THE STELLAR UNIVERSE. 



Dr. "Wollaston, by certain photometric methods, which are con- 

 sidered to have been susceptible of great precision, compared the 

 light of the sun with that of the full moon, and found that the 

 ratio was 801072 to 1 ; or in other words, that to obtain moon- 

 light as intense in its lustre as sun-light, it would be necessary 

 that 801072 full moons should be stationed in the firmament 

 together. 



33. By the combination of these observations of Herschel and 

 "Wollaston, we are supplied with means of bringing into direct 

 numerical comparison the sun and the star a Centauri. Since it 

 appears that the light of a Centauri is 27408 times less than that 

 of the full moon, while the light of the full moon is 801072 times 

 less than that of the sun, it will evidently follow, that the light 

 received by the eye from the sun, is greater than that received 

 from the star in the proportion of 801072 times 27408 to 1. 

 Thus, it appears that the light received from the sun, is in round 

 numbers 21956 million times the light received from this parti- 

 cular star, which has been adopted as a fair average standard of 

 stars of the first magnitude. 



34. It has been demonstrated by theory, and verified by experi- 

 ment, that when a luminous object is removed from the eye to 

 increasing distances, the light which the eye receives from it will 

 decrease in the same proportion as the square of the distance 

 increases : that is, at twice the distance, the light is decreased to 

 one fourth ; at three times the distance, to a ninth ; at four times 

 the distance, to a sixteenth, and so on. 



Now, upon this principle, it will be easy to compute the 

 proportion in which the apparent light of the sun would be 

 diminished by any given increase of distance ; or, what increase 

 of distance would produce any given decrease of light. Let it 

 then, be demanded how far the sun should be removed from the 

 observer, in order that its light should be decreased in the pro- 

 portion of 21956 millions to 1, that is so that its light should be 

 equal to that of the star a Centauri. According to what has been 

 just explained, this increase of distance will be found by taking 

 the square root of 21956 millions, which is 148175. It follows, 

 therefore, that if the sun were removed to 148175 times its pre- 

 sent distance, it would appear as a star precisely similar to the 

 star a Centauri. 



But it has been already shown that this particular star is at a 

 distance 225920 times that of the sun, and, consequently, it 

 follows, that if the sun were removed to that distance, its lustre 

 would be less than that of a Centauri, in the proportion of the 

 square of 148175 to the square of 225920, which is in the propor- 

 tion of 22 to 51. 

 190 



