TRANSACTIONS OF SECTION A. ; 317 
cosmogonic applications. The astronomical difficulties are immediate : different 
corrections for meridian observations in latitude, in longitude, on Moésting A, 
for occultations and for the photographic method, will be required. 
I next turn to a question, the chief interest of which is geodetic rather than 
astronomical. I have mentioned that a certain value of the earth’s ellipticity 
will make the observed motions of the perigee and node agree with their 
theoretical values. This value is 1/293-7 +-:3. Now Helmert’s value obtained 
from gravity determinations is 1/298-3. The conference of ‘ Nautical Almanac’ 
Directors in 1911 adopted 1/297. There is thus a considerable discrepancy. 
Other evidence, however, can be brought forward. Not long ago a series of 
simultaneous observations at the Cape and Greenwich Observatories was made 
in order to obtain a new value of the moon’s parallax. After five years’ work 
a hundred simultaneous pairs were obtained, the discussion of which give 
evidence of their excellence. Mr. Crommelin, of the Greenwich Observatory, 
who undertook this discussion, determined the ellipticity of the earth by a com- 
parison between the theoretical and observed values of the parallax. He found 
an ellipticity 1/294°4+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 determinations 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 observations 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. As far as astronomy 
is concerned, the moon is the only body for which a correct value of this constant 
is important, and it would seem inadvisable to use a value which will cause a 
disagreement between 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 0-02 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 continue 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 Observatory has 
adopted the plan of securing continuous photographic records of the sky with 
particular reference to photometric work. Under Professor Pickering it will 
also continue the photographic record of the moon’s position as long as arrange- 
amg can be made to measure the plates and compute the moon’s position from 
them. 
