753

therefore, assume some quantity as the dift’erence, and compute,agreeably to such assumption, the Moon ’s latitude and longitude :thence, as it is pointed out in the preceding pages, we computetlie distance of the Moon ’s centre, and of the star on its disk:such distance is the Moon ’s semi-diameter. But we can alsodetermine the Moon ’s semi-diameter, by interpolating betweenthe values expressed in the Nautical Almanack, for noon andmidnight, its value corresponding to 1 l h minus the assumed time°f the difference of the longitudes of Greenwich and Cambridge.Should that dift’erence be assumed, as it probably will be,erroneously, the two values of the semi-diameter comparedtogether will not agree. The quantity of their disagreement willbecome an index of the error of the original assumption, and themeans of amending it: and, by repetition of process, of com-pletely correcting it.

% computing the parallaxes in longitude and latitude, wehave, in the preceding pages, deduced the Moon ’s apparentlongitudes and latitudes from her true, and thence the apparentdistance of the Moon from the star. If we reverse the process,"’e may deduce the true distance of the Moon and star: andsome authors make the same use of the true, as, according to theabove explanation, may be made of the apparent, (see Vince,v °l. I. pp. 334, See.)