ASTRONOMY 9S 



Comets. Numerous comets have been recently discovered or recovered, but they 

 have generally been too small for special notice. Halley's Comet was followed up to 

 the beginning of June 1911. Its photographic magnitude as determined at Helwan 

 (Egypt) was the same 394 days after perihelion as 238 days before, showing the long 

 persistence of the exciting effect of the sun. The return of 1910 will be remembered 

 for the accurate prediction by Cowell and Crommelin, and for the transit of the comet 

 across the sun. The calculations of prediction were carried out with such care that the 

 discrepancy with fact was minute: and yet the calculators were able to declare with 

 confidence that even this minute discrepancy must be referred to " some cause whose 

 character is not yet recognised." At the transit no trace of the comet appeared on the 

 sun's disc, but the tail appeared unexpectedly on both sides of the earth simultaneously. 



Another comet of special interest, Encke's, which has a short period of 33 years only, 

 was re-discovered on July 3 1 at Algiers only three degrees above the horizon striking 

 testimony to the climate of Algiers. The subsequent observations led firstly to the de- 

 tection of a clerical error in the computations, and further to a reassurance as to the 

 correctness of the assumed mass of Mercury, on which the 1908 observations seemed to 

 throw doubt. The anomalies in 1908 are probably due to a further change in the 

 acceleration of the Comet's mean motion, but the return of 1914 will probably settle the 

 point. 



It has recently been asserted by E. Stromgren that all the cases of supposed hyper- 

 bolic orbits of comets disappear on rigid examination. If this be so, we may regard all 

 comets as permanent members of the solar system, none being alien intruders. But 

 Stromgren's conclusions have not yet won general acceptance, several computers having 

 quite recently published hyperbolic elements of comets. 



Comet 1911 c discovered by W. R. Brooks on July 20, 1911 became visible to the 

 naked eye and had a tail 25 in length. Some fine photographs were secured, notably 

 by E. E. Barnard. 



A more exact knowledge of the spectra of the tails of comets dates from 1907. The 

 spectrum consists generally of a series of double bands, which were identified by Fowler 

 in 1909 with bands obtained from carbon monoxide at a pressure of about o.oi mm. 

 In one or two cases negative bands of nitrogen have also appeared. The chief bands in 

 the head of a comet are those of cyanogen and the Swan bands of carbon, the latter 

 being possibly derived from carbon monoxide; and it is only necessary to suppose that 

 this gas is expelled from the head in order to explain the spectrum of the tail. The 

 decomposition of cyanogen in the head might account for the nitrogen in the tail. 



New and Variable Stars. A new star of the 4th magnitude was discovered by the 

 Norwegian astronomer Enebo in Gemini on March 12,1912, and was attentively watched 

 and photographed. Considerable interest was excited by a reported discovery of the 

 lines of radium in its spectrum, by Giebeler of Bonn: but the evidence is questioned. 



A feature of recent work has been the extension of the scrutiny of variability to a 

 higher grade of accuracy. Thus E. Hertzsprung, guided by the spectroscopic observa- 

 tions of the pole star which showed its velocity to be variable in a period of close on 4 

 days, detected a minute variation in the light in the same period. So little was this 

 previously suspected that the pole star was used as a standard of (constant) brightness 

 in more than one long investigation. The range found by Hertzsprung from his study 

 of 418 photographs is less than a fifth of a magnitude about 17 per cent of the light: 

 and even this small range was reduced to one-third less than 6 per cent by J. Stebbins 

 using a selenium photometer. Now C. G. Abbot has found variations in the radiation 

 of our own sun at least comparable with these, so that we may regard the gap between 

 our sun and the variables as bridged. Indeed it becomes a question whether every 

 star is not variable within limits of this kind. The beautiful work of Stebbins with his 

 selenium photometer opens up definite prospects of settling this great question. As an 

 instance of the delicacy of this instrument the case of the variable Algol may be quoted. 

 It had already been shown that the variation was due to the eclipsing of a bright body 

 by a dark one, but former observations showed no variation in the light between the 



