THE ISSUES OF TnE LATE ECLIPSE. 
133 
eclipse observation is available for fixing the moon’s approxi- 
mate place in the absence of better material, such as meridian 
determinations. An instance of such application presented 
itself a few months since, when Professor Newcomb, wishing to 
ascertain how, within wide limits of error, the present lunar 
tables represented the moon’s place a century and a half ago, 
resorted to the contacts in the eclipse of 1715 observed by 
Flamsteed, Halley, and Pound. Such rough observations would 
be of little avail now. 
In order to extract useful data from the passage of the moon 
over the sun, the Astronomer Eoyal some years since devised a 
plan of measuring the cusps in their changing positions through- 
out an eclipse in such a manner as to bring out the errors of 
all the numerical elements concerned in the prediction of the 
phenomenon, the most important of which are the co-ordinates 
of the moon’s position. This method was first put in operation 
in 1836, and again, with great instrumental power, during the 
eclipse of July 18, 1860, when the deduced errors of the 
“ tabular places ” — as the places predicted by calculation are 
called — were found to agree closely with those exhibited by 
observations of the moon which were procurable in the ordinary 
manner near to the time of conjunction, though there is neces- 
sarily a small incomparability from the virtual difference be- 
tween the black moon on the bright sun, and the bright moon 
on the black sky ; the effects of irradiation being reversed in 
the two conditions. These observations were repeated at 
Greenwich during the eclipse under notice, and with a similar 
resulting agreement between the observed and calculated data. 
It will be obvious that they do not require the eclipse to be 
total at the place of observation. One distinguished mathe- 
matical astronomer, however, Professor Newcomb, who is under- 
stood to be engaged upon the construction of new tables of tho 
moon’s motions, thought it desirable to make the cusp-measures, 
directly upon the line of totality, and he came from the United 
States for the sole purpose of so making them. He stationed 
himself at Gibraltar, and saw enough of the eclipse in its partial 
stages (the total phase had no scientific interest for him) to 
secure what observations he desired. But he was baulked in 
another way. His measurements would be of no use without 
a very exact knowledge of the longitude of his station from a 
fixed observatory, or from Greenwich; without this, an essential 
datum, the astronomical time at which each was taken could 
not be obtained. Preparations of somewhat elaborate character 
were made to determine this longitude by the method of 
little value ; and they are quite useless where the longitude of the ob- 
serving station is not very exactly known, as is often the case. 
VOL. X. NO. XXXIX. L 
