NATURE 



[June ii, 1903 



OUR ASTRONOMICAL COLUMN. 



The South Polar Cap of Mars. — In an article pub- 

 lishe.d in No. 4, vol. xvii. of the Astrophysical Journal, Prof. 

 Barnard details the results of his observations of the South 

 Polar cap of Mars made at Lick during the close approaches 

 of the planet in 1892 and 1894. He made a series of micro- 

 metrical measures of the cap during each opposition, and 

 the figures obtained during 1892 are set out in a table 

 which accompanies the article. 



Whilst looking over these measures recently it occurred 

 to Prof.' Barnard that if they were plotted with respect to 

 the summer solstice of the Martian southern hemisphere 

 some instructive results might be obtained. This was 

 done, and the two curves, which are reproduced, show that 

 the cap at both oppositions followed the same law of 

 decrease with remarkable fidelity. 



Another important point observed was that the cap 

 appeared to diminish for some time after the summer 

 solstice, that is to say, the highest temperature was not 

 reached until several weeks after the maximum of solar 

 heat ; this may have an important bearing when discussing 

 the existence of a Martian atmosphere similar to the 

 earth's atmosphere. 



In May, 1894, the Polar cap covered an area of about 

 365,000 square miles, by the end of November it had com- 

 pletely disappeared, thus showing that the snow, if snow 

 it be, is not of any very great depth. 



One remarkable phenomenon observed was the appear- 

 ance of a projection from the edge of the cap in the same 

 position and at the same period during each opposition ; 

 this remained behind as a bright strip, and seems to in- 

 dicate the existence of a mountain range which is prob- 

 ably high enough to remain permanently snow-capped. 



Eight drawings of the cap during each opposition, and a 

 drawing of the whole planet, accompany the article, and 

 show the details of the outline of the cap very clearly. 



The Harvard Photographs of the Entire Sky. — In 

 Circular No. 71 of the Harvard College Observatory, Prof. 

 E. C. Pickering gives a description of the photographs taken 

 at Cambridge (Mass.) and Arequipa, which have been 

 obtained so as to furnish a bi-monthly record of the entire 

 sky down to stars of the twelfth magnitude. Each plate 

 measures ten inches by eight, and covers a region of more 

 than 30 degrees square ; they have been obtained with two 

 similar anastigmatic lenses of one inch aperture and 

 thirteen inches focal length. 



Prof. Pickering explains how useful these plates have 

 already proved at Harvard in determining changes of 

 variable stars, the times of the first appearances of Novae, 

 &c., and states that in order to allow astronomical science 

 generally to participate in these benefits, it has been 

 decided to make negative copies on glass of one series of 

 fifty-five plates, and distribute them to all who desire them 

 at a price below cost. The whole set of fifty-five may be 

 obtained for 1500 dollars, and selected sets of ten for 3;i.i 

 dollars ; the balance of the cost is being paid from the 

 " Advancement of Astronomical Science " fund of the 

 Harvard Observatory. Should the demand justify the ex- 

 periment a second set, the centres of which are near the 

 corners of the first set, will be issued later. 



Prof. Pickering gives a catalogue of the plates it is pro- 

 posed to issue, giving full particulars of the regions they 

 cover, the dates of exposure, &c., and in a set of " re- 

 marks " appended to the catalogue he gives details of any 

 special object each plate contains. 



THE ROYAL OBSERVATORY, GREENWICH. 

 'T' HE Report of the Astronomer Royal to the Board of 

 Visitors of the Royal Observatory, Greenwich, was 

 read at the annual visitation on Saturday last. From the 

 record of work done during the year covered by the report, 

 we select a few notes referring to the state of some in- 

 vestigations of especial interest. 



