August, 1913. 



KNOWLEDGE. 



305 



18 d 8 h 16"' 58 s IV. Ec. R. ; 19 d 8 h 10 m III. Oc. D. ; 23 d 6 h 36 m 

 III. Sh. E., 8 h 5 m II. Sh. I., 8 h 22 m II. Tr. E., 9 h 15 m I. Oc. 

 D.; 24" 6 h 27 m I. Tr. I., 7 h 45 m I. Sh. I. 8 h 45 m I. Tr. E., 

 10 h 4 m I. Sh. E. ; 25 d 7 h 17 m 37" I. Ec. R. ; 30 d 7" ll m III. Sh. 

 I., 8 h 5 m II. Tr. I. All these are in the evening hours, the 

 planet setting before midnight. 



Table 56. 



last year are again available, and readers are referred to the 

 corresponding month, of last year. 



Variable Stars.— Tables of these will be given each 

 month ; the range of R.A. will be made four hours, of which two 

 hours will overlap with the following one. Thus the present 

 list includes R.A. 20 h to h , next month 22 h to 2 h , and so on. 



Meteor Showers (from Mr. Denning's List) : — 



Configuration at 8 1 ' c for an inverting telescope. 

 Double Stars and Clusters. — The tables of these given 



NOTES. 



ASTRONOMY. 



By A. C. D. Crommelin, B.A., D.Sc, F.R.A.S. 



THE LIGHT- VARIATION OF EROS.— It will be 

 remembered that at the time of the near approach of Eros at 

 the end of 1910 and beginning of 1911 a large variation was • 

 detected in its brightness ; there was a question whether the 

 period was five hours or two and a half hours ; it appears that 

 the full period is five hours, but that it is made up of a double 

 wave with unequal maxima, like some variables (Beta Lyrae 

 has the maxima equal, the minima unequal). At the opposition 

 of 1903 Eros was far south, and Professor S. Bailey made 

 most careful observations on the variability at the Arequipa 

 Observatory. The visual and photographic methods con- 

 firmed each other, and showed that the five-hour period is 

 correct ; the exact value is 0-2196 day, but this has not been 

 corrected for the changing position of the planet. Taking the 

 minimum brightness as zero, the following tables give the 

 excess of brightness (in stellar magnitudes) over minimum 

 light at intervals of one-twenty-fourth of the period, beginning 

 at principal maximum. The upper set is derived from the 

 visual measures, the lower from the photographic : — 



•63, -57, -44, -27, -10, -01, -00, '06, -19, -34, -47, -55, -56 

 •50, -40, -29, -14, -02, -00, -07, -22, -39, -53, -61. 



•51, -47, -39, -28, -15, -05, -00, -00, -07, -16, -29, -38, -43, 

 •43, -37, -29, -16, -05, -00, -00, -07, >21, -36, -47. 



The two curves are in good agreement, and show a range of 

 fully half a magnitude. The maxima differ by -07; the reality 

 of the double maximum is probable, but it appears possible 

 that there is really only a single wave with half the period. 

 The most probable cause of the variation seems to be 

 irregular shape of the planet. In such a tiny body gravity is 

 very weak, and insufficient to form a regular shape. This 

 seems more likely than the suggestion made by Professor 

 Turner that it may have spots on its surface of very different 

 albedoes. On such a tiny world anything in the nature of cloud 

 or snow is out of the question, for it could not retain an 

 atmosphere. Also organic rocks like chalk are excluded. It 

 seems not impossible that at the near approach of 1931 our 



giant telescopes may be able to see Eros as a tiny disc (it 

 should be quite half a second in diameter) and detect any 

 notable departure from roundness ; double stars half a second 

 apart are easily separated in large instruments. 



There is no reason to think that Eros is the only small 

 planet whose shape is irregular — many have therefore been 

 observed at Arequipa for variability. Three gave definite 

 light-curves: Sirona has a period of d, 403, range one-half 

 magnitude; Celuta period 0-364, range one-half magnitude; 

 Tercidina period d, 366, range one-half magnitude; its 

 variation had already been detected by Wolf and Wendell. 

 Variation was suspected in Hecuba and Urania, but not 

 fully established. It would seem that the asteroids as a 

 class have pretty rapid rotation from the periods of those 

 that vary. 



THE WORK OF HARVARD OBSERVATORY.— We 

 had the pleasure of hearing both Professor E. C. Pickering 

 and Miss Cannon at the meetings of the R.A.S. and the B.A.A. 

 in June, and we formed some idea of the immense scale of 

 their work. Miss Cannon is directing the preparation of a 

 catalogue of the spectra of one hundred thousand stars, 

 which will be practically exhaustive down to magnitude 8, and 

 will contain many fainter stars. This is, of course, made from 

 photographs taken with a prismatic camera ; it would be out of 

 the question to obtain so many spectra with a slit spectro- 

 scope; the latter is, of course, better for large-scale spectra of 

 bright stars, or for motion in the line of sight, which can only 

 be deduced in an indirect and unsatisfactory way from the 

 prismatic camera. This, however, gives a considerable amount 

 of detail even with faint stars, and wave-lengths of lines can 

 be deduced differentially. Miss Cannon is so intimately 

 acquainted with all the types of spectra and their subdivisions 

 that she can assign them to their different classes with great 

 rapidity ; subsequently her assistants identify the stars and 

 prepare the catalogue. No fewer than fifteen Novae were 

 discovered at Harvard by their spectra, ten by the late Mrs. 

 Fleming. Many other stars having bright lines in their 

 spectra have been discovered. Professor Pickering states that 

 with a slit-spectroscope only about two per cent, of the stars' 



