366 Astronomical and Nautical Collections. 



the luminary, and not of its centre. But, in fact, we want for 

 the computation the apparent altitude of the centre, and not of 

 the limb ; it is therefore best to find, in the first instance, the 

 apparent altitude of the centre, by applying the vertical semi- 

 diameter of the sun or moon, reduced to its apparent magni- 

 tude, by subtracting from the semidiameter found in the ephe- 

 meris the correction given in the following table : 



oooooooooooooooo 

 Altitude 5 6 7 8 9 10 11 12 13 14 15 16 18 20 30 45 



Correction 25 19 14 I'i 9 8 7 6 5 4 4 3 3 2 1 1 

 After applying the semidiameter with this correction, as well 

 as that augmentation, for the moon, which depends simply on 

 the sine of the altitude (R. T. IV.) the refraction and parallax 

 may be taken out for the centre of the luminary. Strictly 

 speaking, the refraction ought to be subtracted first, and the 

 parallax then found for the diminished altitude : but the dif- 

 ference, arising from the neglect of this precaution, seldom 

 amounts to a second, and there is no material objection to the 

 practice of taking out both these corrections from the same 

 table. 



B. Of the Horizontal Parallax. 



We must not, however, forget to enter the table of the lunar 

 corrections with the tabular horizontal parallax of the ephemeris, 

 properly reduced for the latitude, on account of the spheroidal 

 figure of the earth ; since the parallax, as set down in the tables, 

 is calculated for the equatorial radius of the earth, and requires 

 to be diminished by a correction which varies as the square of 

 the sine of the latitude, or as the verse sine of twice the latitude. 

 This correction is computed in the annexed table, the ellipticity 

 being supposed -^-g ; and it may be taken out simply for the 

 nearest latitude : thus, for Greenwich, it will be 6" for 53' H.P., 

 and 7" for 55', or more. 



