THE ORIGIN OF THE SOLAR SYSTEM^ 



By J. H. Jeans 

 Secretary, Royal Society of London 



[With 5 plates] 



The astronomer of to-day has at his disposal telescopes which 

 range in aperture from his naked eye, of aperture about one-fifth 

 of an inch, up to the giant Mount Wilson telescope of more than 

 100 inches. If we lived in the midst of a uniform infinite field of 

 stars, or in a field which was uniform as far as our telescopes could 

 reach, the numbers of stars visible in different telescopes would be 

 proportional to the cubes of their apertures. 



In actual fact our naked eyes reveal about 5,000 stars ; with a one- 

 inch telescope this number is increased to about 100,000, with a 

 10-inch to 5,000,000, and with the 100-inch telescope to perhaps 

 100,000,000. These numbers increase much less rapidly than the 

 cubes of the apertures. We conclude that we are not surrounded by 

 an infinite uniform field of stars. We live in a finite universe, which 

 thins out quite perceptibly within distances reached by telescopes 

 of very moderate size. It is estimated that the whole universe con- 

 sists of some 1,500 million stars, our sun being not very far from the 

 center of the system. 



Imagine the various celestial objects in this universe arranged 

 according to their distance from us. Disregarding altogether bodies 

 which are much smaller than our earth, we must give first place to 

 the planets Venus and Mars, which approach to Avithin 26 and 35 

 millions of miles, respectively. Next comes Mercury with a closest 

 approach of 47 million miles, and the sun at 93 million miles. The 

 remainder of the planets follow at distances ranging up to 2,800 

 million miles, the radius of the orbit of Neptune. 



But now comes a great gap. The first objects beyond this gap 

 are the faint star Proxima Centauri at a distance of 24 nwllion 

 million miles, or more than 8,000 times the distance of Neptune, 

 and, close to it, a Centauri at 25 million million miles. Next in 

 order come the faint red star Munich 15,040 at 36 million million 



* Discourse delivered at the Royal Institution of Feb. 1.5. Reprinted by permission 

 from the supplement to Nature, Mar. 1, 1924. 



139 



