REVIEW OF ASTRONOMY FOR THE YEAR 1913. 



By P. PuisEUx, 

 Member of the Institute, Astronomer at tlie Ohficrratory of Paris. 



STUDY OF PLANETS AND COMETS. 



The increasing knowledge of the phenomena of the globe that 

 carries us puts ns in a position to interpret more snrel,v what we 

 observe in the celestial bodies. The astronomer, who gives to the 

 mariner and the geodecist the means for determining their time and 

 precise position, hopes some day to receive some recompense for these 

 services. He is examining now^ the facts which come from tlie scien- 

 tific stations established at diverse latitudes. One of the least ex- 

 pected among these facts is a small annual variation in geographic 

 latitude. This variation had not been predicted by dynamical theory. 

 It takes place as though the center of gravity of our globe were 

 displaced alternately about 3 meters toward the North, and then 

 toward the South Pole. Several explanations come to mind, but have 

 to be abandoned under closer analysis. For instance, the melting of 

 the ice, taking place alternately each six months in the region of 

 the two poles, acts in the right direction, but in order to correspond 

 with the magnitude of the observed change, would have to affect 

 masses of ice very improbable in size. The most-favored opinion, 

 developed by the recent studies of Kimura, Ross, and Biske, assumes 

 that the isobars (lines of equal pressure) of the air vary with the sea- 

 son, oscillating about a mean configuration. There would result, for 

 a series of stations at the same latitude, a variation in the same man- 

 ner of the atmospheric refraction, and an annual, purely apparent 

 oscillation would be mixed with the one of 430 days, the reality of 

 which W'C have no reason for doubting. 



The movements of the magnetic needle show bizarre caprices which 

 would seem to escape all prediction. However, in a long series of 

 means, each magnetic element is seen to be affected by four super- 

 posed fluctuations the periods of which are the day, the year, the 

 synodic rotation of the sun, and the sun-spot-cycle period. From this 

 we conclude that the sun acts upon the earth's magnetism, not only 

 through the unequal heating to which it subjects our globe, but also 

 through a direct action, doubtless the restricted emission of electrified 



1 Translated, by permission, from the Revue gen^rale des Sciences, vol. 25, p. 746, 1914. 



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