212 Transactions of the Boyal Society of South Africa, 
The limit of uncertainty was reduced to 
30,000 miles. 
Surely it is sufficient of honour to have reduced our uncertainty of a 
fundamental constant from 10,000,000 miles to a quantity three hundred 
times less ! Yet this was only one of this man's many achievements. 
Two related and important facts emerged from this consideration of 
the sun's distance. 
The mass of the moon is inseparably bound up in the problem of the 
solar parallax, inasmuch as the earth circles monthly round the centre of 
gravity of the earth-moon system. The value of the moon's mass 
accepted by astronomers prior to 1897, Gill proved to be in error. His 
equations yielded the relation, 
E : M :: 81-60 : 1. 
The other matter lay more in the region of physics than in that of 
astronomy. There was some uncertainty as to the effect that a systematic 
difference of refrangibility would have on the final result. Gill proved that 
his heliometer method of determining stellar or solar parallax eliminates 
the error which otherwise might arise from this source. 
Before we pass on to the last and probably greatest of Gill's 
achievements, his part in the great photographic star map, we may recall 
the rest of the history of the problem of the sun's distance. 
When Gill visited Potsdam in 1891, Vogel showed him some remark- 
able photographs of stellar spectra that he had taken. On examining 
them under a powerful microscope. Gill exclaimed, " Why, here is a fine 
method of determining the solar parallax." 
Later on in the same year he gave a short account of his visit to 
Potsdam to the Eoyal Astronomical Society, stating how impressed he was 
with the accuracy of the spectrographic measurements secured there, and 
urging that a determination of the sun's distance should be attempted by 
this new and promising method. 
As is well known, when spectrographic measurements of stars are 
taken, to reduce these measurements to absolute radial velocities a correc- 
tion has to be applied for the earth's orbital movement. Until very 
recently the margin of error in spectrographic determinations was so great 
that to reverse the operation and determine the earth's velocity, and con- 
sequently its distance from the sun, from the spectrograms was utterly 
futile. But with the great advance made in spectrography during the 
past ten years. Gill's hope became possible of realization. 
There is something very fitting in the fact that it was at the Cape, and 
with the McClean telescope, on the perfecting of which Gill spent sa 
