Astronomy. 569 



III. Astronomy. 



1. On the Theory of the Moon's Motion. — The address of 

 Professor E. W. Brown before the sub-section of cosmical physics 

 at the Australian meeting of the British Association starts with a 

 brief review of the history of the problem of the lunar motion. 

 He points out that for the past two hundred years there has been 

 a constant effort for the theory to account for the observed path. 

 The success which has been obtained up to the present time is 

 the main object of his address. His theory was completed some 

 seven years ago, and since that time the chief object has been to 

 compare it with the observations so as to secure the best values of 

 the arbitrary constants and to find out in what respects the Moon 

 deviates from its computed position. There are two great series 

 of observations, one consisting of occultations gathered from 

 observatories all over the world by Simon Newcomb, the other 

 the Greenwich meridian observations recorded almost without a 

 break since the year 1750. Newcomb's results from the occulta- 

 tions were published not long ago and the amount of work neces- 

 sary to compare it with the latest theory has been quite small. 

 The reduction of something like twenty thousand Greenwich 

 observations has been partly done by Airy and P. H. Cowell. 

 Professor Brown has taken their results and correcied them in every 

 possible way so as to get a direct comparison with the new 

 theory. The net result is very satisfactory. The agreement 

 with Newcomb's values has been unexpectedly close and the 

 different es which do occur are neither very large nor very impor- 

 tant. On the whole, the meridian observations give better results 

 in most cases than the occultations. The great value of the 

 latter series is that it gives us a criterion which enables us to say 

 that certain deviations of the Moon from its theoretical orbit 

 are not due to errors in the observations. 



The principal deviations consist first of the great inequality 

 discovered by Simon Newcomb having a period of some 270 

 years and amplitude of about 12 seconds of arc. This is a devia- 

 tion of i he Moon in front of and behind its theoretical position. 

 Superposed on this are smaller deviations which appear to have a 

 main period of between (50 and 70 years and a coefficient of 3 or 

 4 seconds. But it is pointed out that this is not a periodic 

 change in the proper sense, for it has been found impossible to 

 analyze it. It is these deviations which have formed the subject 

 of a number of hypotheses. It appears now to be fairly certain that 

 they cannot be accounted for by any gravitational cause ; the 

 effects of all the bodies in the solar system have been very care- 

 fully computed and the observed fluctuations are very large in 

 comparison with most of the gravitational effects which come 

 from distant bodies. At the end of his address Professor Brown 

 examines a number of hypotheses which have been advanced to 

 explain either or both of these deviations. He rejects many of 



