Oblateness of a Rotating Planet. 711 



of the body, or what would be such were the mass distributed; 

 in other words, on the planet's spin and mean density only. It is 



somewhat less than 7, and increasingly, so as the spin 



increases and the density diminishes. 



If now we compare the observed oblateness with its possible 

 extreme values, we can get a criterion of how the matter is 

 distributed. For each planet respectively the limiting values 

 and the actual oblateness in terms of are: — 



Earth . . 1-25 



md *50 



-9730 



Mars . . 1-250 



„ -50 c/) 



1-145 



Jupiter . . 1*25 



„ -487 



•72 



Saturn . . 1*25 



„ -476 



'66 



! the percentages of rji in terms of 77- 



-772 are: 



Earth . . . *63 



Jupiter 



. . '38 



Mars ... '87 



Saturn 



. . -24 



Here a marked distinction shows itself between the major 

 planets and the terrestrial ones. The major are from a quarter to 

 three-eighths way from central concentration to homogeneity, 

 while the terrestrial ones show the opposite relation. Now 

 when we consider the small mean density of both Jupiter and 

 Saturn we see that this means that tenuous matter occupies a 

 large part of their volumes ; that they consist of a more or 

 less dense fluid kernel surrounded by a vast shell of gas. 

 Also that we never see what may by courtesy be called the 

 surface of their globes. For the dark cherry-red portions 

 of what we perceive in the telescope lie very nearly as high 

 as the brighter portions, and cannot therefore be of much 

 denser material. 



A secondary consideration from the table implies that the 



observed flattening of Mars n is probably a little too large 



and that t ^ n or even ~rj is nearer the truth. This seems to 



be corroborated by the fact that its oblateness as deduced by 

 H. Struve from the motions of the apsides of the satellites 



is _. and for some unexplained reason the values of the 



J. J\ ' 



oblateness of planets so deduced seem always to be a little too 



