and thence that of the Sun. 



51 



velocity of Venus in her orbit, led to the exact determination of 

 the sun's parallax. 



" Now the counterpart of the above is the simultaneous obser- 

 vation at different parts of the earth's surface, of the time occu- 

 pied by a superior planet, when near opposition and near the 

 node, in passing through a certain interval of space, say about 

 half a degree (the sun's diameter) ; but as this happens at night, 

 comparison stars are to be used, and the assumed interval to be 

 nearly equivalent to their distance. Thus, ex. gr., if the nearest 

 planet (Mars) be the object observed, and at Greenwich x mi- 

 nutes are occupied by it in describing an arc which it requires 

 only y minutes to describe at the Cape of Good Hope, then will 

 the difference x—y properly applied give the parallax of Mars, 

 and hence that of the Sun. 



" It is, in short, the observation for a like end which an observer 

 at Venus would have made of the Earth at the former's transit. 

 That such observations on Mars may differ ten or twelve minutes 

 in time, the following investigation will possibly show. Accord- 

 ing to Vince's 'Astronomy,' part 3, we had in 1840 — 



1840. 



Venus. 



Mars. 



Planet. 



Earth. 



Planet. 



Earth. 





75° 12 48 



309 08 06 



75 51 00 



75 13 04 



76 40 50 

 9-857570 

 9-857508 



1 25 52 

 12 // -28 



204s-84 



33"-30 



/ // 



100 76' 39 



O 7 // 



48 19 24 



153 07 57 



38 10 00 



38 17 36 



38 50 38 



0-168847 



0169232 



1 31 03 



5"-96 



94*26 



18' -12 



O 1 II 



100 10 39 



38 17 36 



39 17 41 

 9-995103 

 9-995304 





Min. long, hypoth 



True eel. long 



75 13 04 



76 14 03 

 9-993322 

 9-993269 









Hor. par. from © 



Described with geoc. 1 

 motion in J 



" ',',[ 





Hor. par. from © 





" Maskelyne's formula, in O. Gregory's ' Astronomy,' p. 383, as 

 applied to the transit of Venus in 1 769 for the parallax-caused 

 differences in times of ingress and egress, is representable by 

 204 s# 84 (A + B) ; and for the counterpart observations of Mars 

 when passing through the same arc in the opposite part of the 

 heavens, the above formula is modified to 94 s 26 (A + *4879B). 

 The preceding Table was calculated for the mean parallax 8"-80. 



204-84 A oc parallax =141 7-051. for 8"-80, f 1412-221 for 

 204-84 B a (parallax) 2 = -10-10 J and \-10 05/8"-83 



Therefore 94 s -26 A = 649 s -81, and 94*26 x *4879B = -2 s -25 for 

 parallax 8"-80. Hence 649 s 81 -2 S -25 = 647 S 56 = 10 m 47 8 *56 



E2 



