188 



NATURE 



[October 15, 1914 



He found an ellipticity 1/294-41 1-5 closely agreeing 

 with that which I have obtained. Finally, Col. 

 Clarke's value obtained from geodetic measures was 

 1/293-5. We have thus three quite different deter- 

 minations ranging round 1/294 to set against a fourth 

 determination of 1/298. The term in the latitude of 

 the moon which has often been used for this purpose 

 is of little value on account of the coefficient being also | 

 dependent on the value of the obliquity of the ecliptic ; 

 such evidence as it presents is rather in favour of the 

 larger value. I omit Hill's value, obtained from 

 gravity determinations, because it is obviously too 

 large. 



Here, then, is a definite issue. To satisfy the ob- 

 servations of the moon in at least three different parts, 

 a value near 1/294 must be used; while the value 

 most carefully found from gravity determinations is 

 1/298. So far as astronomy is concerned, the moon 

 is the only body for which a correct value of this con- 

 stant is important, and it would seem inadvisable 

 to use a value which will cause a disagreement be- 

 tween theory and observation in at least three different 

 ways. It is a question whether the conference value 

 should not be changed with the advent of the new- 

 lunar tables. 



In looking forward to future determinations of this 

 constant, it seems to be quite possible that direct 

 observations of the moon's parallax are likely to 

 furnish at least as accurate a value of the earth's 

 shape as any other method. This can be done, I 

 believe, much better by the Harvard photographic 

 method than by meridian observations. Two 

 identical instruments are advisable for the best 

 results, one placed in the northern and the other 

 in the southern hemisphere from 60° to 90° apart in 

 latitude, and as nearly as possible on the same 

 meridian. On nights which are fine at both stations, 

 from fifteen to twenty pairs of plates could be 

 obtained. In a few months it is probable that some 

 400 pairs might be obtained. These should furnish 

 a value for the parallax with a probable error of 

 about 002", and a value for the ellipticity within 

 half a unit of the denominator 294. It would be still 

 more interesting if the two instruments could be set 

 up on meridians in different parts of the earth. The 

 Cape and a northern observatory, Upsala, for 

 example, would furnish one arc; Harvard and 

 Ariquipa or Santiago another. If it were possible to 

 connect by triangulation Australia with the Asiatic 

 continent, a third could be obtained near the meridian 

 of Brisbane. Or, accepting the observed parallax 

 and the earth's ellipticity, we could find by observation 

 the lengths of long arcs on the earth's surface with 

 high accuracy. 



In any case, I believe that the time must shortly 

 come when the photographic method of finding the 

 moon's place should be taken up more extensively, 

 whether it be used for the determination of the moon's 

 parallax and the earth's ellipticity or not. The 

 Greenwich meridian observations have been, and con- 

 tinue to be, a wonderful storehouse for long series 

 of observations of the positions of the sun. moon, 

 planets, and stars. In the United States, Harvard 

 Observatorv has adopted the plan of securing con- 

 tinuous photographic records of the sky, with par- 

 ticular reference to photometric work. Under Prof. 

 Pickering it will also continue the photographic record 

 of the moon's position so long as arrangements can 

 be made to measure the plates and compute the 

 moon's position from them. 



In spite of the fact that Harvard Observatory has 

 undertaken to continue for the present the work of 

 photographing the moon's position, I believe that 

 this method should find a permanent home In a 

 national observatory-. It has already shown Itself 

 NO. 2346, VOL. 94] 



capable of producing the accuracy which the best 

 modern observations of Greenwich can furnish, and 

 no higher praise need be given. If this home could 

 be found in the southern hemisphere, and more par- 

 ticularly in Australia, other advantages would accrue. 

 But we should look for more than this. In an 

 observatory the first duty of which might be the 

 securing of the best daily records of the sky, the 

 positions of the sun, stars, planets, a couple of plates 

 of the moon on every night when she is visible would 

 be a small matter. What is needed is an organisa- 

 tion so constructed as to be out of the reach of 

 changing governmental policy, with a permanent 

 appropriation, and a staff of the highest character 

 removed from all political influences. It could render 

 immense service to astronomers, not only in the 

 Empire, but all over the world. The pride which 

 every Englishman feels who has to work with the 

 records of the past furnished by Greenwich would in 

 course of time arise from the work of a similar 

 establishment elsewhere. Those of us who live in 

 a community which, reckoning by the age of nations, 

 is new, know that, In order to achieve objects which 

 are not material, sacrifices must be made ; but we 

 also know that such sacrifices are beneficial, not only 

 In themselves, but as exerting an indirect influence 

 in promoting the cause of higher education and of 

 scientific progress in every direction. In saying this, 

 I am not advocating the cause of the few, but of 

 the majority; the least practical investigations of 

 yesterday are continually becoming of the greatest 

 practical value to-day. 



No address before this section Is complete without 

 some speculation and a glance towards the future. 

 I shall indulge in both to some small extent before 

 closing. I have shown you what the outstanding 

 residuals in the moon's motion are : they consist 

 mainly of long-period fluctuations in the mean longi- 

 tude. I have not mentioned the secular changes 

 because the evidence for them does not rest on 

 modern observations, but on ancient eclipses, and 

 these are matters too debatable to discuss in the 

 limited time allotted to me for this address. It rnay 

 be said, however, that the only secular motion which 

 is capable of being determined from the modern 

 observations, and is not affected by the discussion of 

 ancient eclipses — namely, the secular motion of the 

 perigee — agrees with its theoretical value well within 

 the probable error. With this remark I pass to the 

 empirical terms. 



These unexplained differences between theory and 

 observation may be separated Into two parts. First, 

 Newcomb's term of period between 250 and 300 years 

 and coefficient 13", and, secondly, the fluctuations 

 which appear to have an approximate period of sixty 

 to seventy years. The former appears to be more 

 Important than the latter, but from the investigator's 

 point of view It is less so. The force depends on 

 the degree of inclination of the curve to the zero line, 

 or on the curvature, according to the hypothesis made. 

 In either case the shorter period term is much more 

 striking, and, as I have pointed out on several 

 occasions, it Is much more likely to lead to the sources 

 of these terms than the longer period. It Is also, at 

 least for the last sixty years, much better determined 

 from observation, and is not likely to be confounded 

 with unknown secular changes. 



Various hypotheses have been advanced within the 

 last few vears to account for these terms. Some of 

 them postulate matter not directly observed, or matter 

 with unknown constants; others, deviations of the 

 Newtonian law from Its exact expression ; still others, 

 non-gravitational forces. M. St. Blancat ' examines 



fi AnnaUs de la FeuuUi eies Sciences deToulouse , 1907. 



