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Astronomy. — ''Prdbniiuirn /'nvest/j/tttioii into the motion of the 

 pole of the earth in. J 907." Hy Dr. H. .). Zwikks. (Coinmnni- 

 ciited 1)) Pi'of. K. F. VAN dk Sandk Bakhuyzkn.) 



Ill 1910 I started investigations into tlie motion of the momentary 

 rotation-pole of the eartii since tlie l)eginiiing of tlie year 1H90. 

 Besides the general scientitk* importance of such an analysis, especi- 

 ally for the future explanation of the rather complicated phenomenon, 

 another reason prom|)ted these investigations, i.e. the desire to arrive 

 at a safe basis for a (piick reduction of some observations of 

 declination with the Leyden meridian circle. For often enough 

 starplaces must be reduced soon after the day of observation, 

 even before aiiytliiiig is known about the momentary value of the 

 latitude, which is one of the most im[)ortant elements of re<luction. 



The general results of these investigations I hope to |)ublish before 

 long. At present I desire only to communicate some preliminary 

 results about a perturbation in the regular motion of the pole, which 

 must have taken place in the course of 1907. 



Early in my investigations I found, that, while the motion of the 

 pole up to the beginning of 1907 could be represented by simple 

 formidae with tol<3ral)le accuracy, later observations showed great 

 deviations. Originally only the results of the observations u|> to 

 1908.5 were known from Prof. Albkkcht's dilferent pui)licati()iis, 

 and the lapse of time after the moment of the perturbation was too 

 short to determine accurately its nature and the orbit described 

 afterwards. In N". 4414 of the Astvonoinii^che Nnchricltten Albrecht 

 gave the polar co-ordinates for the period 1908.0 until 1910.0, and 

 in the first days of June 1911 in Astron. iVachr. N^ 4504 a con- 

 tinuation of the table of these co-ordinates was jMiblished by him, 

 as far as the commencement of 1911. This enabled me to investigate 

 more closely into the time and the probable nature of the disturbance. 



Without dealing with it in detail, I must first give here some 

 results of my earlier investigations. 



It api)eared, that from 1890.0 until early in 1907 the motion of the 

 pole could be analysed into a yearly ellipse and an approximately 14- 

 monthly circle, the motion in both being from W. to E. The polar 

 co-ordinates msiy be thus represented by : 



, \_ (i) 



in which the indices 1 refer to the co-ordinates in the yearly ellipse, 

 the indices 2 to the 14-monthly circle, while 5 and ^ represent the 

 co-ordinates of the mean i)ole. 



