3i8 



NA TURE 



[March 9, 1922 



Our Astronomical Column. 



Saturn. — This planet is now very favourably 

 situated for telescopic study. The luminous rings 

 are now only slightly incUned, as seen from the 

 earth, and present but a small extent of surface 

 and detail. The ball, however, with its various 

 dusky bands and bright zones of different intensities, 

 will furnish interesting features under high magnify- 

 ing powers. Occasionally, white spots and other 

 irregularities are to be seen in the belts, and mark- 

 ings of this kind are important and should be utilised 

 for redetermining the rotation period. Mr. W. F. 

 Denning points out that Saturn is akin to Jupiter 

 in presenting a number of surface currents which 

 differ considerably in their relative velocities. In 

 1903 a number of light and dark spots became visible 

 in the north temperate region of the planet, and these 

 indicated a rotation period of lo hours 37 minutes 

 and 52 seconds. 



Saturn will be in opposition to the sun on March 

 25, and situated at a distance of 794 milUons of miles 

 from the earth. 



Meteoric Fireballs. — A magnificent fireball is 

 described as having passed over the southern hemi- 

 sphere on January 1 1 last. Its flight was witnessed 

 from the Liverpool liner Vauban, which arrived at 

 New York on February 20. The fireball is described 

 as being as large as the full moon and moving very 

 slowly from 10 degrees above the western horizon to 

 the eastern horizon. It occupied three and a half 

 minutes in its flight and all the while emitted a blaze 

 of Ught sufficiently powerful to illumine the sea and 

 ship in an extraordinary degree. 



On February 17, at 11-32, a brilliant meteor was 

 observed from many places, inchiding London, Barnet 

 (Herts), Stowmarket, Droitwich, and Scunthorpe, 

 Lincolnshire. As seen from places not remote from 

 the object it appeared to be many times brighter 

 than Venus, and its flight was fortunately witnessed 

 by several observers who apply themselves to celestial 

 studies, including Mr. A. King, Mr. J. P. M. Prentice, 

 Mr. A. N. Brown, Mr. Gheury de Bray, and others. 

 Computation shows that the meteor had a radiant at 

 about i25°+i3° in Cancer, and that its height was 

 from 62 to 29 miles from over Yarmouth to Win- 

 chester, Hants. Its luminous course extended over 

 41 miles and its velocity was 14 miles per second. 

 It is remarkable how many fireballs displaying ex- 

 ceptional characteristics have appeared during the 

 period from February 7 to 22 in different years. 



Comparison of Speed of Blue and Yellow 

 Light. — Harvard College Observatory Bull. No. 763 

 contains an investigation of the difference in the 

 times of the phases of the short-period variables in 

 the globular cluster Messier 5 in Libra, as determined 

 from photographic plates sensitised for blue and 

 yellow light respectively. On the average the times 

 were later in the blue light by 35 seconds, with a 

 probable error of 70 seconds. The distance of the 

 cluster was found by five different methods to be 

 about forty thousand hght-years, making it follow 

 the Hercules cluster, Messier 13,, as the second in 

 nearness of those north of the equator. Accepting 

 this distance, the speeds of blue and yellow Hght in 

 the intermediate space do not differ by more than 

 one part in ten thousand milhon as a maximum 

 possible. Since any absorbing medium would cause 

 the speeds to differ, this affords an upper limit to its 

 amount. 



The Illumination of the Eclipsed Moon. — The 

 B.A.A. Journal for January contains an important 

 article by L. Richardson discussing the action of 

 the terrestrial atmosphere in refracting sunlight on 

 to the eclipsed moon. Tables and diagrams indicate 

 the amount of refraction of light at various heights 

 above the earth. The values at heights of o km., 

 10 km., 20 km., 30 km., are 68', 22', 5', and i' re- 

 spectively ; thus to reach the centre of the shadow 

 the sunlight has to pass fairly near the earth's surface, 

 and high mountains or clouds would intercept a 

 good deal of it ; an irregularity in the outline of 

 the shadow in the eclipse of 1888 was plausibly 

 attributed to cloud in the Amazon basin, or else to 

 the Andes. The strange distortion that the sun would 

 undergo to an observer on the moon is described. 



The author deduces from theory that the centre 

 of the shadow should be slightly brighter than 

 the surrounding regions, and finds some support in 

 the observations of May 1920. He constructed a 

 model lens of concave section in printers' roller 

 composition, with an opaque disc in the middle ; 

 when this was placed over a source of light, the 

 brilUant ring could be seen round the dark disc, also 

 the increase of illumination near the centre of the 

 shadow. The bluish or greenish fringe often seen 

 in the outer parts of the shadow is explained by 

 stating that the sunlight that has passed high above 

 the earth's surface would be much less reddened than 

 that which passed low down. It is also pointed out 

 that the varying distance of the moon from the 

 earth is an important factor in altering the illumina- 

 tion in different eclipses. When the moon is in 

 apogee it is further from the earth's " black shadow," 

 and gets more Hght. After allowing for these factors, 

 and for the mountain ranges that he along the earth's 

 terminator, the illumination of the moon should 

 afford a useful index of the clearness of the zone of 

 atmosphere that Ues near the terminator. 



Parallaxes and Proper Motions. — Mr. Van 

 Maanen deals with this subject in Contributions from 

 Mt. Wilson Observatory. No. 204 contains two 

 important investigations, the first being a set of 

 parallax determinations of specimen objects of 

 various types made with the 60-inch reflector. The 

 terms to reduce to absolute parallax have been de- 

 rived from comparison with the spectroscopic 

 parallaxes of Adams, etc. The mean parallax of 11 

 planetary nebulae is of the order of O'oi", the mean 

 absolute magnitude is 8-4, and the mean diameter 

 0'o6 Hght-year. Two Cepheids give small parallaxes 

 of the same order as those found by Shapley from 

 the proper motions ; T Cassiopeiae, a long-period 

 variable, has the considerable parallax 0-027" » ^^s 

 absolute magnitude varies from 3-9 to 9-7 ; two 

 stars. Boss 500 and RR Lyrae, are notable for their 

 high velocities, each about 200 km. /sec, their absolute 

 magnitudes being near o ; the value 0-019" found 

 for Nova Aquilae is nearly the same as the accepted 

 value for Nova Persei, 1901, while Campbell's 

 hydrogen-envelope star (type O) and Boss 3322 

 (type N) are assigned parallaxes of 0-005" ^^^ ~ 0-002". 

 The radial velocity of the double cluster in Perseus 

 is found to be -40 km. /sec. and its proper motion 

 + 0-003" ill R.A. and +0-003" in Decl. These 

 values are so small that it is impossible to pick out 

 cluster stars with certainty unless they are bright 

 enough to permit their radial velocity to be deter- 

 mined. Tables are given of the individual motions 

 and magnitudes of over 1500 stars. 



NO. 2732, VOL. 109] 



