The Evolution of the Sciences 



tances deduced of the stars from the solar group. 

 This distance is, however, not given in ordin- 

 ary units; an incalculable number of ciphers 

 would be required to express it even in millions 

 of kilometres. The unit adopted is the light- 

 year, that is to say the distance travelled by 

 light, at the rate of 300,000 kilometres per 

 second, in one year, or, roughly, ten trillion 

 kilometres : 



Star. Magnitude. Parallax. Light-years. 



8 

 163 



4 

 6 



47 



22 

 63 



Our knowledge of the parallaxes of stars 

 enables us also to form a sufficiently approximate 

 idea of the actual dimensions of the stars. If, 

 for example, we imagine our sun removed to 

 the distance at which Sirius is situated, it will 

 be easy to calculate the amount of light sent to 

 us from its new position. As the sun and 



Sirius are comparable stars, giving the same 



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