NEW MEASUREMENTS OF DISTANCE OF SUN. 



115 



From hdiometer observations of Victoria, 

 Iris, and Sappho. 



7r = 8'802 seconds zbO '005 second. 



Front i^95 pJiotograplis of Eros. 



TT — 8-797 seconds ±0 '0047 second. 



Distance of sun in miles. 



92,875,000 

 92,928,000 



I can not refrain from calling yonr attention to a by-product of 

 this work which has for me a singular interest, because it seems to 

 exhibit in a favorable light the accuracy which we may obtain with 

 these photographic methods. After Eros had been under observation 

 for some time it was discovered that its li^ht was varving in a short 



Fig. 5. — Light curve of Ero.s. Whole period 5 h. lO m. 



period, which was at first thought to be 2h. 38m. Afterwards it was 

 found that the alternate minima of light was unequal, and that the 

 true period should be reckoned as 5]i. 16m., two equal maxima and 

 two unequal minima being included within that space of time 

 (fig. 5). The variation appears to be continuous, without sensible 

 pause, which precludes the idea that the planet is double and that the 

 minima are due to eclipses of one body by the other. We must find 

 some other cause. There are two Avhich suggest themselves quite 

 naturally — irregularity of shape and irregularity of surface l)right- 



y- 'Oso 



'OSO. 



Flu. 6. — Residuals grouped in eigliths of the whole perioil. 



ness. For our purpose the important point is this — that either of 

 these causes might produce an apparent oscillation in the ])lace of 

 the jilanet. To discover if this Avere so, I grouped the residuals in 

 my equations of condition according to their epoch into eight columns, 

 corresponding to successive eighths of the whole period of 511. Kim., 

 and took the means for each column. If there were a sensible 

 oscillation in a period of 5h. 16m., these would lie on a sine curve. 

 They obviously do not; there is no sensible oscillation in that })(M'iod 

 (fig. 6). But notice that if we add together the first and fifth, tlie 

 second and sixth, and so on — rhat is to say, if we seai'ch for the half 

 period of 2h. 38m. — wegetquite strong evidence of pei'iodicily {'(\^^- T). 



