June 24, 1922] 



NA TURE 



815 



THE celebration of a centenary is an occasion for 

 retrospect over the past and for hopeful outlook 

 towards the future. We have here representatives 

 of many different sides of astronomy who view it from 

 many different aspects, and I should not be surprised 

 if there are wide differences of opinion as to which are 

 the outstanding landmarks in these hundred years. 

 Like selecting the hundred best books, the selection 

 of, let us say, the six great landmarks of astronomical 

 progress in the century is a pastime which need not 

 be taken too seriously. I shall venture to try my 

 hand at a selection : — 



(i) i839.' The first determinations of stellar parallax 

 of 61 Cygni and a Centauri, giving for the first time a 

 definite idea of the scale of the stellar universe. 



(2) 1846. The discovery of Neptune. An event 

 perhaps more highly celebrated outside astronomical 

 circles than among professional astronomers, but 

 producing an incalculable moral effect. 



(3) 1864-68. The early spectroscopic discoveries of 

 Huggins and Lockyer, and the rise of spectroscopic 

 astronomy. 



(4) 1882-87. The beginnings of stellar photography, 

 starting with Gill's photograph of the great comet of 

 1882 and leading to the inception of the astrographic 

 chart in 1887. 



(5) 1904. Kapteyn's discovery of the two star- 

 streams, the beginning of the modem era of investiga- 

 tions of the sidereal system. 



Coming to events so near to the present that we 

 cannot yet put much trust in our perspective, I would 

 very tentatively include 



(6) 1920. The measurement of the angular diameter 

 of Betelgeuse by Michelson's interferometer method. 

 I would not venture to predict how great or how im- 

 mediate may be the influence of this last on the progress 

 of astronomy ; but it seems to me to be worthy of 

 a place in this select hst as a triumph of scientific 

 achievement which is second to none. It is one of 

 those signal instances which convince us that the 

 word " impossible " must be banished from the 

 vocabulary. 



This is a record of continuous advance — not in great 

 waves followed by periods of exhaustion. A new 

 impetus has always been found before the last one 

 has begun to fail. Even the alHed science of physics 

 has not, I think, had such a continuous record. I 

 am told that there was a period shortly before X-rays 

 and electrons came to the fore, when the physicist 

 had given up anticipating any radical advance ; he 

 thought that the big discoveries were already garnered ; 

 and the feeling, so present with us to-day, that we are 

 on the verge of something greater than our dreams 

 can shape, had not yet disturbed his placid progress. 



The centre of most rapid progress has shifted from 

 time to time, and the various branches of astronomy 

 have had their ups and downs. I suppose that in 

 recent years the department of planetary astronomy 

 has been in the depression of a wave. At least it 

 seems to be so in comparison with the more sensational 



' From the presidential address delivered before the Royal .Astronomical 

 Society on May 30. 



NO. 2747, VOL. 109] 



A Century of Astronomy.^ 



By Prof. A. S. Eddington, F.R.S. 



progress in our knowledge of the sun and stars. Whether 

 we regard the physical observation of the surfaces of 

 the planets or the study of their motions, the openings 

 for advance seem to be few and difficult. But the 

 depression has by no means reached stagnation. We 

 have the remarkable advance in planetary photography, 

 exhibited at several recent meetings of the Society ; 

 the discovery of new satellites, including Jupiter's 

 two pairs of twins, and the specially significant 

 phenomenon of the retrograde motion of the outermost 

 satellites of Jupiter and Saturn ; the determination 

 of the rotation period of Uranus by Slipher ; and the 

 Trojan group of minor planets, the principal merit 

 of which is that they have beneficently prevented the 

 once great science of dynamical astronomy from grow- 

 ing altogether rusty. Renewed interest is added to 

 the exact and regular observation of the positions of 

 planets by Einstein's explanation of the anomalous 

 motion of Mercury ; the same observations reveal 

 interesting irregularities in the longitudes of the planets 

 which perhaps reflect inequalities in the rotation of 

 the earth as standard time-keeper. These observa- 

 tions, which else might have seemed to be mere 

 survivals of traditional routine, are seen to be full of 

 importance for the future ; and for the same reason 

 we welcome the revival of observations of occultations 

 oi stars by planets. On the theoretical side, we have 

 Taylor's important investigation of tidal friction in 

 the Irish Sea, which, true to its name, is responsible 

 for a considerable proportion of the friction and 

 dissipation of energy on this planet ; and Jeans's 

 researches have given us new ideas of the origin of 

 the planets which attend the sun, and of the singular 

 (perhaps even unique) character of this system. Many 

 other researches in this field could be mentioned. If 

 the department of planetary astronomy is now the 

 Cinderella of our science, she yet has dreams that her 

 Prince is waiting for her. 



It is startling to-day to read a passage from Huxley's 

 " Essays " which runs : — " Until human life is longer 

 and the duties of the present press less heavily, I do 

 not think wise men will occupy themselves with 

 Jovian or Martian natural history." Martian — and 

 I almost fear to mention it — lunar natural history are 

 no doubt thorny subjects, but notwithstanding Huxley's 

 censure, probably the most sceptical among us would 

 admit that the observation of seasonal changes of 

 what is presumably some kind of vegetation on Mars 

 is a recognised astronomical pursuit. 



In reviewing the general advance of astronomy 

 during the century, we cannot but be struck by what 

 I may call its centrifugal tendency — the tendency to 

 leave the little system ruled by the sun and penetrate 

 deeper and ever deeper into the vast world outside. 

 In the older books, the author leads us deliberately 

 through the planets one by one, and it is with difficulty 

 that the account of the stellar universe can be spun 

 out to any respectable length. Before the first meeting 

 of this S<)ciety in 1820 an introductory address was 

 circulated, which contains the paragraph : 



Beyond the limits of our own system, all at present 



