35. 



NOTE ON THE ELLIPTICITY OF MARS, AND ITS EFFECT ON THE 



MOTION OF THE SATELLITES. 



[From the Monthly Notices of the Royal Astronomical Society, Vol. XL. (1879).] 



ONE of the results of Professor Asaph Hall's able discussion of his 

 observations of the satellites of Mars is to shew that the orbits of both 

 the satellites are at present inclined at small angles to the plane of the 

 planet's equator. It becomes an interesting question to inquire whether 

 this state of things is a permanent one. The plane of Mars' orbit is 

 inclined to its equator at an angle of 27 or 28. If then the planes of 

 the orbits of the satellites retain constant inclinations to the orbit of the 

 planet, as they would do if the Sun's disturbing force were the only force 

 tending to alter those planes, their inclinations to the plane of Mars' 

 equator, and still more their inclinations to each other, would in time 

 become considerable. 



In No. 2280 of the Astronomische Nachrichten, Mr Marth has found 

 the motions of the nodes of the orbits of the satellites on the orbit of 

 the planet due to the Sun's action, and he concludes that, if there is no 

 force depending on the internal structure of Mars which counteracts or 

 greatly modifies the Sun's action, the nodes of the orbits will be in 

 opposition to each other a thousand years hence, when the mutual inclination 

 of the satellites' orbits will amount to about 49. 



In this case the near approach to coincidence between the planet's 

 equator and the planes of the orbits of the satellites, which is observed 



352 