Longitude Operations. — The second stage of the redeter- 

 mination of the Paris-Greenwich longitude was completed 

 in the autumn of last year. As in the first stage carried 

 out in the spring and referred to in the last report, observ- 



NO. 1754, VOL. 68] 



ations were made simultaneously by two French and two 

 English observers at adjacent stations. The observations 

 of both the French and English observers were made in 

 three groups of three, six, and three full nights (or their 

 equivalents in half nights), the observers with their instru- 

 ments being interchanged between the first and second and 

 again between the second and third parts. In the deter- 

 mination made in the autumn the stands were also inter- 

 changed with the instruments. 



The reduction of the observations made by the English 

 observers is completed with the exception of slight correc- 

 tions which may have to be made in a very few instances to 

 the assumed right ascensions of the stars. 



The determination made in the spring of last year gave 

 for the dift'erence of longitude between Cassini's meridian 

 and that of the Greenwich transit-circle 9m. 2o-974S., and 

 for the difference of personal equation D — H =00415. The 

 determination made in the autumn gave 9m. 2o-909s. and 

 the difference of personal equation D — H =00495. In the 

 first series, if the level determination had been based 

 entirely on observations of the striding levels, the result 

 would have been 9m. 20-9825., and if entirely on the 

 observations of nadirs 9m. 20-969S. In the second series 

 the difference between the results from " striding levels " 

 and " nadirs " was only 0002s. In the first series the 

 probable error of the difference of longitude determined 

 from one full night's observations was + 0040s., and in the 

 second series only +o-oi8s., giving for the probable error 

 of the determination made in the spring +001135., and for 

 that made in the autumn ±000473. In each series there 

 was a double interchange of observers, so that the probable 

 error includes any change of personal equation between the 

 first and third parts, and this would appear to account to 

 some extent for the larger probable error found for the 

 first series. 



The International Geodetic Association, considering it 

 desirable that a redetermination of the difference of longi- 

 tude Potsdam-Greenwich should be made with their lately 

 adopted Repsold registering micrometer, the longitude 

 pavilion was placed at their disposal, and the Post Office 

 authorities have given all the telegraphic facilities desired. 

 Prof. Albrecht and Herr Obst installed their instruments in 

 the last week in April, and the observations are now in 

 progress. 



Lunar Tables. — The need for improved tables of the moon 

 has been emphasised during the past year by the discussion 

 of the results of Greenwich observations in the last ten 

 years, which was taken up primarily in connection with 

 the delimitation of an Anglo-German boundary, and may 

 perhaps be advantageously extended with a view to its use 

 in the formation of improved tables of the moon. In the 

 same connection Prof. Newcomb, who has devoted so much 

 attention to the subject, has urged that a fresh comparison 

 should be made between theory and the Greenwich meridian 

 observations from 1750 to the present time. It is a question 

 for consideration whether it would be practicable to carry 

 out this work at the Royal Observatory in such a form as 

 would facilitate the preparation of improved tables and 

 materially advance the lunar theory. 



Stellar Observations. — The progress made in the observ- 

 ation of the reference stars for the astrographic plates, for 

 which more than 10,000 stars are to be observed three times 

 above and twice below pole, has been very satisfactory. 



The observations of these stars were commenced in 1897 

 and will be completed at the end of 1906. In 635 years. 

 633 per cent, of the observations have been secured, oi 

 which 115 per cent, were contributed in the last year. 

 From a comparison of the observations above and below 

 pole for the stars from N.P.D. 0° to 5°, which have been 

 completely observed, it appears that the probable error of 

 a catalogue place (five observations) does not exceed 

 + o"-23 in R.A. or N.P.D. 



As the photography for the Greenwich Zone (Dec. + 64° 

 to the Pole) has been completed, only a few photographs 

 have been taken with the astrographic equatorial to replace 

 some which appeared to be inferior to the general standard. 

 Altogether 116 photographs were taken during the year; 

 these include 16 plates for the Astrographic Chart, 21 for 

 the Catalogue, 48 of Nova Persei, 11 of Comet b 1902, 

 6 of Comet a 1903, and 8 for the adjustments of the instru- 

 ment. 



