April 7, 1910] 



NATURE 



169 



OUR ASTRONOMICAL COLUMN. 



OccuLTATioN OF Mars, April 13. — An occultation of 

 Mars by the moon will take place at 10.28 p.m. on .April 13, 

 the planet disappearing behind the dark limb of the moon 

 in fMjsition-angle 0°. Emersion will take place at iih. 4m. 

 p.m. in position-angle 278°, the angle in each case being 

 reckoned from the zenith point of the moon towards the 

 east. 



Comet 1910a and Halley's Co.met. — From an article 

 by Mr. Knox Shaw, published in No. 40 of the Cairo 

 Scientific Journal, we learn that photographs of comet 

 1910a w-ere secured at the Helwan Observatory. The 

 comet was first seen ten minutes after sunset on 

 January 20, clouds having prevented earlier observations. 

 The Reynolds reflector w'as not ready for photographing 

 objects at such low altitudes, but some good photographs 

 were secured with a 4-inch Cooke lens on January 24, 25, 

 27, and 28 ; more cloudy weather then intervened. The 

 photographs show the twin tails and also the southern 

 secondary tail, which is much fainter, and can only be 

 traced to a distance of 40' from the head. 



Mr. Shaw also publishes a useful diagram of the path 

 of Halley's comet, with regard to the sun and the earth, 

 during the period February i to May 29. A photograph 

 of this object, obtained at Hehvan on Januarj- 28, showed 

 faint traces of a tail about 18' in length. 



Three excellent photographs of 1910a, and one of 

 Halley's comet, appear in No. i, vol. iv., of the Journal 

 of the Royal Astronomical Societ}' (Canada). They were 

 taken at the Dominion Observatory, Ottawa, on January 

 25, 28, 31, and February 10, respectively. On the last- 

 named date the negative of Hallev's comet showed a tail 

 1° long. 



-A brief message from M. Jean Mascart informs us that 

 he is at Teneriffe, w-here, at an altitude of 2700 metres, 

 he intends making observations of Halley's comet. M. 

 Mascart's station is very near that occupied by Piazzi 

 Smith during his sojourn, for astronomical observatioriS, in 

 the island. 



Sun-spots and Faculje in 1909. — Prof. Ricco's usual 

 annual summary of the sun-spots and faculaj observed at 

 Catania during 1909 appears in the February number of 

 the Metnorie di Astrofisica ed Astronomia of the Societi 

 degli Spettroscopisti Italian! (vol. xxxix., p. 17). On the 

 whole, the activity displayed during 1909 was markedly 

 less than that of 1908. In April, 1909, there was a sudden 

 decrease of spots, the mean frequency becoming 25 instead 

 of 41 as it was in March. This low value continued for 

 six months, but in October there was a renewal of activity, 

 the mean frequency again rising to 43, a value which it 

 maintained until the end of the year. Thus, although the 

 quarterly values of the frequencies were 41, 2-4, 2-3, and 

 43 respectively, the half-yearly values were more nearly 

 equal, at 31 and 32, the latter also being given as the 

 mean frequency for the whole year. The frequency values 

 for faculae vary in the inverse to those of spots, the 

 quarterly values being 12, 14, 21, and la ; the meao 

 ' for the year is i-6. 



!!E Nature of Comets' Tails. — In the course of an 



le on the present position of the problem of the forma-- 



and constitution of comets' tails, which appears in 



Physikalische Zeitschrift for March 15, Dr. L. Zehnder 



"s and extends a theory he first put foward tw'enty- 



ears ago in the pages of Kosmos. .According to this 



y, as the swarm of meteorites which constitutes a 



t approaches the sun, the meteorites nearer the sun 



1 to give out gases and vapours which arrange them- 



^ as atmospheres about single or about groups of 



al meteorites. These atmospheres refract the light 



i the sun, and, according to their densities, concentrate 



tr sun's rays to foci at different distances behind them- 



s- vf^s. If a meteorite is present at a focus it may be 



I rendered visible, or even be heated sufficiently to produce 



' combustion of any hydrocarbons present in it. The 



orites thus heated surround themselves in turn with 



spheres which concentrate the sun's rays on still more 



^-uiote meteorites, and the visible tail of a comet is, accord- 



j ing to the theory, the locus of the successive foci. Dr. 



