M 14, 15] The Planets \ 5 



Mercury is never seen except occasionally near the horizon 

 just after sunset or before sunrise, and in a climate like 

 ours requires a good deal of looking for ; and it is rather 

 remarkable that no record of its discovery should exist. 

 Venus is conspicuous as the Evening Star or as the 

 Morning Star. The discovery of the identity of the 

 Evening and Morning Stars is attributed to Pythagoras 

 ,(6th century B.C.), but must almost certainly have been 

 made earlier, though the Homeric poems contain references 

 to both, without any indication of their identity. Jupiter is 

 at times as conspicuous as Venus at her brightest, while 

 Mars and Saturn, when well situated, rank with the brightest 

 of the fixed stars. 



The paths of the planets on the celestial sphere are, as 

 we have seen ( 13), never very far from the ecliptic ; but 

 whereas the sun and moon move continuously along their 

 paths from west to east, the motion of a planet is some- 

 times from west to east, or direct, and sometimes from east 

 to west, or retrograde. If we begin to watch a planet when 

 it is moving eastwards among the stars, we find that after 

 a time the motion becomes slower and slower, until the 

 planet hardly seems to move at all, and then begins to 

 move with gradually increasing speed in the opposite 

 direction ; after a time this westward motion becomes 

 slower and then ceases, and the planet then begins to move 

 eastwards again, at first slowly and then faster, until it 

 returns to its original condition, and the changes are 

 repeated. When the planet is just reversing its motion it 

 is said to be stationary, and its position then is called a 

 stationary point. The time during which a planet's motion 

 is retrograde is, however, always considerably less than that 

 during which it is direct; Jupiter's motion, for example, 

 is direct for about 39 weeks and retrograde for 17, while 

 Mercury's direct motion lasts 13 or 14 weeks and the retro- 

 grade motion only about 3 weeks (see figs. 6, 7). On the 

 whole the planets advance from west to j^ast and describe 

 circuits round the celestial sphere in periods which are 

 different for each planet. The explanation of these irregu 

 larities in the planetary motions was long one of the great 

 difficulties of astronomy. 

 .15. The idea that some of the heavenly bodies are 



