aS REPORTS FROM THE SECTIONS. 
On the Longitude of Sydney Observatory. 
By H. C. Russert, B.A, F.R.AS. 
[Read before the Astronomical Section, 3 May, 1878.] 
Tur members present this evening are probably all aware of 
the extreme difficulty of finding the longitude by astronomical 
observations alone, that is, by ohsarvadjone of the moon’s change 
of position, due to her motion. To those who have not been in the 
- habit of discussing lunar observations, it may be worth while to 
say that an error mmade in observing t the moon’s right ascension 
(taking the moon’s average change in right ascension) is magnified 
some twenty-five times in the resulting error of longitude ; ; thus, an 
error of observation of the moon of only one-fifth of ‘a second 
would produce an error of 5s. in the longitude, and if the observa- 
tion were made when the moon was changing her right ascension 
slowly the error would be much greater. All the refinements of 
the moon pointed out by the Astronomer Royal so long since as 
1848, which might, I think, be called view personality. During 
that year it was found that between Mr. Dunkin and Mr. Breen, 
two of the Greenwich observers using the altazimuth, there 
existed a difference of 0.38s, in determining the moon’s right 
agoenslon 5 and the Astronomer Royal remarks :— 
e circumstances under which the large errors given by Mr. 
hs occurred were so various, in respect of moon’s age.and 
moon’s position in her orbit, and the intermixture of the observa- 
tions had been so complete that there was no doubt whatever that 
this was the result of a difference in the mode of observation. 
“« And ng was not the result of personal equation usually so 
called ; for it was known from the medley eye of perso: 
equation as exhibited i in the clock errors given n by stars (which are 
confirmed by similar investigations made to the fer of 1849) that 
the personal equation was small. Neither was it a different es- 
timation of the moon’s diameter, for the difference of errors © 
moon’s R. A. is nearly the same, and in the same direction, 
whether the first limb or the second limb be observed. It is 
strictly speaking a difference between the personal equation for 
the moon and that for the stars ; or, it. may be thus stated, that 
the duration of the impression on the nerves of the eye, or the 
time occupied in bringing into comparison the impressions on the 
