J.^NUARY, 1915. 



KNOWLEDGE. 



15 



la"" 8" -7 m W., W^ S=> -1 m S., 20'' 10'' -3 »i E., 24'' 10'' -1 

 m I., aS"* 7'' -2 m W. ; Japetus 9'' midnight I. 



Uranus is invisible. In conjunction with Sun February 

 1st. 



Neptune was in opposition January 20th, diameter 2". 



Double Stars and Clusters. — The tables of these, 

 given three years ago, are again available, and readers are 

 referred to the corresponding month of three years ago. 



Varl\ble Stars. — Stars reaching their maxima in or near 

 January, 1915, are included. The lists in recent months may 

 also be consulted. (See Table 3.) 



Meteor Showers (from Mr. Denning's List) :— 



Table 4. Occultations of Stars by the Moon visible at Greenwich. 



*The asterisk indicates the day following that given in the date column. 

 From New to Full disappearances take place at the Dark Limb, from Full to New reappearances. 

 Attention is called to the tangential occultations of 21 Piscium and BD + 15°'2027. 

 Probably for some points in England these stars will escape occultation. 



ASTRONOMY. 



By A. C. D. Crommelin, B.A., D.Sc, F.R.A.S. 



PROFESSOR E. W. BROWN ON THE MOON'S 

 MOTION. — Professor Bro%vn was Chairman of the Cosmical 

 Physics Section at the recent meeting of the British Asso- 

 ciation. Nature for October 15th contains his address, 

 which brings out the fact that the study of the Moon's 

 motion has now reached a point where it overlaps ^\-ith the 

 study of the physics of the solar system, forces of electrical 

 or magnetic character being invoked as possibly responsible 

 for the residuals that stUl remain in the observed places of 

 the Moon. 



The growth of human knowledge on this subject forms 

 a long and glorious chapter of difficulties patiently sur- 

 mounted. As far back as the time of Hipparchus we find 

 a knowledge of the eccentricity of the orbit and of the 

 change in the direction of the major axis, which revolves 

 in about nine years. The Evection, the largest of the 

 disturbances produced by the Sun, was discovered by 

 Ptolemy ; the Variation, the next in order of magnitude, 

 seems to have been found by the Arabian, Aboul Wefa, 



about A.D. 975 : it was rediscovered by Tycho Brah6. The 

 ancient astronomers also knew of the inclination of the 

 Moon's orbit to the ecliptic, and the retrograde motion 

 of the line of nodes. They could predict eclipses by the 

 Saros cycle, but could not predict the region w-here an 

 eclipse would be total. So far, all the discoveries had been of 

 a purely empirical nature ; no explanation could be given of 

 the cause of the various irregularities. That was reserved 

 for Newton, who applied the question as a searching test 

 of his law of gravitation. His wonderful mathematical 

 powers enabled him to explain the larger lunar irregulari- 

 ties by geometrical methods ; these, however, proved 

 inadequate for such intricate problems, and Ne\vton 

 laid the foundations of the calculus and of analytical 

 methods, which were firrther developed by Clairaut, d'Alem- 

 bert, and Euler, followed a century later by Hansen and 

 Delaunay, and, still more recently, by Newcomb and 

 Professor Bro%vn himself, who, following up a suggestion 

 of the late Dr. Hill, has made a new computation of the 

 gravitational effect of every known body in the solar system 

 on the Moon's motion to a degree of accuracy beyond 

 that which observation can at present attain. In spite 

 of this notable advance in accuracy, no such illusions are 

 entertained about the new tables as were put forward 



