611 



JUPITER 



The mean distance of Jupiter from the Sun, expressed in 

 fractional parts of the Earth's distance from the central body, 

 amounts to 5-202767. The true mean diameter of this planet, 

 the largest of all, is 77,176 geographical miles; equal, there- 

 fore, to 11-255 terrestrial diameters, about one -fifth greater 

 than the diameter of the more remote Saturn. His sidereal 

 revolution occupies lly. 314d. 20h. 2m. 7s. 



The flattening of Jupiter, according to the measurements 

 by Arago with the prismatic micrometer (which were intro- 

 duced into the Exposition du Systeme du Monde, p. 38), is as 

 167 : 177, consequently yy.y, which agrees very closely with 

 the later determination (1839) of Beer and Madler," who 

 found the flattening to be between -j-i-.y and T {.-^. Hansen 

 and Sir John Herschel give the preference to -fa. The 

 earliest observation of the flattening, by Dominique Cassini, 

 is older than the year 1666, as I have already pointed out 

 elsewhere. This circumstance has an especial historical im- 

 portance, on account of the influence which, according to Sir 

 David Brewster's acute remark, the discovery of this flatten- 

 ing by Cassini exercised upon Newton's ideas as to the figure 

 of the Earth. The Principia Philosophic Naturalis bears 

 witness to this, but the epochs at which the Principia and 

 Cassini' s observation of equatorial and polar diameters of 

 Jupiter appeared, might excite chronological doubts.* 



* Beer and Madlcr, Beitraye zur Phys. Kenntniss der 

 Himl. Korper, pp. 104-106. Older and less certain obser- 

 vations by Hussey gave ^. Laplace (Syst. du Monde, p. 266) 

 found it theoretically between -^ and ^-, with increasing 

 density of the strata. 



* Newton's immortal work Philosophies Nafuratis Prin- 

 cipia Mathematica, appeared as early as May, 1C87, and the 



