32 PRESIDENTIAL ADDRESS SECTION A. 



STELLAR DISTANCES, MAGNITUDES AND MOVEMENTS. 



By Joseph Lunt, D.Sc, 

 Royal Observatory, Gape of Good Hope. 



Presidential Address to Section A, delivered July 11, 1921. 



Although Astronomy is the most ancient of all the sciences, 

 it is only two hundred years ago — a mere matter of yesterday 

 astronomically speaking — that Halley first discovered that certain 

 of the brighter stars had perceptibly changed their positions with 

 respect to neighbouring stars since they were recorded by Ptolemy 

 in the Almagest from observations made nearly 2,000 years ago. 



We now know that, just as Jupiter and Saturn with their 

 attendant satellites move in their well-determined orbits round the 

 sun — a phenomenon which must be seen to be fully appreciated — 

 so our sun with all its attendant planets is hurtling through space 

 at a speed twenty times as great as that of a modern rifle bullet, 

 and that all the so-called "fixed stars," of which our sun is an 

 example, are in rapid motion. 



Thanks to the labours of generations of astronomers, who 

 have accurately mapped the stars for us, we now see on either side 

 of the sun's path and above and below it, a gradual but unmis- 

 takable drift of stars slowly filing past us as we proceed on our 

 journey. 



The spectroscope has revealed the further fact that the stars 

 ahead of us are approaching and those behind are receding with 

 velocities which — in the mean — are but a reflex of our own motion 

 in space. 



Astronomy has now, in common with most other sciences, 

 become so complex that the recently-formed International Astro- 

 nomical Union has found it necessary to divide it into no less than 

 32 sections, almost any one of which may easily provide enough 

 material for a man's life-work. I need not, therefore, apologise 

 for speaking about Astronomy again this year after the address 

 given by my predecessor in office last year on the same subject. 



In a short Presidential Address it is only possible to give 

 an impressionist sketch of some of the main principles involved 

 in a study of stellar distances, magnitudes and movements; details 

 I must leave for your further reading. 



In these studies, spectrographs methods are so essential that 

 a brief survey of their development may come first. 



Spectrographic Methods. 



The birth of spectroscopy mav be said to have taken place in 

 1666, when Newton first showed that the white light from the sun 

 is really composed of seven different colours and that these colours 

 could be re-combined to produce white light. It was, however, 

 two hundred years later before the subject had been sufficiently 

 advanced to make astronomical applications possible. 



