APPENDIX 



217. Determination of Latitude. In Fig. 121 consider 

 the sun as vertically above the point where our meridian 

 crosses the equator and the lines AB and ED as repre- 

 senting rays from the sun. The line FI tangent at the 

 point A will represent a level surface at that point. Draw 

 the line CH through the point A. It will be at right 

 angles to the tangent line FI. The latitude of the point 

 A is measured by the 

 angle EGA, as this angle 

 measures the number of 

 degrees of latitude be- 

 tween the point E, 

 which is on the equator, 

 and the point A. 



It can be proved by 

 geometry that the angle 

 HAB is equal to the 

 angle HCE, since if the 

 sun is vertical, the line 

 CED is a straight line, 

 a right angle, or 90 



Fig. 121. 



The angle HAB is equal to 

 minus the angle BAI. As the 

 angle BAI can be easily found by measuring the elevation 

 of the sun above a horizontal plane, it is not a difficult 

 thing to find the latitude of a place when the sun is ver- 

 tical at the equator. 



As the sun is vertical at the equator but twice in a year, 

 on March 21 and September 23, this method can be used 



455 