I Zehnder considers the forms of the refracting atmospheres 



which would produce the various types of tails now 



I known. 



NO. 2 no, VOL. 83] 



Periodic Errors in Right Ascension of Standard Star 

 Catalogues. — .\ comparison of the periodic errors of the 

 right ascensions of the Newcomb, Auwers, and Boss 

 standard catalogues is published by Dr. Downing in No. 

 420 of the Observatory. The comparisons were made with 

 the *■ Standard Mean Right .Ascensions of Clock Stars for 

 19000, based on Twelve-hour Groups," published in the 

 Greenwich " Second Nine-year Catalogue," the places 

 there given being, presumably, free from periodic errors 

 depending upon right ascension. 



The differences found are verj- small in amount, but most 

 interesting in their distribution. There is a distinct drop 

 at R.A. 4h. and a rise at R..A. 2oh. which are too per- 

 sistent, throughout the catalogues, to be entirely due to 

 accidental errors. It is suggested that the peculiar dis- 

 tribution of magnitude through R.A. may account for some 

 of, but not all, the discordances in question. 



Observations of Southern Double Stars. — The first 

 number of the Circular of the Transvaal Observatory is 

 devoted to the measures of a number of double stars dis- 

 covered by Mr. Innes, with the 9-inch Grubb refractor, 

 south of declination —19°. Experience shows that this 

 instrument, used at the altitude (5900 feet) of the Trans- 

 vaal Observatory, is capable of resolving very close 

 doubles (03') discovered by Prof. Hussey with the 36-inch 

 refractor at Lick, and 11 per cent, of the 268 stars (Innes, 

 433-700) now given are separated by not more than 05' ; 

 43 per cent, have distances of 10* or less. Mr. Innes also 

 gives a list of stars which have been wrongly identified by 

 other obsen.'ers. 



The " Gazette Astronomiql-e. " — We regret to learn 

 from the current number of the Gazette Astronomique, 

 published by the Antwerp .Astronomical Societj', that, until 

 further financial support is forthcoming, this ver>' useful 

 journal for amateur astronomers will only be published 

 alternate months, instead of monthly, as heretofore. The 

 Gazette always contains ephemerides, notices of pfienomena, 

 &c., in addition to interesting accounts of observations ; the 

 subscription is 3 francs per annum, post free. 



AURORAL DISPLA YS. 



"DRILLIANT displays of aurora were reported from many 

 -'--* different parts of Scotland on the nights of March 27, 

 28, and 29, and aurora was also observed in Ireland and 

 the northern portion of England. At Aberdeen aurora was 

 seen each night between 8 and 9 o'clock. The Westminster 

 Gazette gives an account of a brilliant display seen at 

 Edinburgh early on the morning of March 28, stating that 

 two separate displays were seen before 2 a.m., and there 

 was a third shortly before 2.30 a.m. One of the first 

 indications of the coming of this third display was a long, 

 luminous shaft stretching upwards and intersecting the 

 constellation Cassiopeia at a point near the star 

 8 Cassiopeiae. For some seconds it remained motionless 

 and alone, like the tail of a great comet. Then the sudden 

 flashing forth of a myriad quivering shafts and sheaves of 

 light, exquisitely and delicately tinted, outlined a wide arch 

 of striking beauty. 



Mr. Wilfred C. Parkinson, writing from Eskdalemuir 

 Observatory, Scotland, gives the following interesting 

 details of the display on ^larch 28 : — 



8.10 p.m. — Luminous band first observed in N. rising 

 slowly like a bank of light cloud. 



8.14-8.38. — Gradually assuming a cur\'ed form io°-i2° 

 above horizon at middle point, which was rather to the 

 W. of N.. and about 8° in width. Length about 140°. 



8.40. — Band very bright and w^ell defined, very intense at 

 top edge, gradually thinning out towards the lower edge. 



8.53. — The lower edge of the main band had formed a 

 distinct band by itself, running parallel to the higher band, 

 but not so wide, long, or intense. Higher band of uniform 

 intensity throughout. 



8.54.— Vertical streamer gradually forming, and also 

 smaller ones, fluctuating in length and brilliancy. 



8.56. — Vertical streamer very intense, especially where 

 the curved bands cross it. 



8.59. — Lower horizontal band gradually disappearing. 

 Upper band growing faint and ill-defined. Vertical 

 streamers growing more numerous. 



