OBSERVATIONS FOR TIME AND LONGITUDE. 
191 
Sum of App. Alts. Nat. Cosine .. = *178802 
O / // 
App. Dist. 53 16 58 Nat. Cosine .. = *597865 
(3rd Term) .. = *776667 
1*178734: 1*167203 :: *776667 : •769069 = 0; 
Sum True Alts. Nat. Cosine = *167204 
x = *769069 
o I II _ 
True Distance 52 59 47 Nat. Cosine = *601865 
To compute the Altitude of a Heavenly Body. 
It frequently happens that, at the lime when a lunar distance is 
required, the altitude of one, or both, of the bodies may be so high or 
so low as to prevent their being taken in an artificial horizon, in which 
case the altitude should be computed, the error of the watch on M. T. 
at place having been previously determined; and since the Altitudes 
employed in clearing the lunar distance are not required to the same 
degree of precision as those used in finding the time, it will be sufficient 
if they are computed within 20" or 30" of the truth. 
Rule. —Having taken from the ‘ Nautical Almanac 5 the declination, 
E.A., Sidereal Time, Semi-diameter, Horizontal Parallax, &c, as required, 
correct the same for the approximate Greenwich Date. 
Find the Hour Angle as follows:— 
For the © the apparent time from Noon is the Hour Angle. If p.m. 
the mean time at place converted into app. time with the equation of 
time will be the hour angle, but if a.m. the apparent time thus found, 
expressed astronomically, must be subtracted from 24 hours to give the 
hour angle. 
For the Moon, Star, or a Planet:— 
To the Sidereal time at noon on the given day (page ii. N. A.) accelerated 
fot Greenwich date (Table XXXI.) add the mean time at place, this 
sum will be the Right Ascension of the Meridian; subtract from the R. A. 
of the Meridian the R. A. of the object, and the result will be the west 
hour angle of the object; which subtract from 24 hours when the east 
hour angle is required. 
The True Altitude may now be computed as follows:— 
To find arc 1 . —To the log cosine of the object's hour angle add the log 
