** 33-351 Spherics 37 



point on the celestial sphere vertically above the observer ; 

 the verticals, or great circles through the zenith, meeting the 

 horizon at right angles ; and the declination circles, which 

 pass through the north and south poles and cut the 

 equator at right angles. Another important great circle 

 was the meridian, passing through the zenith and the poles. 

 The well-known Milky Way had been noticed, and was 

 regarded as forming another great circle. There are also 

 traces of the two chief methods in common use at the 

 present day of indicating the position of a star on the 

 celestial sphere, namely, by reference either to the equator 

 or to the ecliptic. If through a star s we draw on the 

 sphere a portion of a great circle s N, cutting the ecliptic r N 

 at right angles in N, and another great circle (a declination 

 circle) cutting the equator at M, and if T be the first point of 

 Aries ( 13), where the ecliptic crosses the equator, then 

 the position of the star is completely defined either by the 

 lengths of the arcs r N, N s, which are called the celestial 

 longitude and latitude respectively, or by the arcs r M, M s, 

 called respectively the right ascension and declination.* 

 For some purposes it is more convenient to find the 

 position of the star by the first method, i.e. by reference 

 to the ecliptic ; for other purposes in the second way, by 

 making use of the equator. 



34. One of the applications of Spherics was to the con- 

 struction of sun-dials, which were supposed to have been 

 originally introduced into Greece from Babylon, but which 

 were much improved by the Greeks, and extensively used 

 both in Greek and in mediaeval times. The proper gradua- 

 tion of sun-dials placed in various positions, horizontal, 

 vertical, and oblique, required considerable mathematical 

 skill. Much attention was also given to the time of the 

 rising and setting of the various constellations, and to 

 similar questions. 



35. The discovery of the spherical form of the earth 

 led to a scientific treatment of the differences between the 

 seasons in different parts of the earth, and to a correspond- 

 ing division of the earth into zones. We have already 

 seen that the height of the pole above the horizon varies in 



* Most of these names are not Greek, but of later origin. 



