SOLAR LIGHT AT PLANETS. 



principles. It has been already stated that the apparent diameter 

 of the sun's disk is less in exactly the same proportion as the. 

 distance of the observer from that luminary is greater. Since, 

 therefore, the distances of Jupiter, Saturn, Uranus, and Neptune 

 are severally five, nine, eighteen, and twenty-eight times the earth's 

 distance, the apparent diameters of the sun's disk, as seen from 

 them, will be |, , ^ and of its diameter, as seen from the earth. 



Fig. 2. Fig. 3. 



If the white circle E (fig. 2) be imagined to represent tbe 

 apparent disk of the sun, as it is seen by an inhabitant of the 

 earth, then J (fig. 3) will represent its appearance to an inha- 

 bitant of Jupiter, s its appearance to an inhabitant of Saturn, 

 u to an inhabitant of Uramis,and N to an inhabitant of Neptune. 



The light and heat which it would supply to each of these 

 planets would be in the exact proportion of the apparent surface 

 of the solar disk, and since the areas of circles are as the squares 

 of their diameters, it would follow that the solar light and heat 

 at Jupiter is 25 times, at Saturn 81 times, at Uranus 324 times, 

 and at Neptune 784 times less than at the earth.* 



19. In considering the question of the habitability of these 

 globes, it might appear from these numbers that the illuminating 

 and heating power of the sun would be so diminished by distance 

 as to be incompatible with the existence of organised races, at 

 least on the more distant of those planets. It must, however, be 

 considered that the illuminating power of the sun would be the 

 same as at the earth, if only the pupils of the eyes were enlarged 

 in the same ratio as the apparent superficial magnitude of the 



* These numbers are not the exact ratios, but are near enough for the 

 present illustration. For more precise results see "Handbook of Natural 

 Philosophy and Astronomy" (2,994). 



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