OCCULTATIOU OF A STAR BY MARS, 20l 



tion of Mars, was at this time not quite two feconds of a de- Occultation of 

 1 . 1-1 , I . £• . 1 rt 1 • ^ fixed ftar by 



gree in three minutes, his place (that or the itar being accu* j j^^ pj^^g^ j^jjj^ 



rately known)j may certainly be determined by this obferva- 

 tion within a fingle fecond ; with a view, therefore, of afcer- 

 taining the accuracy of the tables of Mars, given in the laft 

 edition of De la Lande*s Aftronomy, the longitude and lati- 

 tude of Mars were computed with care as follow ; 



1796, April l?*^ 15** 12' 18''' apparent time at Greenwich. 

 Gcoc. Long. Geoc. . Lat. Auft. 



Mars 8' 27 "* 7^ 37" 0^ 20' ^l'' 40"' uncorreded, 

 for aberration or parallax. And April, IS** 15^ 12' 18" his 

 place was 8* 27° 23' 2" 0° 22' 53" 18'". 



His daily motion in longitude^ was therefore 0° 15' 25", 

 and in latitude 0° 2' 16" 38'" increaiing. 



His aberration in longitude at the time of obfervation, 

 was ~ 3".97, and his horizontal parallax, that of the Sun- 

 being taken at 8''.72, was 11 ".7 12. 



The parallax iii longitude and latitude, computed by the 

 tables of the Nonagefimal for Greenwich, given in the Con-^ 

 naijfarice des Terns for 1775 is. Par. Long. + 1".772, and Par. 

 JLat. -I- 1 1".2. 



The apparent place of Mars at the time of the occultatiorv ' 



was, therefore, by thefe tables, 



Geoc. Long, Geoc. Lat. Auftt 



g. 27© 7/ 34/^.8 0° 20' 52''.9. 



But the place of the ftar, as fettled at Greenwich, was' at 

 the time of occultation. 



Long. Lat* Auft. 



8* 27° 6' 19" 0^ 20' 41" 



And as the centre of the planet pafled about two fifths of 

 his feraidiameter fouth of the ftar, and the apparent diameter 

 of Liars at that time was very nearly fourteen feconds and an 

 half; the difference of latitude between the ftar and Mars, 

 roay be eftimated at three feconds, without poifible error of 

 above half a fecond. 



The apparent place then of Mars, as found by obfervation, 

 was, 



Geoc. Loo^. Geoc. Lat. Auft» 



8* 27 *» 6' 19'' 0° 20' 41" 



; and the error of the tables is in longitude 4- l' 15". 8, and 

 in latitude + 1 2". 



Had 



