160 OUTLINES OF NATURAL PHILOSOPHY. 



a first approximation to the distance of Venus, 

 supposing her orbit a circle, with its centre in the 

 sun. 



b. From the synodical revolution of 584 days, the 

 periodic time of Venus is found nearly equal to 224 

 days. For if v be the angular velocity of the 

 planet in her orbit, or her diurnal motion, that of 

 the earth being 59' 8". 3 ; then the arch described 

 by the sun, during a synodical revolution of Ve- 

 nus is (59' 8".3) 584; and that described by Venus 

 in the same time is 360-f-(59' 8".3) 584 ; there- 

 fore the diurnal motion of Venus in her orbit 



360-f(59'8".3) 584 __ , , 360 



584 ~ "~ . T ' 



~ 1 35' 56" ; and the periodic time = 

 360 



35'. 56 



=r 224 days, nearly. 



159. Mercury, like Venus, accompanies the 

 sun, and never recedes from him farther than 

 28. The greatest apparent diameter is 

 11".275, the least 5 



a. Hence Mercury describes an orbit round the sun, 

 at the mean distance, (computed as in the case of 

 Venus), of 3.7 of the parts whereof the Earth's 

 distance from the sun is 10. 



&. The synodic revolution of Mercury is 115^.877 

 nearly. 



i The 



