86 



THE POPULAR SCIENCE MONTHLY. 



Professor Newcomb gives, for the period of revolution of the inner 

 satellite around Mars, about 7'65 hours, or 7h. 39m., and 30-25 hours, 

 or 30h. ISm., as that of the outer moon. Both of them, like our moon, 

 revolve around the primary from west to east. Mars rotates on its 

 axis from west to east in 24*623 hours, or 24h. 37m. 23s. ; this is the 

 duration of the Martial day. We have seen that the period of revo- 

 lution of the inner satellite is less^ while that of the outer is greater, 

 than a Martial day. It is evident, therefore, that, as seen from the 

 surface of the planet, the apparent motion of the satellites will be in 

 opposite directions, the inner rising in the icest and setting in the east, 

 the outer (like our moon) rising in the east and setting in the west. 

 This anomalous condition of things must have greatly perplexed the 

 primitive astronomers of Mars, and probably led them to the invention 

 of cycles and epicycles to account for these appearances. 



It follows that the phenomenon of two moons meeting in mid- 

 heavens will be no unusual occurrence to the observers on the surface 

 of Mars. The apparent motion of the fixed stars from east to west, 

 produced by the rotation of the planet upon its axis, is at the rate of 

 14-62° per hour. The real motion of the inner satellite among the 

 stars from west to east is at the rate of about 47*06° per hour, while 

 that of the outer one is at the rate of 11*90° per hour. Hence it fol- 

 lows that the api^arent motion of the inner satellite from west to east 

 across the heavens, to an observer on Mars, will be at the rate of about 

 32*44° per hour, while the apparent motion of the outer moon from 

 east to toest will be at the rate of nearly 2*72° per hour. 



It likewise follows from the preceding calculations that the time 

 elapsing between two successive meridian passages of the inner ^tel- 

 lite will be about 11*09 hours, and the time elapsing between two suc- 

 cessive conjunctions of the inner with the outer moon will be about 

 10*24 hours ; consequently two conjunctions will occur in less time 

 than it takes for Mars to rotate on its axis, or than a Martial day. 

 This satellite completes more than three orbital revolutions in a Mar- 

 tial day. 



As the apparent motion of the outer satellite from east to west is 

 at the rate of only about 2*72° per hour, it is obvious that the time 

 elapsing between two successive meridian passages of this moon will 

 be about 132*35 hours ; so that there will be no less than twelve con- 



