NATURE 



[August 24, iyi6 



precessional rotation and sometimes of changing its 

 amplitude — that is to say, altering the distance be- 

 tween the pole of rotation and the mean pole — accord- 

 ing as it is acting parallel to, perpendicular to, or 

 at any intermediate angle to the direction of the pre- 

 cessional rotation. 1 shall revert to this question 

 later, and show how it is possible by a simple graph- 

 ical construction to separate out this irregular motion 

 and construct a diagram of it which should be helpful 

 in elucidating its cause. 



While it is thus to Chandler that the credit of dis- 

 tx)vering the 428-day period should be ascribed, it is 

 to Kijstner that we owe the first real proof that there 

 is an actual variation in the latitude of a point upon 

 the earth. 



Kiistner's observations were made in the same years 

 as Chandler's, 1884-5, ^*"<J were designed to determine 

 the constant of aberration, a class of observation 

 identical with those which would be used to determine 

 the latitude of the place. Upon reducing these ob- 

 servations the results were at first sight anomalous 

 in that they gave an impossibly small value of the 

 aberration constant. The anomaly was not due to any 

 instrumental cause ; it could not be due to any 

 seasonal change in the refraction, as the morning 

 observations of 1884 were not accordant with the 

 morning observations of 1885, nor could it be explained 

 by any possible error in the proper motions of the 

 stars. Kiistner was thus enabled to state positively 

 that the latitude of the place of observation had 

 actually changed. It must be admitted that the years 

 1884-5 were particularly favourable ones, and that 

 both these astronomers were in a sense lucky in 

 having chanced upon them. The movement of the 

 pole happened at that time to be exceptionally rapid. 

 I do not, however, mention this as detracting in any 

 way from the merit of their achievements ; they deserve 

 to be remembered as simultaneous but independent 

 discoverers of this important and interesting pheno- 

 menon, and should be honoured, Chandler esj^ecially 

 for his courageous rejection of mathematical theory, 

 and Kiistner for the very high skill and exquisite 

 refinement of his observational work. 



The importance of Kiistner's discovery was at once 

 recognised upon the Continent, and a proposal was 

 made to the International Geodetic Conference to 

 establish a chain of stations for carrying on a series 

 of simultaneous observations and thus deducing the 

 true law of this latitude variation. The suggestion 

 was soon carried into effect. Six stations were chosen, 

 all at the same latitude, 39- 1° N.^ — Carloforte, in an 

 island close to Sardinia ; Mizusawa, in Japan ; 

 Gaithersburg in Maryland, and Ukiah in California — 

 all new stations, where special observatories had been 

 built for the purpose ; a new observatory, established 

 by the Russian Government at Tschardjui, in Russian 

 Asia ; and the existing observatory at Cmcinnati. The 

 reason for selecting stations at the same latitude was 

 that identical sets of stars could be observed at each 

 place, and thus any errors due to defective knowledge 

 of star places are similar for all. These began work 

 in 1899. Later, two stations in the southern hemi- 

 sphere, at latitude 315° S.- — Bayswater in Western 

 Australia, and d'Oncatwo in the Argentine— were 

 added. 



The results were reduced and discussed by Prof. 

 Albrecht at the Geodetic Institute, Potsdam, and pub- 

 lished with a diagram showing the actual polar move- 

 ment as deduced from the mean of the observations 

 at all the stations, from time to time. 



It was not long before these observations yielded a 

 result of the highest interest. The observatory which 

 devoted itself most whole-heartedly to the work, and at 



NO. 2443, VOL. 97] 



which the observations were most extensive and most 

 precise, is that in Japan. This was under the able 

 direction of Prof. Kimura. By a searching discussion 

 of the whole series of observations he showed that 

 they became far more consistent if a new term were 

 introduced into the expression for the Jatitude varia- 

 tion, this term having an annual period, but being 

 independent of longitude and having the same value 

 for all the stations at the same date. 



It will be readily seen that this term differs com- 

 pletely from those we have been considering hitherto, 

 it is not a shift of the earth's axis or a movement of 

 the pole of rotation; as it affects all places along a 

 parallel of latitude equally the pole evidently does not 

 move, but something which has an effect exactly the 

 same as if the centre of gravity of the earth were 

 shifted a few feet up and dow-n, northward and south- 

 ward, from its mean position. 



The great difficulty in elucidating the Kimura term 

 lies in its extremely small magnitude and in the con- 

 sideration that there are so many possible sources of 

 error affecting observations of this class which might 

 have annual periodicities that their separation and 

 evaluation are extraordinarily complicated questions. 

 This is not the place to attempt any complete discus- 

 sion, but a mention of some of the lines along which 

 a solution has been sought may detain us for a few 

 minutes. 



The magnitude of the term at the latitude of 39° is 

 about 6/iooths of a second of arc, or 6 ft. on the 

 earth's surface. It has the same value and phase 

 for every station on the same parallel and is zero on 

 about March 9 and September 12, and maximum and 

 minimum on June 10 and December 10, i.e. about ten 

 days before the equinoxes and solstices respectively. 

 It cannot be accounted for as a real shift of the 

 earth's centre of gravity. It is true that in the alter- 

 nate melting and accumulation of ice and snow at the 

 two poles we have a periodic factor at work which 

 does do this, but the amount is far too small. If was 

 pointed out long ago by Van de Sande Bakhuysen 

 that to fit in with the observed value of this term the 

 apparent path of the centre of gravity must have an 

 amplitude of 3 metres, which, if translated into terms 

 of polar ice, would mean that a cap of ice one kilo- 

 metre thick and 244 square degrees in area would 

 have to form and disappear each year. This is ob- 

 viously quite impossible. There are certain possible 

 errors in the accepted values of the proper motions 

 and parallaxes of the fixed stars w^hich might produce 

 an apparent variation in the observed latitude of this 

 nature. As all parallaxes are based upon differential 

 measures we cannot with certainty say that such 

 errors are impossible ; we can only say that they appear 

 to us very unlikely, and that, if they were actually 

 proved to exist, our ideas of the stellar universe would 

 be profoundly modified. 



If there were a yearly term in the refraction which 

 had the effect of a periodic change in the apparent 

 zenith we should get a corresponding periodicity in 

 the observations. If, for example, there were a solar 

 atmosphere, even of a quite tenuous nature, which 

 extended into space beyond the earth's orbit, we should 

 get a seasonal change due to the varying angular 

 distance of the sun from the zenith of the place of 

 observation. An atmosphere which could bend rays 

 of light to the requisite amount, though undoubtedly 

 extremely rare, would, however, be dense enough to 

 offer an amount of resistance to a planet, or o fortiori 

 to a comet, inconsistent with observed facts. It is, 

 however, quite possible that the changing declination 

 of the sun may curve or tilt the mean isobaric surfaces 

 in the upper atmosphere in such a way that the 



