MATHEMATICAL PAPERS i5 
The parallax in longitude muft be added to the moon's true 
longitude, to give the vifible, when her true longitude is to the 
eaft of the nonagefimal degree, otherwife, it is to be fuübtracted. 
If the moon’s true Jatitude and longitude are to be found 
from the vifible, the above rules, for Apt the Lone 
muft be pirical 
- 
for the moon's eren or - vifible femi-diameter according 
to her altitude or zenith diftance. 
The vifible diameter of the moon, to a fpectator on the fur- 
face of the earth, is continually enlarging, from the horizon to 
the zenith ; becaufe the femi-diameter of the earth bears a fen- 
fible proportion to the moon's diftance. When the moon 
is in Sons sim of < an nee gate on ees ms moe fhe is 
dun when the ; is in at [ng m ie fee “therefore. 
the moon’s vifible diameter muft be the leat, and in the zenith 
the greateft of all, —. * 
"The wifible. diameter of any planet i is inverfely as it’s. diftance 
from us. therefore, to id the moon's augmented. finda 
meter, EL her borizontal, at a particular zenith ditare, we 
muft fay c he moon's diftance from the earth’s furface at the - 
given. vifible zenith diftance : her, horizontal diftance, or dií- 
tance from. the. earth’ s center} :: her horizontal femi-diameter : : 
her augn sented or vifible femi-diameter at the given zenith dif- 
tance. but fides of Tum Gan are mieafared by the Gries 
PY WD 
+ The moon's diftance from a ipiis at the ides tenter is tlic fame, a$ eve 
ay altitude, as her horizontal diftancc. 
