[ 43 ° ) 
which, converted into time, gives fh 23' f" ; whence 
the no&urnal arc = 13 11 13' fo". 
* ' " 
Sun’s oblique atcenfion 341 07 38 
Oblique afcenfion of Ardturus ifa 57 of 
Difference 188 10 33 
h ' " 
This, converted into time, gives 12 00 42 
Nodturnal arc 13 13 50 
Difference 1 13 08 
Add Semidiurnal arc f 23 05 
Time of the riling of Ardturus 6 36 13 
Time of Sun-fetting at Athens f 23 Of 
Ardturus therefore rofe, after Sun-fet there, 1 13 08 
Let us now fuppofe, farther, that twilight ends 
when the Sun is 18 deg. below the horizon; and 
therefore, in the figure, 
where H O reprefents the 
horizon, P O the height of 
the pole at Athens, we have 
Z P the complement of 
PO= fi°ff, PS =the 
diflance of the Sun from 
the pole, = 10 1 0 33' f 6", 
Z S = the Suns diflance 
from the zenith, = iob°. 
Then, 
Z S 
