386 



NA TURE 



[March 23, 1922 



Our Astronomical Column. 



The Approaching Opposition of Mars. — Mars 

 will be closer to the earth next June than it has been 

 since 1909 ; the opposition of 1924 will, however, be 

 still closer, the distance being then almost the absolute 

 minimum. The high south declination next June of 

 26°, making its meridian altitude at Greenwich only 

 12°, will prevent any useful work from being done 

 in this country. The nearest approach to the earth, 

 0-45 astronomical units, is on June 18 ; a week later 

 the autumnal equinox of the northern hemisphere 

 will occur, so that both polar caps should be visible. 

 The earth will remain to the north of the Martian 

 equator till mid-September. 



Planetary Observations at Setif. — M. Jarry- 

 Desloges established an Observatory at S6tif, N. 

 Africa, specially for planetary and lunar observa- 

 tions, and he has lately published a large illustrated 

 volume containing studies of the moon and all the 

 planets. Mercury was found a fairly easy object 

 by day, the spots being nearly as well-defined as those 

 of Mars. The results confirm those of Schiaparelli 

 and Lowell, making the period of rotation 88 days, 

 equal to that of revolution. Other observers have 

 concluded that the low albedo, and the absence of an 

 external ring of Ught when the planet is entering on 

 the sun in transit, negative the idea of an appreciable 

 atmosphere; this volume, however, supports the 

 presence of occasional mist or cloud veiling some of 

 the markings and altering their aspect. It is pointed 

 out that the light and lieat received from the sun 

 at perihehon and aphelion are in the ratio of 9 to 4, 

 which would make much difference in the precipita- 

 tion or dissipation of cloud. Most of the markings 

 are broad, curved, dusky streaks, some 60° in length ; 

 there are a few larger spots. The colour of the disc 

 was generally rosy. 



Drawings of Uranus show markings not unlike 

 those of Saturn ; there is a bright equatorial belt, 

 and fairly bright belts in each temperate zone, with 

 darker regions between them and round the poles. 

 The markings are much inclined and curved, but 

 exact measures are not given. It was noted that 

 the direction of the belts changed during the night, 

 showing that they cannot be quite parallel to the 

 equator. Dark belts were also seen on Neptune, 

 making in 1914 an angle of some 40° with the east- 

 west line, and slightly curved. Neptune's satellite 

 Triton was generally easier to see than Mimas ; two 

 fainter stars were seen on February 15, 191 4, between 

 Triton and Neptune. 



There are also interesting drawings of Saturn, 

 showing notches in the outhne of the Cassini division, 

 and in that of the crepe ring. The markings seen on 

 Venus were so vague and difficult that no deduction 

 was made of the rotation period. 



Stars of Class A in the Solar Cluster. — Both 

 Sir J. Herschel and Dr. Gould noticed a zone of 

 bright stars, the medial line of which makes a small 

 angle with the Milky Way. Later on, the local 

 cluster of B stars studied by Prof. Charlier was found 

 to mark out nearly the same great circle. Dr. Harlow 

 Shapley and Miss Annie J. Cannon, in Harvard 

 Circular No. 229, describe the distribution of the 

 stars of spectral type B8, B9, Ao, A2, A3, of magnitude 

 6-5 or brighter. The stars, 2450 in number, are 

 plotted in galactic co-ordinates on an equal-area 

 projection. The median galactic latitudes of the stars 

 in each 10° of longitude are then found and marked 



with crosses. The resulting smoothed curve shows 

 maxima and minima as follows : long. 50°, lat. + 5° ; 

 long. 195°, lat. -6°, long. 300°, lat. +7° ; long. 335°, 

 lat. - 4°. These results are considered to confirm 

 the existence of the local cluster, but also to show 

 the presence of some disturbing factor, possibly a 

 separate cluster about the region of Corona Austrina. 

 It is intended to pursue the investigation with stars 

 of " later " spectral types, for which spectroscopic 

 parallaxes can be found. 



The Perth Section of the Astrographic Cata- 

 logue. — The publication of Zone —35° of this 

 catalogue by Mr. Curlewis was noticed in this column 

 a few weeks ago. Zone -33° has followed it after 

 a very short interval, the arrangement being in all 

 respects similar. It appears in 4 parts, each contain- 

 ing 6 hours of R.A. ; they contain respectively 7393, 

 25,882, 21,163, ^•J^d 16,365 stars. The variation in 

 star-density with galactic latitude is again very strik- 

 ing ; it will be remembered that the south galactic 

 pole is in Decl. -29°, so that these zones embrace 

 practically all galactic latitudes. The ratios of the 

 numbers of stars in each volume to those given in 

 the same areas in the Cape Durchmusterung are 

 3'5. 57. 5'7» 4"6 respectively. Actually the extreme 

 ratios are 2 in poor fields near the galactic pole and 

 8 in rich galactic fields. 



The places of the reference stars have been taken 

 from the recent Perth Catalogue ; the tables for re- 

 duction from rectangular co-ordinates are in the same 

 form as those in the Oxford Astrographic Catalogue. 



The Light-Curve of Nova Cygni, 1920. — An 

 exhaustive discussion both of the light-curve and of 

 the colour variation of this Nova is given in Publica- 

 tions of Urania Observatory, Copenhagen, 2nd Series, 

 No. 3. The Nova was of special interest from its 

 comparatively slow rise to maximum and its eai'ly 

 visual detection, which enabled observations to be 

 made on the up-slope of the curve : two photographs 

 taken before discovery, at Kvistaber, by Mr. Tamm, 

 and at Harvard, fully confirm the leisurely nature of 

 the increase of light. The apex of the curve, at 

 mag. 1-8, is very sharp. The fall of light was at 

 first very rapid, amounting to i\ mags, in ten days ; 

 it then became slower but still uniform for 3 months. 

 Here it began to be oscillatory ; after another 3 

 months the oscillations grew larger and the diminution 

 of light slower; in the year ending 192 1 Sept. the 

 mean magnitude fell from about 8-6 to 9-7. 



The colour determinations are much less consistent 

 than those of magnitude, but they suffice to indicate 

 that at discovery the colour was less than 2, while 

 three months later it rose to 6 or 7, on a scale ex- 

 tending from o (white) to 10 (red). The colour at 

 maximum was yellow. A table of photographic 

 magnitudes for the first six months is also included 

 in this publication. This indicates a fall of light 

 from magnitude 2-12 (at maximum) to magnitude 10 ; 

 a comparison of this table with that giving the visual 

 magnitudes fails to indicate the rapid increase in 

 redness after maximum which the observers noted. 



Slides of Photographs taken at Yerkes Ob- 

 servatory. — The beauty of the slides taken with the 

 40-inch refractor at Yerkes Observatory is well known, 

 and a selected list of a hundred slides is now being 

 offered for sale. The price asked is 62 1- dollars in 

 addition to carriage ; 75 cents is charged for single 

 slides, and double this amount for coloured slides. 



NO. 2734, VOL. 109] 



