March 8, 1900 J 



NA TURE 



447 



The reality of a distinct, if minute, influence exerted by 

 the changes of solar activity on the earth's motion 

 cannot therefore be doubted, though we are as yet com- 

 pletely in the dark as to the physical causes of this 

 peculiar perturbation. 



Now the question arises as to whether traces cannot 

 be discovered of a similar influence upon the motion of 

 the earth-spheroid synchronous with the eleven-years 

 cycle of solar activity. The result obtained on this 

 point receives additional importance from the fact that it 

 throws quite a new light on the theory of a peculiar 

 phenomenon, which has now greatly attracted the atten- 

 tion of astronomers, viz. the variation of latitude. The 

 conclusion to be drawn from our investigation points to a 

 close relationship between the amplitude of the motion 

 of the terrestrial pole and the period of solar activity. It 

 may be taken to be clearly established that the radius of 

 the circle described by the pole of instantaneous rotation 

 is greatest at times of sunspot-minima, and smallest at 



spot spectra during a spot-cycle, the maxima and minima 

 of the spectroscopic curves showing indeed, so far as 

 observations go, a perfect synchronism with those of the 

 curve of latitude-variation. 



Judging from these curves the conclusion may be 

 drawn that a very marked influence on the motion 

 of the terrestrial pole of rotation is exerted by a force 

 varying synchronously with the display of spots on the 

 solar surface. Chandler's data previous to 1856 have 

 not been included owing to their incompleteness. But it 

 ought to be mentioned that the correspondence with 

 regard to the positions of the maxima and minima is 

 quite as certain as in the interval exhibited in the above 

 curves. The sun-spot maximum in 1838 is followed by a 

 minimum of the semi-amplitude in 1840, while the ne.xt 

 sun-spot minimum in 1843 is succeeded by a very pro- 

 nounced maximum of the semi-amplitude in 1845. 

 Judging from the epochs of the maxima, the amplitude 

 of the latitude variation completes three full periods irk 



Fig. 3. — Curves of variation of terrestrial latitade, and of solar activity. 

 The upper curves represent semi-amplitudes of latitude-variation. __^_ Chandler. 



Nyr^n. 



times of maximum-displays of solar spots. This corre- 

 spondence is found to hold true for the whole interval of 

 about sixty years now covered by Dr. Chandler's inves- 

 tigations. The subjoined diagram may help to give a 

 clear idea of this peculiar relation, the first curve showing 

 the semi-amplitudes of the latitude-variation for every 



year from 1856 to li 



as deduced from Chandler's 



curves in Astrofi. Journ. Nos. 277 and 446, and from 

 Dr. Nyr^n's values communicated in Publications de 

 rObservatoire Central Nicolas, Serie ii. vol. ii. ; while 

 the second curve indicates the spot-frequency according 

 to Wolf during the same space of time. As the latitude- 

 phenomenon has been found to lag behind the spot-curve 

 by an interval of about 1-5 years, the latter curve has 

 been shifted one and a half years in the forward direc- 

 tion, in order to establish an agreement between the 

 positions of the maxima and minima of the two curves. 



Attention may here be drawn to Sir Norman Lockyer's 

 discovery that a similar lag can be traced in the curves 

 representing the changes in the lines widened in sun- 



NO, 1584, VOL. 61] 



thirty-four years ; while the epochs of the minima make 

 this figure only slightly less, viz. thirty-two years. Hence 

 the period of the amplitude is found to be eleven years. 



On the whole, then, we are confronted by the fact, sa 

 distinctly brought out by observation, that the motion of 

 our planet reveals traces of the action of a force, the 

 intensity of which can be measured by the state of activity 

 on the solar surface. No doubt, the perturbations caused 

 by this force are extremely minute as compared with the 

 gravitational effects exerted on the earth-spheroid. But 

 still, in the present state of our theoretical knowledge 

 regarding planetary motion, and with the high degree of 

 perfection now attained in the art of astronomical observ- 

 ation, such minute quantities are of considerable import- 

 ance. This is, for instance, sufficiently illustrated by the 

 derivation of the solar parallax from the secular variations 

 of the obliquity and the node of Venus. The value for 

 this constant, as found after eliminating the perturbing 

 effect of the new solar force from the secular variation of 

 the obliquity, is tt = 8""8o2, a result which is in perfect 



