20Q OCCULTATION OF A STAR BY MARSr 



of time. At the emerfion the ftate of the air was rather more 



favourable than at the immerfion ; and as the ftar emerged 



from behind the dark limb of Mars at a fenfible diftance from his 



enlightened edge, this obfervation is much more certain than 



' that of the immerfion ; and is probably true, to five, or ten 



feconds at mod. 



EfFcft of the The fiar at its firft emerfion, certainly appeared much fainter 



ttmofphere of jjj^^ jj jj^j f^^^y afterwards ; but I cannot preteBd to decide, 



Mars, probably. , , . .... /•.,., 



that this apparent dimmution ot its light was owmg to any 



other caufe than its great proximity to the fuperior light of the 

 planet. Yet it was at firfl fo faint as to be very doubtfully 

 viiible, and in lefs than a minute was very confiderably 

 brighter. As in this interval of time, its difiance from the 

 planet was not very much increafed, it is probable that the 

 atmofphere of Mars might have been the caufe, at lead in 

 fome degree, of this appearance. 



Occultatlon of a Occultation of h f by the Planet Mars, April 17, 1796, 

 ^tXr'' atPeUrJham. 



Immerfion of b ^ per clock 15'' 4' 48''' 



Uncertain to nearly one minute. 



Emerfion of b ^ per clock ..... 15** 24' 28^ 

 Certain to five, or ten feconds at moft. 

 ' The flar pafied to the north of the centre of Mars. The 



immerfion was about one third, or at moft two fifths of his 

 femidiameler north of the centre. The emerfion uot quit« 

 / half the femidiameter north of the centre, by eftimation. 



The clock, at the time of the occultation, 



was faft on mean time 0** 4' 2l|" 



The immerfion was therefore at . . .13'' 0' 26|'^ 



The emerfion at . . li** 20' 6f'^ 



Mean time at Peterfliam. 



And, Greenwich being 1' 12'' in time eaft of Peterfbam^f 

 the obfervations reduced to tlie meridian of Greenwich are. 

 Mean Time. Apparent Time. 



Immerfion ... 15'' V 3}^'^ 15^ 2' 28i" 



Emerfion ... Id'' 21^ 18|" 13" 22' 8|" 



Although this obfervation is fubje61, from the circumfiances 

 ilaled before, to an uncertainty of half a minute in time, a$ 

 to the middle of the occultation ; yet, as the geocentric mo- 

 tion 



