230 FROM NEBULA TO NEBULA 



Of the intensity of the sun's brightness an idea may 

 be gained by a comparison of the following facts: At 

 the distance of Neptune the sun subtends an angle of 

 slightly more than a minute of arc, or, to cite an illustra- 

 tion, it subtends the same angle as a cent piece does at 

 the distance of 200 feet obviously too small an area to 

 be recognized as a disc. Now, it has been repeatedly 

 estimated, with care, that we derive 600,000 times as 

 much light from the sun as we do from the full moon; 

 hence putting two and two together, it follows that the 

 solar spark yields as much light to the inhabitants of 

 Neptune as would a battery of 666 full moons such as our 

 own. 



To the novice it might seem a comparatively easy 

 matter to determine the temperature of the sun, but that 

 it is by no means so is shown by the great diversity in the 

 estimates made at various periods by recognized authori- 

 ties; varying, as they do, between 6,000 and upwards of 

 a million degrees. In his recent work, The Sun (pp. 109- 

 116), after describing the four methods which he deems 

 the most dependable, Doctor Abbot says: " Hence we 

 conclude that there is a high probability that the average 

 temperature of the apparent photosphere exceeds 5860, 

 or even 6260, on the absolute Centrigrade scale, and may 

 be as high as 7,000 absolute Centrigrade ' '. In passing 

 let me record my substantial agreement in this estimate. 



However, that there is still a mystery behind this one 

 of temperature yet to be explained away, will appear 

 from the following considerations : Construing the earth's 

 disc as part of the surface of a sphere 186,000,000 miles 

 in diameter (i. e., double the earth's distance from the 

 sun), and comparing this surface with that of the sun, 

 whose diameter is 865,000 miles, and, further remember- 

 ing that the surfaces of spheres are to each other as the 

 squares of their diameters, it develops that the radiation 

 of each square foot of the sun's area must answer for the 

 heating of each 46,000 square feet of the earth's area, 

 considered as a disc. We should not forget, however, 

 that the sun 's rays fall on the same hemisphere only half 

 of the 24 hours and that it is a hemisphere, and not a flat 



