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I. The path of the earth's projection on the sun. 

 The probable origin of the 11-year period. 



If it be true that sun-spots, faculae and prominences are effects of 

 ray-curving, it stands to reason that their form and situation will 

 depend in a far greater measure on the position occupied by the observer, 

 than would be tlie case, if they were themselves light-emitting bodies. 



A correct idea of the movement of the Eartli Avith respect to the 

 reA'olving body of the Sun nnist therefore be the basis of our invest- 

 igations. Ihiforüiuately it is impossible to give an absolutely exact 

 idea of this relative motion, for iu)t only are we in ignorance of the 

 exact period of the Sun's rotation, but it is extremely difticult to 

 define the meaning of that term, because we take the Sim to be a 

 mobile gaseous mass. ()n tlie other hand it is quite evident that we 

 are dealing with a pcriotHcal pliciioiuenou ; the only (jueslioii tlierefoi-e 

 is, whether we shall succeed in selecting from the various \alues 

 on record, the one which lias the greatest significance from our 

 point of view on Earth. 



As a matter of course we select a synodical period of re\'olutiou. 

 It is a known fact that difrerent values for the |K'riod are obtained 

 from the movement of spots aiul faculae, varying from 2G to 30 days 

 according to their heliograi)hic latitude. By the a|)plicati()n of Doppler's 

 principle, Duner found that near the equator, the i)eriod of rotation 

 of the photosphere was 25,4() days and at 75° latitude up to 38,55 

 davs. In 1871 Hornstein observed in the deviations of the magnetic 

 declination at Prague a period of about 26 days, which other invest- 

 i'>ators have found also in various meteorological phenomena. The 

 results obtained led to the conclusion, that the rotation of the ecpia- 

 torial regions of the Sun exercises a greater inlluence on the Earth 

 than that of the other zones. 



From the following table it will appear how iiidelijiile as yet is 

 our knowledge of the period of the Sun's rotation : 

 Stratonoff (faculae near the equator) 26,06 ') 



Carrington (smi-spots near the equator) 26,82 ^) 



DuNER (photosphere near the equator) 25,46 ') 



Hornstein (magnetic observations at Prague) 2(),5 ^) 



Ad. Schmidt (most probable value deduced from the mag- 



i),Arrhenius, Lehrb. d. kosmisclicn Pliysik, p. 148. 



2) This value is communicated by Dünér as being the sidereal period of rotation, 

 and appears to have been generally accepted as such. Prof. J. (J. Kapteyn, however, 

 kindly informed me that in Dunér's interpretation an error has slipped, and that 

 he result must be taken as the synodical period. 



