16, > ASTRONOMICAL Ax» 
of the oppofite angles ; thus in plate I, figure III, fine angle 
BCA, the moon's true zenith diftance, meafures the fide AB, 
the moon's diftance from a {pectator on the earth's fur- 
face ; and fine angle A B C, or which is the fame, fine angle 
DBA, the moon's vifible zenith diftance meafures the fide A C, 
the moon’s diftance from the earth’s center — her horizontal 
diftance. We may therefore fay, Sine angle AC B, the moon's 
true zenith diftance : fine angle D B A, her vifible zenith dif- 
tance :: her horizontal femi-diameter : her augmented or vifi- ` 
ble femi-diameter.* ; | 
For the vifible difference of longitude between the center of 
| | the fun and moon. > = = 
To obtain this, there are given the fum of the fun’s hori- 
zonfal femi-diameter, and the moon's augmented or vifible femi- 
diameter, at the time of the obferved beginning or ending of 
the eclipfe, for which.the calculation s-made, and the vifible 
atitude of the moon from the fun, found by properly apply- 
l ing the parallax of latitude from the fum, by which.a. plain 
right-angled triangle may be formed. Thus in the plain right- 
angled triangle GE», plate L figure IV, there will be given 
. the hypotenufe ©) = the fum of the femi-diameter of the fun 
or moon,+ or vifible diftance of their center, and the fide Ep, - 
ae P c “the 
* The augmentation of the moon's femi-diameter to her altitude or zenith dif- 
tance may be found in fome aftronomical tables ; but I thought it beft to give the 
—— voces | fe | 
T This fum of the femi-diameter ought to be diminifhed, on account of dif- 
fra&ion, or the infle&ion of the rays which pafs the limbs of the moon, fuppofed to 
be caufed by her atmofphere. When an eclipfe or occultation would appear to us 
to be juk begup, were there no inflexion, this caufes her limb to be vifibly diftant 
s from 
