Methods of correcting Lunar Observations. 367 



Diminution of the Horizontal Parallax. 



C. Of the Oblique Semidiameter. 



The observed distance requires also to be corrected, when the 

 altitudes are small, for the effect of refraction on the oblique 

 semidiameters, which may amount to several seconds, espe- 

 cially in low latitudes, where the ecliptic approaches most fre- 

 quently to a vertical position. The magnitude of this correction 

 depends on the angle of the triangle concerned at the Itiminary 

 in question, which is required to be known when we employ the 

 tables of this correction which have been published. It will, 

 however, be more convenient to have an easy method of de- 

 ducing the correction more immediately from the elements 

 observed ; and for this purpose the minutes and seconds may 

 be neglected, the nearest degrees only being employed for 

 entering the annexed tables. The first table affords, by adding 

 these numbers contained in it, a logarithm of the sine of half 

 the angle at the luminary, multiplied by the cosine of the alti- 

 tude, and the second table being entered with this argument 

 and with the altitude, gives the proper correction for the oblique 

 semidiameter. 



The arguments of the first table are d the distance or its sup- 



