AND THEORY OF THE HEAVENS. 105 



direction of their track when still far off and to rise 

 over the planet in an elongated sweep. And because 

 these particles have a greater degree of velocity than the 

 planet itself when they are made to fall by its attraction, 

 they give to their rectilinear fall, and also to the fall of 

 the other particles, an inclination from west to east ; 

 and this slight deviation is all that is needed to cause 

 the circular motion into which the falling movement 

 started by the attraction passes, to take this direction 

 rather than any other. For this reason all the moons 

 will agree in their direction with the direction of their 

 revolution. But neither can the plane of their path 

 deviate far from the plane of the orbits of the planets, 

 because the matter out of which they are formed from 

 the same reason that we have adduced for their direction 

 is likewise inclined to this precise determination of 

 these orbits, namely, to coincidence with the plane of 

 the principal orbits. 



From all this we see clearly what are the circum- 

 stances and conditions under which a planet can obtain 

 satellites. The force of its attraction must be great, 

 and consequently the range of its sphere of attraction 

 must be widely extended, in order that the particles 

 moving through a long fall towards the planet may, 

 notwithstanding what is destroyed by resistance, yet 

 acquire sufficient velocity for them to revolve freely, and 

 also in order that sufficient matter may be present in 

 this region for the formation of moons; and this cannot 

 be realized with a feeble attraction. Hence only those 

 planets which are of great mass, and at a far distance, 

 are provided with satellites. Jupiter and Saturn, the two 

 greatest, and also the most distant among the planets, 



