M.-P.— Vol. I.] DAVIDSON— APPARENT PROJECTION, ETC. 93 



of the disc; and several cloud-like areas actually detached. 

 The breadth of the thin border is about 6", and the whole 

 factitious breadth must be more. 



It seems a physical impossibility that we should be able 

 to detect any irregularities of even a dense atmosphere at 

 the border of the Sun. If that body were to decrease 45 

 miles in diameter, our instrumentation would not be able to 

 measure it under present conditions. Much less could we 

 detect it if there were actually rolling billows of the exterior 

 matter of the Sun 45 miles high around its border. They 

 would subtend but 0.1" in height; a quantity covered by the 

 finest spider thread in the telescope. Much less could we 

 detect any changes of atmospheric conditions at the dis- 

 tances of Jupiter, Mars, Venus, or Mercury. Some great 

 deep Sun spot just on the edge of the Sun would doubtless 

 show a depression, but not any imaginable storm disturbance 

 of the densest atmosphere, as we understand storms and 

 waves. 



Official Recognition of the Factitious Borders of 



the Moon and Sun. 



In still further confirmation of the existence of the 

 spurious limbs of the Moon and Sun under unfavorable 

 atmospheric conditions, it is only necessary to appeal to the 

 representative of astronomical authority in the American 

 Ephemeris or Nautical Almanac. In that government pub- 

 lication an empirical correction of 2". 5 is applied to the 

 Moon's semi-diameter in order to represent the observed 

 value in meridian instruments. But this " constant is 

 omitted in the computation of solar eclipses and occultations 

 as due to telescopic and ocular irradiation" (p. 5 2 ^' 

 1899). This 2". 5 of increased semi-diameter may represent 

 the average measure of the factitious border, but certainly 

 it is frequently much greater. 



In many years of experience upon the Pacific Coast of 

 the United States in observing lunar transits, we have 

 learned to expect wild results when a markedly factitious 

 bright limb of the Moon was observed, the resulting longi- 

 tude being always affected as if the diameter of the Moon 



