2io THE POPULAR SCIENCE MONTHLY 



bodies closer, in effect, by means of telescopes, bnt the reduced distances 

 are still heroic. The stars, some of which are many millions of kilo- 

 meters in diameter, are still seen as mere points of light in our most 

 powerful telescopes, even though the telescopes magnify 3,000-fold. 



Secondly, the evolutionary processes are exceedingly deliberate. We 

 do not know that any progressive changes have ever been noted in any 

 celestial body, except in the comets and meteorites, in the Earth's surface 

 strata, and possibly in the so-called new stars. We observe changes in 

 the clouds of Jupiter, changes in the surface features of the Sun, and 

 some 4,000 stars are known to vary in brightness; but all these are 

 short-period changes, and they do not indicate that progressive or per- 

 manent changes are involved. 



We can get no help in our problem by waiting for any star to show 

 signs of change in physical condition — we should probably have to wait 

 tens of thousands, and perhaps millions, of years. We must take the 

 heavenly bodies as they are, try to fit them into an orderly series repre- 

 senting the various stages of evolutionary development, and justify our 

 arrangement by means of the evidence collected. 



We need, first of all, to comprehend as thoroughly as possible what 

 the individual heavenly bodies are, how they are arranged in space, and 

 how they are related to each other, both physically and geometrically. 

 At the cost of telling you many things you have already learned I shall 

 recall a few features in the structure of the solar system and of the stellar 

 system, and describe briefly the characteristics of each class of objects 

 with which we have to deal. 



The Solar System. 



In the solar system we have the great central body, our Sun, around 

 which revolve the 8 major planets and their 26 moons, the 800 minor 

 planets or asteroids discovered to date, the zodiacal-light materials, the 

 comets and the meteors. The Sun is one of the ordinary stars. It 

 seems very large, very bright and very hot, because it is relatively near 

 to us, and we receive from it our entire supply of energy; but, com- 

 pared with the thousands of other stars visible on any clear night, it is 

 merely an average star. Nevertheless, the Sun is a very large body; if 

 it wore a hollow shell of its present diameter we could pour more than 

 a million Earths into it and still leave empty the space between the 

 earth-balls. Traveling outward from the Sun we come, first, to the 

 small planet Mercury, its diameter a little more than one third the 

 Earth's diameter, which revolves once around the Sun in 88 days; sec- 

 ondly, to the planet Venus, just a shade smaller than the Earth, with 

 period of revolution 225 days; and thirdly, to the Earth and its moon, 

 which revolve around the Sun in one year. Fifty per cent, farther out 

 than the Earth is Mars, its diameter a trifle more than one half the 

 Earth's, with two tiny moons, and period of revolution 1.9 years. Next 



