BAEORVIOR Ay SOLID ISORKROPIC HART L, 47 
R-z% 
R sin AFC 
COS @é= 
so that instead of e starting at o for 9? =o and increasing / to 77/2 
for || Or, e begins at 7/2 for @=0, reaches a minimum when 
AFC =77/2, and then increases to 7/2 for =7. The minimum 
is given by 
—h 
R i AN}. 
cos €=cos 9=5— or sin o/2=(4 4) 
This point is important in attempting to determine % from 
observations. As an example if Z=10km., R=6370 km., we 
set 0= 3°12’, 4=356 km., and the corresponding apparent 
angle of emergence @=22°. But for 4=1000 km. the error 
in e made by supposing F to coincide with E would only be 
about 4°. 
