PRESIDF.NT S ADDRESS. 5 



the stars and the future conditions of the planet upon which he 

 has his being. 



The Distances of the Stars. 



The problem which will be more closely discussed in this 

 address is that of the distance of the stars. The most direct 

 way of finding these is by the parallactic displacement of the 

 stars caused by the motion of the earth round the sun. In this 

 enquiry the Union can have a local pride, as the first paral- 

 lax* certainly found was that of Alpha of Centaurus by Hen- 

 derson, the Cape Astronomer. The late Sir David Gill, our 

 first President, continued Henderson's work, and perhaps one 

 might say finished it in that form. Gill was an organiser, and 

 when the parallax campaign, initiated by himself and completed 

 with the aid of Dr. Elkin and others, had come to an end. it 

 was apparent that the most directf method of finding parallaxes 

 which was available would only yield a small crop, because the 

 stars are at such enormous distances from the sun that the avail- 

 able base-line for measurement, the diameter of the earth's orbit, 

 some 186,000,000 miles, or 300,000,000 kilometres, is vanishingly 

 small at the distance of all but a few near stars. Alpha Centaurus 

 is the nearest known, and almost certainly the nearest to the 

 svm, yet at its distance the diameter of the earth's orbit subtends 

 an angle of but i Yi seconds of arc — an angle which is described 

 by the minute hand of a clock in a four-thousandth part of a 

 second of time. An angle so small is difficult to observe directly 

 with accuracy, so that at best the measures must become differen- 

 tial — that is, the stars are measured from neighbouring stars 

 supposed to be at a much greater distance away ; such stars are 

 called comparison stars. 



Professor Eddington estimates that there are thirty stars 

 with a parallax of o".20 or greater, of which nineteen are already 

 known. This means that within a distance nearly four times 

 as great as that of Alpha Centaurus there are but thirty stars 

 in all. This is the limit of visual work such as was done by 

 Gill, but photographic methods, especially with the enormous 

 telescopes used in America, carry the direct attack further. 



The delicacy, or, if you prefer, the accuracy, of any measure- 

 ment is limited by its probable error. The probable error of a 

 parallax measured visually under good circumstances (such as 

 with the Cape heliometer) is about o".io (a tenth of a second 

 of arc), and this is already, small as it is, a quantity larger than 

 the quantity to be measured except in the cases of a hundred or 

 so stars. The same method of parallactic displacement of stars 

 on photographic plates has a much smaller probable error. The 



"^ Cut not the first announced. Bessel in 1838 announced the measure- 

 ment of the parallax of 61 Cygnus two months earlier than Henderson 

 whose delay was caused by his removal to Europe. 



t The only direct parallax found was that of ^ Centaurus, by Hen- 

 derson. All other parallaxes of any certainty depend on an indirect 

 method involving the assumption, nearly true, that all the stars with a few 

 exceptions have very minute parallaxes. 



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