216 OCCULTATION OF ALDEBARAN 



1764, says that an inflection of l" 8. and a diminution of the se- 

 midiameter of the moon of l" 5 agreed with some of the observa- 

 tions, but he could come to no linal conclusion upon this point. 

 To determine the quantity of the inflection it is necessary to 

 know precisely the following data, viz. the precise diameter of 

 the moon, (he beginning and end of the occultation, the true 

 difference of latitude, the parallaxes in longitude and latitude, 

 and the horary motions of the two bodies. 



Let us suppose the diameter observed to be less than tiiat cal- 

 culated by the tables 0" 8, as in the present case, and that in 

 other respects the elements that have been made use of are cor- 

 rect, we cannot on that account suppose it to be the effect of 

 the irradiation, it being certain that the doubt respecting the 

 lunar diameter, measured by different astronomers, is much 

 greater than the above difference. 



The diameter of the sun has been frequently the object of 

 the attention of astronomers, and although it is much more ea- 

 sily determined than that of the moon, there is notwithstand- 

 ing, a great difference in the various determinations. 



The Apogee diameter of the Sun by, Picard. in 1670=31 38 



Mouton. 31 31 



Louville. 1724 31 33 



Gentil, Lemonnier and La CaUle 1750 31 34 5 



Bradley. ■ 31 30 5 



Lalande. 1764 31 31 



Maskelvne. 31 29 



Short. ' 31 28 



If we confine Ourselves even to the determination of Lalande, 

 Maskelyne, Bradley, and Short we find a difference from 

 2" to 3" and there is reason to believe that the uncertainty of 

 the diameter of the moon is much greater, consequently we 

 may well doubt whether the diminution of l" or 2" resulting 

 from the observations of the moon by eclipses of the sun and 

 occultations, is the effect of the irradiation or of an error in 

 the diameter represented in tables. 



Remark on the elements of the tables, 



I have calculated the place of Aldebaran taking the right 

 ascension from the catalogue of Maskelyne and the declination 



