204 H. Crew — Period of Rotation of the Sun. 



Art. XXYII. — On the Period of Rotation of the Sun ; by 

 Henry Crew, Ph.D., Instructor in Physics in Haver- 

 ford College. 



Peters* was probably as early as anyone to remark that the 

 period of rotation of the sun, without farther limitation, was a 

 meaningless term. In a series of observations on sun spots, 

 taken at Naples during the years 1845-6, he found, among his 

 results, discrepancies larger than could be explained by errors of 

 measurement ; but what is more striking, these discrepancies 

 were always in one direction. In short, he showed that each 

 heliocentric latitude has its own period of rotation. 



The law, according to which this velocity varies with the dis- 

 tance from the equator, Carringtonf has placed beyond doubt. 

 His work, however, is all confined to the photosphere or the 

 immediately underlying region in which the sun spots have their 

 seat. 



A few years later Braun and Hornstein,;}; independently, dis- 

 covered in the magnetic elements a pretty well marked variation, 

 having a twenty-six-day period. This at once suggested a means 

 of getting the rotation period of the solid (?) nucleus of the sun. 

 For Braun (Phil. Trans., 1876), from two years' observations, 

 finds that the large disturbances of the horizontal component 

 of the earth's magnetism were nearly all confined to the days 

 on which one of three solar meridians was' presented to the earth. 

 But this needs confirmation, for the total number of disturb- 

 ances observed was not large. However, the phase% of this 

 disturbance, unlike those of the annual and secular variations, 

 is the same in all parts of the earth, seeming to indicate that we 

 have here to deal with solar action which is direct and not inter- 

 mediate. On the other hand, solar temperatures appear to pre- 

 clude the possibility of the sun being a permanent magnet. So 

 that the region of sun whose period Braun and Hornstein and, 

 later, LiznarJ have determined remains completely unknown, 

 but with the probability of its being below the photosphere. 



A fairly pronounced twenty- six-day variation in the daily 

 range and height of both the thermometer and the barometer 

 has been known for a long while. But no theory, in any degree 

 tenable, has been offered to explain the connection of this vari- 

 ation with the sun. We are, therefore, also unable to assign 

 this period to any definite solar height. But there are reasons 

 (Encycl. Brit., art. Meteorology) for thinking that it is the rota- 



* Proc. Amer. Assoc. Adv. Sci., ix, p. 81, 1855. 



f Carrington, Observations on Solar Spots, London, 1863. 



i Hornstein, Ber. Akad. Wien., lxiv, p. 62, lxvii, p. 385, 1873. 



§ Braun, C. R., lxxvi, p. 698. 



IfLiznar, Ber. Akad. Wieu., Bd. 91, p. 454. 



