14 



SCIENCE 



[N. S. Vol. XLII. No. 1970 



telescopes are available and photographic 

 methods have been developed, we may ex- 

 pect that in the next few years very rapid 

 progress will be made. 



For many centuries astronomers had 

 speculated on the distances of the stars. 

 The Greeks measured the distance of the 

 moon; they knew that the sun and planets 

 were much further away, and placed them 

 correctly in order of distance, guessing 

 that the sun was nearer than Jupiter be- 

 cause it went round the sky in one year 

 while Jupiter took twelve. The stars, from 

 their absolute constancy of relative posi- 

 tion, were rightly judged to be still more 

 distant — but how much more they had no 

 means of telling. 



In 1543 Copernicus published ' ' De Revo- 

 lutionibus Orbium Ccelestium, ' ' and showed 

 that the remarkable movements of the plan- 

 ets among the stars were much easier to 

 understand on the hypothesis that the earth 

 moved annually round the sun. Galileo's 

 telescope added such cogent arguments that 

 the Copernican system was firmly estab- 

 lished. Among other difficulties which were 

 not cleared up at the time one of the most 

 important was this: If the earth describes 

 a great orbit round the sun, its position 

 changes very greatly. The question was 

 rightly asked : Why do not the nearer stars 

 change their positions relatively to the 

 more distant ones? There was only one 

 answer. Because they are so extremely dis- 

 tant. This was a hard saying, and the only 

 reply which Kepler, who was a convinced 

 believer in the earth's movement round 

 the sun, could make to critics was "Bolus 

 erat devorandus." 



Although no differences in the positions 

 of the stars were discernible to the naked 

 eye, it might be that smaller differences 

 existed which could be detected by refined 

 astronomical measurements. To the naked 

 eye a change in the angle between neigh- 



boring stars not more than the apparent 

 diameter of the sun or moon should be 

 observable. No such changes are perceived. 

 The stars are — it may be concluded — at 

 least two hundred times as distant as the 

 sun. "With the instruments in use in the 

 seventeenth century — before the telescope 

 was used for the accurate measurement of 

 angles — angles one twentieth as large were 

 measurable, and the conclusion was reached 

 that the stars were at least four thousand 

 times as distant as the sun. But no posi- 

 tive results were obtained. Attempts fol- 

 lowed with the telescope and were equally 

 unsuccessful. Hooke tried to find changes 

 in the position of the star y Draconis and 

 failed. Flamsteed, Picard and Cassini 

 made extensive observations to detect 

 changes in the position of the pole star and 

 failed. Horrebow thought he had detected 

 slight changes in the position of Sirius due 

 to its nearness in a series of observations 

 made by Romer. He published a pamph- 

 let, entitled "Copernicus triumphans," in 

 1727, but the changes in the position of 

 Sirius were not verified by other observers, 

 and were due to slight movements of 

 Romer 's instruments. 



Thus in Halley's time it was fairly well 

 established that the stars were at least 20,- 

 000 or 30,000 times as distant as the sun. 

 Halley did not succeed in finding their 

 range, but he made an important discovery 

 which showed that three of the stars were 

 at sensible distances. In 1718 he contrib- 

 uted to the Royal Society a paper entitled 

 ' ' Considerations of the Change of the Lati- 

 tude of Some of the Principal Bright 

 Stars." While pursuing researches on an- 

 other subject, he found that the three 

 bright stars — Aldebaran, Sirius and Are- 

 turus — occupied positions among the other 

 stars differing considerably from those as- 

 signed to them in the Almagest of Ptolemy. 

 He showed that the possibility of an error 



