The Sun 



teen days, to start on a second, sometimes even 

 a third and a fourth revolution before it vanishes 

 entirely. The earliest observers had attributed 

 these phenomena to the transit of planets in 

 front of the sun, but from the time of Galileo 

 this hypothesis has been abandoned because 

 several co-existing spots are frequently seen at 

 the same time to cross the sun on parallel lines, 

 and in addition the faculae appear to follow 

 the same rotary motion. Only one explanation 

 can account for all the details of this move- 

 ment, viz.: the luminous surface of the sun 

 rotates as a whole round an axis which is the 

 line between the poles of the sun. The period 

 of the rotation T is about 25 J days. 



This rotation of the solar surface has been 

 confirmed by experiments based on an entirely 

 different principle, which may be rendered com- 

 prehensible by the following parallel. When 

 two trains are moving in opposite directions and 

 pass one another, a traveller in one of the trains 

 would notice that the whistle is of a higher pitch 



1 Not twenty-eight days as might be inferred from the 

 duration of their apparent movement from edge to edge of the 

 sun, because the translation of the earth during that period 

 must be taken into account. 



K 165 



