164? OUTLINES OF NATURAL PHILOSOPHY. 



therefore be nearly in the centre of the orbit of 

 Mars. The same is true of Jupiter, Saturn, 

 &c. 



e* Mars appears with his disk perfectly round, both 

 at the opposition and the conjunction. In the in- 

 termediate positions, he is found to want some- 

 thing of perfect rotundity on the side turned from 

 the Sun. 



Observations made at the opposition of a planet, 

 and, when possible, near the conjunction, are 

 valuable, because the planet is then seen from the 

 Earth in the same place it would be seen in from 

 the Sun ; that is, the geocentric and heliocentric 

 places, either coincide, or differ exactly by 180 

 degrees. 



164. The superior planets are not always pro- 

 gressive, or do not always move from west to 

 east ; but, like the inferior, they become sta- 

 tionary, and also retrograde. They are pro- 

 gressive at the conjunction, and for a consider- 

 able distance on each side of it ; they are re- 

 trograde at the opposition, and at certain points 

 between, are stationary, passing gradually from 



one of these states to another. 







a. Thus Mars, when he emerges from the Sun's rays, 

 a few days after the conjunction, and is seen to 



rise 



