2 Introductory Chapter [OH. i 



A similar uniformity is found in the satellites of the planets. The modern 

 astronomer knows that the system of Saturn as well as that of Jupiter is a 

 small-scale replica of that of the Sun, while the systems of the smaller planets 

 differ only in having fewer satellites. With a few exceptions, it is found that 

 throughout the whole complex system formed by the sun, its satellites, the 

 planets, and the satellites of the planets, the motion is uniformly in the same 

 direction and in nearly circular and nearly coplanar orbits. 



The exceptions occur on the outermost edges of the solar system, and on 

 the outermost edges of the systems of Jupiter and Saturn. They are as 

 follows : 



Neptune has only one satellite, and this has retrograde motion. 

 Uranus has four satellites, whose orbits are highly inclined to the 



plane of the ecliptic. 



Saturn has nine satellites*, of which the outermost (Phoebe) revolving 

 at a mean distance of 209 diameters of Saturn, has retrograde 

 motion and high eccentricity of orbit. 



Jupiter has nine satellites of which the two outermost move with 

 retrograde motion. 



S^me of the asteroids also have considerable inclinations and eccentricities. 

 Thus Pallas has an inclination of 34 43', and Zerline (531) one of 34 33', 

 these being nearly five times the greatest inclination observed among the 

 planets (7 0', the inclination of Mercury). Juno has an eccentricity of 

 0*257 and Pallas one of 0'239, while a few smaller asteroids are supposed, 

 although with less certainty, to have eccentricities of about J. 



Binary Stars 



2. We do not know whether uniformity of this kind extends to other 

 systems in space, or whether it is a peculiarity of our own system. When it 

 was first realised that the so-called fixed stars were essentially suns more or 

 less similar to our own, it was natural to conjecture that they also might be 

 the centres of planetary systems similar to that of our sun, but the further 

 growth of knowledge has shewn the need for caution in such conjectures. 



Of the nineteen stars whose parallaxes are less than O20" i.e. the nine- 

 teen stars which happen at the present moment to be within 96 x 10 12 miles of 

 our sun no fewer than eight, or 42 per cent, of the whole, are quite certainly 

 binary stars f. Although there is no special reason for thinking that these 

 nineteen stars are not likely to be a fair sample of the whole, it is obviously 

 desirable to try to get evidence from other regions of space. Of fifteen stars 



* Excluding the tenth (Themis) discovered photographically by W. H. Pickering in 1904, 

 but not seen since. 



t Eddington, Stellar Movements, p. 41. 



