582 
On Certain Errors in Lunar Observations. [October, 
the apparent Zenith distance of the Star. M S the angular 
measured distance between the Star and the centre of tie 
Moon. „ „ , ,, „ , r. j 
We have three sides — viz., Z M, Z S, and M S to find 
the angle at Z. 
Having found the angle at Z, corredt the Zenith distance 
Z M for parallax and refraction, and the side Z S for re- 
fradtion. 
2 
Both parallax and refradtion will be manifested in the 
diredtion of the vertical lines ZM and ZS, and will not 
therefore affedt the angle at Z. , 
Then with the two sides Z M and Z S corrected , and the 
included angle at Z, find the value of the third side, which 
will be greater or less than M S, according to the position of 
the Moon and Star, and the third side, so found, will be the 
true distance of the Moon from the Star at the instant o 
observation. 
By comparing the true distance thus found with the dis- 
tance given in the “ Nautical Almanac for a certain time, 
we obtain the Greenwich mean time of the observation. 
I have always found this method of teaching Lunars 
more readily understood than if I employed a sort of blind 
formula, — such, for example, as the following : 
M the apparent] j^udes of the Moon. 
in the true J 
S the apparent l a ltit U de of the Sun or Star. 
s the true j 
D the measured distance. 
