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ANNUAL ADDRESS TO THE MEMBERS 
OF THE SOUTH AFRICAN PHILOSOPHICAL SOCIETY, 
ON SEPTEMBER 27TH, 1893, 
- By THE PRESIDENT, D. Git, LL.D., F.RS. 
It has become the custom for the President of the Society, at the 
conclusion of his first year of office, to deliver an address on some special 
subject, and at the conclusion of his second year, before resigning the 
presidential chair, to review the work of the Society. 
Last year I was prevented by many official engagements from 
delivering the usual address, and thus on the present occasion both 
duties devolve upon me. 
For an astronomer occupied with original research it may not be 
difficult to express in a few words, perhaps even in three or four figures, 
the result of many years of strenuous labour. A considerable portion 
of my own astronomical life has been devoted to a single problem. 
The address which I had the honour to deliver from this chair on 
July 30, 1880, was occupied exclusively with the history of that 
problem, and if I again revert to it, I trust the Society will forgive me, 
not alone on the ground of the scientific importance of the matter, but 
because it happens to be again the subject which chiefly occupies my 
_working hours. 
The determination of the mean distance of the earth from the sun 
has been described by the late Sir George Airy as ‘the noblest problem 
in astronomy.’ It is the fundamental unit of the science; upon its 
exact determination depends our estimate of the scale of the solar 
system, nay, of the universe itself, and in a greater or less degree our 
exact knowledge of the foliowing astronomical constants, or at least 
the harmony of their adopted values with physical relations and with 
Newton’s laws of gravitation. These constants are : 
The equatorial and polar diameters of the earth. 
The mean density and surface density of the earth. 
The length of a pendulum beating seconds of mean time in different 
latitudes. 
The length of the year and of the sidereal day. 
The masses of the earth, the moon, and all the planets. 
_ The solar parallax. > 
The eccentricity of the earth’s orbit. 
