ASTRONOMY 93 



"Hitherto we have assumed the spot to be single. The typical spot-group originates 

 through the development of a second spot to the east (following) the primary spot, the 

 line joining the two making only a small angle with the equator. The polarities of the 

 two principal spots (smaller companions are usually present) are opposite, and the hydrogen 

 (Ha) flocculi surrounding a bipolar group resemble the lines of force about a bar magnet. 

 Two alternative hypotheses may be offered to account for these phenomena. According to 

 the first, the lower extremity of the vortex of the primary spot, lagging behind the upper 

 extremity turns up to meet the photosphere thus producing a horse-shoe vortex. As in the 

 case of single (unipolar) spots, the hydrogen and other high-level gases flow toward the two 

 members of the group along the lines of force. According to the second view, the two vortices 

 are independent with nearly radial axes. The necessary criteria of selection can probably 

 be found by a comparative study of the principal members of a bipolar group, special atten- 

 tion being directed to the inclination of the axes of the vortices to the line of sight (as given 

 by the Zeeman effect), the direction of flow (inward or outward) of the vapours at different 

 levels, and the relative temperature and pressure of the two spots." 



The details of work at the Mount Wilson Observatory by W. S. Adams on the sun's 

 rotation have been published in an important memoir. The observations are all satis- 

 fied by a law in which the velocity of rotation increases outwards from the surface, and 

 the equatorial acceleration decreases. 



The Moon. The prediction of the precise place of the moon in the heavens at any 

 given moment has been a problem of increasing interest from the earliest times till now. 

 It attracted the attention of the ancients from their desire to predict eclipses: of astrono- 

 mers before Newton from their desire to help sailors to find their longitude: of astrono- 

 mers since Newton from their desire to test the accuracy of the law of gravitation with 

 the utmost refinement. Recent work has shown that the law of gravitation alone is 

 insufficient to explain the moon's movements. The situation was thus summed up 

 shortly before his death (in July 1909) by Simon Newcomb, who had spent much of his 

 life on the problem: 



"I regard these (outstanding) fluctuations (of the moon) as [the most enigmatical phe- 

 nomenon presented by the celestial motions, being so difficult to account for by the action of 

 any known causes that we cannot but suspect them to arise from some action in nature 

 hitherto unknown.". 



This crisp statement was only rendered possible by an enormous amount of devoted 

 work on the part of many people; centuries of observation at the observatories of Green- 

 wich and Paris; careful scrutiny of these records by Newcomb and more recently by 

 Co well; great labour in working out the theory of the moon's movements by various 

 eminent mathematicians, and the formation of tables, especially those of Hansen; and 

 a recent reconstruction of the theory by E. W. Brown, following the methods of G. W. 

 Hill. Brown is proceeding, with financial help from Yale University, to the construction 

 of new tables of the moon, which may be confidently expected to isolate these puzzling 

 anomalies from various defects in the older tables. Meanwhile new tables have been 

 published in France (in 1911) which represent the completion of a project begun by 

 Delaunay half a century earlier. Delaunay was drowned by the upsetting of a pleasure 

 boat at Cherbourg in 1872, but his work was carried on by Tisserand, Schulhof, Andoyer, 

 Radau and others. In completing the tables full use has been made of the opportunities 

 afforded by recent work. They will be used in the French predictions, which will give 

 a valuable check on predictions made by Brown's tables when ready. It now remains 

 to supplement this considerable improvement in the means of prediction by correspond- 

 ing improvements in the observation of the moon's place. The way has already been 

 shown by the successful photographic observations taken at Harvard and reduced by 

 H. N. Russell of Princeton. By an ingenious device the moon is. photographed along 

 with the stars surrounding it. The difficulty is of course to avoid fogging the plate with 

 the bright moonlight, and so blotting out the tiny star images. The device used by E. 

 S. King of Harvard is to cut off the moonlight during the greater part of the exposure by 

 a screen placed at some distance outside the lens, past which the rays from surrounding 

 stars can edge to reach the lens. 



Planets and Satellites. H. Struve has published the discussion of measures on Mars 



1 At the British National Physical Laboratory. 



