354 Sir Frank Watson Dyson [April 29, 



WEEKLY EVENING MEETING, 



Friday, April 29, 1921. 



The Hon Sir Charles A. Parsons, K.C.B. J.P. M.A. D.Sc. 

 LL.D. F.R.8. M.R.I., in the Chair. 



Sir Frank Watson Dyson, LL.D. F.R.S., 

 The Astronomer Royal. 



Advances in Astronomy. 



In the past ten years a number of the large telescopes of the world 

 have been applied to the determination of stellar parallax. The 

 principle of the method is well known and is extremely simple, merely 

 consisting in the detection of the small annual movement of a near 

 star with reference to more distant stars caused by the different 

 position occupied by the observer in consequence of the earth's annual 

 revolution round the sun. The whole difficulty consists in the 

 extreme minutenes of the angle to be measured. If two railway 

 lines, starting at King's Cross, instead of remaining parallel, met at 

 Newcastle the angle between them would be of the order of the angle 

 to be measured in finding the distances of the nearest stars. To 

 form an idea of what is now being done by large telescopes using 

 photographic methods, imagine two plumb-lines 5 ft. apart. They 

 are sensibly parallel, but actually meet at the centre of the earth, and 

 the angle between them is 0*05". An angle of this size is measured 

 with an accuracy of ±0*01". Results of this high value were first 

 obtained by Prof. Schlesiuger at the Yerkes Observatory. At the 

 present time the observatories of Allegheny, Greenwich, McCormick, 

 Mount Wilson, Yerkes, and a number of others are engaged on a 

 comprehensive programme. At Greenwich we determine the 

 parallaxes of fifty stars a year ; at some of the American observatories 

 many more. 



Necessarily, a good deal of care is required both in taking the 

 photographs and in measuring them. The image of a star may have 

 a diameter of 2" or 3", and the position of its centre should be 

 measurable to between ^th and T ^o tn °f tn i s amount. The methods 

 of measurement present some points of interest which need not be 

 described now, but a word or two about the precautions to be observed 

 in taking the photographs may be of interest. The images must be 

 as circular and uniform as possible. (1) The guiding of the telescope 



