* 8 7 J 



The Mot 'urn of the Planets 



117 



motion of the planet in a circle round the sun, while the 

 sun moves round the earth, or, more simply, the earth 

 round the sun. 



The synodic period of a superior planet could best be 

 determined by observing when the planet was in opposition, 

 i.e. when it was (nearly) opposite the sun, or, more 

 accurately (since a planet does not move exactly in the 

 ecliptic), when the longitudes of the planet and sun differed 

 by 180 (or two right angles, chapter n., 43). The 



FIG. 46. The relative sizes of the orbits ot the earth and of a 

 superior planet. 



sidereal period could then be deduced nearly as in the case 

 of an inferior planet, with this difference, that the superior 

 planet moves more slowly than the earth, and therefore loses 

 one complete revolution in each synodic period ; or the 

 sidereal period might be found as before by observing 

 when oppositions occurred nearly in the same part of the 

 sky.* Coppernicus thus obtained very fairly accurate 



* If p be the synodic period of a phmet (in years), and s the 



sidereal period, then we evidently have - + I = - for an inferior 



II PS 



planet, and I - - = - for a superior planet. 



