Sir David Gill. 
211 
displacement of their path, as viewed from different stations in the Northern 
and Southern Hemispheres, would afford data for a determination of their 
parallax, and consequently of the sun's distance. 
Thus, simply put, was the problem to be solved. But the investigation, 
even in its preparatory stages, was surrounded by difficulties. This was 
from the beginning more fully appreciated by Gill than by any other man, 
and so he allowed no less than five years, 1883 to 1888, in which to 
consider and make preparations for the proposed scheme. 
A new instrument had to be built, for the old 4-inch heliometer had had 
its day. This new 7-inch heliometer has already been referred to when 
speaking of Gill's work on stellar parallax. It was begun in 1884 from 
plans and drawings made by Gill ; but it w^as not completely ready till 
1887, when Gill w^ent home to test its accuracy. 
While on this errand he attended the i\strographic Congress at Paris, 
where he was elected, by a large majority, its senior member. 
Besides a new instrument, co-operation was needed on the part of 
other observatories and w^orkers, and it speaks eloquently for the personal 
esteem in which Gill was held, and the common faith in his genius and 
skill, that no fewer than twenty-two observatories gave promise of help in 
the undertaking. One of Germany's leading astronomers, Dr. Auw^ers, of 
Berlin, came out to the Cape to assist Gill ia taking his observations and 
making his reductions. So many observatories co-operating necessitated 
endless correspondence, for men were very anxious to help, and their 
eagerness made them timorous. This load, no light one, Gill took entirely 
upon his own shoulders. 
The actual and necessary observations of Iris were made in 1888, of 
Victoria and Sappho in 1889. 
As the reduction of the observations proceeded, it was found that the 
orbital places of the planets were not accurate enough for Gill's purpose. 
And so their places, from day to day, had to be recomputed, this time 
by means of 8-figure logarithms, the ordinary 7-figure tables not being 
refined enough. 
The reductions occupied nine years. The large volumes, one of them 
running to close on 1,000 pages of packed matter, that reveal this final 
essay to settle the centuries-old problem of the sun's distance will always 
remain a classic achievement in astronomical science. In these tomes 
the genius of the man stands out clear and inspiring. We may mark the 
unerring wdsdom, wisdom akin to instinct, in deahng with difficulties, the 
exquisite skill in securing and in reducing observations, the indomitable 
patience in combating and rendering innocuous instrumental errors, poor 
observations, and faulty methods. 
Gill's final value of the sun's parallax was 
8-804" + 0-005". 
16 
