20 ASTRONOMICAL AND: 
eclipfe or occultation, according to obfervation.* Then we 
muft fay, This whole motion : the whole time of the duration :: 
the difference of longitude at the beginning or ending : the 
diftance in time between the obferved beginning or ending and. 
the true ecliptic conjunétion.. 
When the beginning or ending only has been obferved; at 
the places for which we made the calculations, we muft ufe the 
moon’s horary motion from the fun, as given by the tables, for 
the firft term, and one hour or 3600", for the fecond term. 
The time thus found is to be added to the obferved time of 
the beginning of the eclipfe or occultation, when the moon is. 
behind the true conjunétion ; but if fhe has paffed it, then. 
it muft be fubtraéted ; and the fum or difference will be the: 
time of the true ecliptic conjunction according to obfervation.. 
* If this whole motion is confiderably different from that given by the tables, . 
and we make dependance upon the obfervations of the beginning and end of the 
eclipfe or occultation, we are to conclude, thatthe moon's latitude, by the tables, 
is not exact ; and the correct. latitude mutt be fought, for- the beginning and end... 
Thefe being obtained, the vifible difference of longitude between the fim’s and 
moon's center is to be found, conformably thereto. By this it is fuppofed, that 
the moon's horary motion, by the tables, is true; which may be concluded. 
to be generally the cafe, in eclipfes and occultations ; and indeed it is much more- 
likely that the latitude fhould be given too fmall or too large, in the tables, than. 
that the horary motion fhould be confiderably erroneous by them. In more than. 
five hundred longitudes of the moon, calculated from Mayer’s printed tables, by 
- Lemery, which he compared with correfponding longitudes, deduced from 
thelunar obfervations of the late accurate and celebrated Dr. Bradley ; which com- 
parifons are publifhed in Connoifance des T. emps pour Année, 1783, Lnd that the- . 
error in the moon’s horary motion in longitude rarely amounts to 2”. It is not 
common for it to exceed 1”; and it is generally but a few tenths of a fecond. We- 
may N seats 1 A P : 
4 
great dependance upon the horary motion given by thefe tables... 
