10 The Outline of Science 



ami. It' they do. we may suppose that system to be 

 permanent in its general features; if not, we must look fur- 

 ther for our eonelusions. 



The Heavenly Bodies 



The heavenly hodies fall into two rery distinct classes so far 

 ns their relation to our Karth is concerned; the one class, a very 

 small one, comprises a sort of colony of which the Earth is a 

 mem her. These hodies are called planets, or wanderers. There 

 are eight of them, including the Earth, and they all circle round 



tin. Their names, in the order of their distance from the 

 sun. are Mercury. Venus, Earth, Mars, Jupiter, Saturn, Uranus, 

 Xeptune. and of these Mercury, the nearest to the sun, is rarely 

 seen hy the naked eye. Uranus is practically invisible, and 

 Xeptune quite so. These eight planets, together with the sun, 

 constitute, as we have said, a sort of little colony; this colony 

 lied the Solar System. 



The second class of heavenly bodies are those which lie 

 tiutsidt' the solar system. Every one of those glittering points 

 we see on a starlit night is at an immensely greater distance from 

 us than is any member of the Solar System. Yet the members 

 of this little colony of ours, judged by terrestrial standards, are 

 at enormous distances from one another. If a shell were shot in 

 a straight line from one side of Neptune's orbit to the other it 

 would take five hundred years to complete its journey. Yet this 

 distance, the greatest in the Solar System as now known (except- 

 ing the far swing of some of the comets), is insignificant com- 



1 to the distances of the stars. One of the nearest stars to the 

 earth that we know of is Alpha Centauri, estimated to be some 

 million millions of miles away. Sirius, the brightest 

 star in the firmament, is double this distance from the earth. 



\Ve must imagine the colony of planets to which we belong 

 a* a compact little family swimming in an immense void. At 

 distances which would take our shell, not hundreds, but millions 



