534 Brown — Effects of Magnetic and Gravitational 



Synodic month, 27 d, 33l7, 

 Anomalistic month, 25 d '6217, 

 Nodal month, 25 d '3254, 



and one of these is to differ from the sidereal period of rota- 

 tion of the sun by about d *0038, since the angles present in 

 the disturbing function are double the differences, and the 

 period of the empirical term is about 3600 months. 



The nodal month is nearest to the values found for the mean 

 solar rotation period by Carrington (25 d *38) and Spoerer 

 (25 d, 23) from observations of the motion of sun-spots. W. 

 S. Adams,* observing the displacements of certain lines of 

 the spectrum, obtains periods ranging from 24r d, 57 at the equa- 

 tor to 35 d, 26 in latitude 79°. He inclines to the view that the 

 equatorial velocities give a period approximating more closely 

 to the actual period than those of higher latitudes. The evi- 

 dence so far obtained indicates a possibility that the true rota- 

 tion period may be close to that of the nodal month relative to 

 the earth's mean radius vector. 



A coincidence with the nodal month involves an effective 

 ellipticity of about 1/2200. This might be detected over a 

 long series of observations if the visible surface of the sun 

 were solid. It is extremely unlikely that such, an ellipticity 

 would persist as far as the photosphere, which ought on the 

 average to be a level surface. An indirect method may, how- 

 ever, furnish evidence one way or the other. If the ellipti- 

 city exists, it is probable that the solar activity would be more 

 marked in two sectors on opposite sides of the solar axis than 

 in the other two sectors. The average period derived from 

 observations of sun-spots or other evidence of abnormal 

 activity would therefore furnish the material to be examined. 

 It is significant that the nodal period lies between the results 

 for the solar period obtained by Carrington and Spoerer. 

 The hypothesis opens up a wide field of speculation and 

 investigation as to its physical consequences, especially on the 

 motion of the outer liquid or gaseous layers of the sun's mass. 

 The belief that the eleven -year period is due to forces within 

 the sun is perhaps less difficult with the hypothesis of such an 

 inequality in the distribution of the mass of the nucleus. 



8. Hypothesis of a magnetic attraction hetween the earth 

 and the moon. — For the purposes of this section it will be 

 sufficient to consider the magnetic fields of the earth and moon 

 as equivalent to fields produced by straight solenoids sym- 

 metrical with respect to their respective centers of figure. 

 The directions of these solenoids will, in the first instance, be 

 taken arbitrarily, but the main results of the discussion depend 

 on the directions which may be assigned to them. 



* Astrophysical Journal, March, 1909. 



