the 

 horizon ; and the equator of the heavens divides the visible 

 vault into two equal portions. It follows that at the equi- 

 noxes the sun rises at the east point and passes in a circular 

 arch directl}' overhead to the west point. 



The observer at the equator sees theoretically every star in 

 the heavens in the course of the year. To the same observer 

 the sun's daily course is like that of the stars perpendicular to 

 the horizon, whatever be his declination. It follows that he is, 

 on any given day, as long a time below the horizon as he is 

 above it, and that day and night are equal throughout the year. 

 It is for this reason that the equator is called the equinoctial. 



It is a necessary consequence of all this and of what has 

 been said concerning the starry sphere as seen at the poles, 

 that at all points between the equator and the poles there are 

 certain stars near the one pole that never rise and others near 

 the other pole that never set. These last are said, somewhat 

 grandiloquently, to be within the circle of perpetual appari- 

 tion ; the others within the circle of perpetual occultation. 

 They are also called, more simply, circumpolar stars. The 

 greatest possible number of circumpolar stars is, of course, 

 one half of the whole. 



This word "number" is the most convenient under the 

 circumstances, and, though inexact, is hardly calculated to 

 mislead. What is meant is merely those contained within the 

 confines of one half of the heavens. The number of these 

 may or may not be equal to that of those contained within 

 the other half. 



The circumpolar stars at any latitude are readily distin- 



