EVOLUTION OF THE STABS 213 



satellites, and all the asteroids with a very few exceptions, would per- 

 form their motions entirely within the box. The exceptional asteroids 

 and the majority of the comets would dip out of the box because the 

 planes in which their orbits lie make considerable angles with the 

 central plane of the solar system. 



It is an equally remarkable fact that the eight planets and the 800 

 asteroids are all revolving around the Sun in the same direction, which 

 we call west to east. Likewise, the Sun rotates on its axis from west 

 to east, and so also do Mercury, Venus, the Earth and its Moon, Mars, 

 Jupiter and Saturn. Our moon, Mars's two moons, the seven inner 

 moons of Jupiter, Saturn's rings and eight of its moons, revolve around 

 their plants from west to east. From Jupiter out to Neptune we come 

 upon exceptions to the rule. The eighth and ninth moons of Jupiter 

 go around the planet from east to west. The ninth moon of Saturn is 

 similarly reversed in direction. The four moons of Uranus move in a 

 plane making an angle of 98° with the principal plane of the solar 

 system; that is, nearly at right angles to the principal plane. The 

 one moon of Xeptune moves in a plane inclined 145° to the plane of 

 the system; in effect, from the east toward the west. The equatorial 

 planes of Uranus and Xeptune are. without doubt, essentially coinci- 

 dent with their satellite planes. 



The Stellar System. 



Our solar system is very completely isolated from other systems. 

 Light travels from our Sun out to Xeptune in less than 44 hours, yet it 

 requires 44 years to travel from our Sun to the nearest star, a Centauri. 

 Stating the case differently, the nearest star is more than 9.000 times 

 as far from our system as our farthest planet, Xeptune, is from the 

 Sun. We should have to go 7 light-years from our Sun in another 

 direction to reach the second-nearest star. It is 9 light-years in a still 

 different direction to Sirius. The average distances between neigh- 

 boring stars, at least in our part of the universe, is 6 or 7 or 8 light- 

 years. We can see that the stars themselves occupy very little space, 

 and that they have an abundance of room to move about. Recalling, 

 further, that the average speed of the stars is about 26 kilometers per 

 second, which means that about 80,000 years would be required for the 

 average star to travel over the average distance to its neighbor, we can 

 see that collisions of two stars must be exceedingly rare; and that close 

 approaches of two stars, approaches so close as to disturb each other 

 violently, must also be rare. However, when we consider the number 

 of stars in the stellar system, we should perhaps expect a few close 

 approaches to occur in a human life time; possibly also a grazing colli- 

 sion, but probably no full collision. 



The universe of stars — our stellar system — is believed by students 

 of the subject, all but unanimously, to occupy a limited volume of space 



