136 



NA rURE 



[June 7, 1906 



ol' applying the corrections to the star-places due to the 

 variation of latitude. The value found for 1904, with 

 Bessei's refractions, is 38° 31' 21 "•74. 



The second nine-year catalogue, for epoch 1900, which 

 was completed last year, will be divided into two sections, 

 one containing the fundamental and zodiacal stars, the 

 other the astrographic reference stars. For the second 

 section the places (for 1900) of the stars within 10° 

 of the pole have already been determined, and a com- 

 parison of these with the places given in Carrington's 

 Kr'dhill catalogue should discover a number of proper 

 millions hitherto undetermined, thereby providing new 

 material for the discussion of the solar motion. 



Mr. Cowell has completed the discussion of the Green- 

 wich meridian observations of the moon from 1750 to the 

 present time, and has found the necessity of introducing 

 three empirical terms, of which the third has a period of 

 about 300 years. Because the introduction of this term 

 renders the determination of the secular acceleration of the 

 moon from modern observations impossible, Mr. Cowell 

 has worked up the conditions for six ancient eclipses of 

 which the historical records seem to be fairly authentic. 

 By introducing accelerations of eleven seconds per century 

 for the moon and four seconds for the sun, he found it 

 possible to bring the conditions of every one of these 

 eclipses into agreement with the historical records of the 

 phenomena attending them. By treating ten of the lunar 

 eclipses recorded in the Almagest in the same way, addi- 

 tional evidence for the existence of the.se accelerations was 

 obtained. At first glance the acceleration for the sun was 

 difficult to account for, and Mr. Cowell hypothecated a 

 resisting medium through which the earth travels; but 

 more recently he has found that a lengthening of the day 

 by the two-hundredth part of a second per century would 

 account for the quantity required for this acceleration. 

 As one of the principal features of Mr. Co well's discussion 

 was the employment of the day as the unit of time, the 

 lengthening of that unit would produce the apparent 

 acceleration. 



Owing to the re-mounting and re-polishing of the object 

 glass the altazimuth was out of use from July 12 to 

 August 30, but for the remainder of the vi'ar it was 

 employed for observations of the sun, moon, "planets, and 

 fundamental stars. The lunar crater Mosting a' was 

 observed whenever the conditions were favourable, and, as 

 the same kind of observations are being made at the Cape 

 Observatory, the results will serve to determine anew the 

 paralla.\ of our satellite. The value obtained from the 

 discussion of the two sets of observations should be more 

 trustworthy than that previouslv obtained, which de- 

 pended solely upon observations 'of the moon's limb a 

 much more dilBcult feature to " set on " than the crater. 

 Mdsting A was also observed with the transit circle when- 

 ever possible, and the mutual agreement of the two sets 

 of results was very satisfactory. 



Eight hundred and twenty-three double and twenty-four 

 single observations of various stars were made with the 

 reflex zenith tube, and the results have been reduced up to 

 March 31. 



The weather was not favourable during the year for 

 observations of difficult double stars with the 28-inch re- 

 fractor, but the time was utilised in completing the 

 measures of neglected doubles in .Struve's " Mensura; 

 Micrometrica; " ; the total number observed was 606. of 

 which 158 have their components separated by less than 

 i"-n_, and seventy by less than o"s. The diameters of 

 Jupiter and his satellites were also measured with this 

 instrument. Both the polar and equatorial diameters of 

 Jupiter were observed, first with the filar micrometer and 

 then with the double-image micrometer, on each night, 

 and it was found that the mean of the results of the two 

 methods produced a very good value for the diameter. 

 The error caused by irradiation in the filar micrometer 

 observation is apparently exactly corrected by the error 

 introduced in the second method by the fact' that when 

 the two images are apparently in .'contact thev actually 

 overlap to a slight extent. 



The 26-inch refractor was employed on twenty-eight 

 nights in obtaining seventy-two photographs of Neptune 

 and its satellite, using the occulting shutter as in previous 

 years. These photographs are now being measured. 



NO. 1 9 ID, VOL. 74I 



A number of photographs of Jupiter's newly-discovered 

 satellites vi. and vii. were obtained with the 30-inch re- 

 flector. This success is remarkable because it was the 

 expressed opinion of the discoverer of the satellites that 

 vii. was too faint to be photographed through our British 

 atmosphere. Yet nineteen photographs of this object were 

 secured at Greenwich on fifteen nights, and eighty-six 

 negatives of satellite vi. were taken on thirty-six nights. 

 The 30-inch reflector was also employed for obtaining 

 photographs of twenty-three minor planets, five comets 

 Nova Aquilae, and several nebulae. 



The reduction of the Eros plates is complete, and the 

 results have been communicated to M. Lcewy. 



At the date of last year's report the measurement ol 

 the Greenwich plates for the Astrographic Catalogue was 

 complete, but a number of the measures have been re- 

 peated, and the press copy has been prepared for the seven 

 zones 80° to 86°. The measures of the eight zones 77° 

 to 84° have been printed during the year, and include 

 46,329 separate stars covering an area of 450 square degrees 

 of sky. The remaining 78-5 square degrees between Ss** 

 and the pole will include about 6000 stars. .An interesting 

 table given in the report shows the number of stars which 

 have been measured, and will be contained in the Green- 

 wich section of the catalogue, and compares it with the 

 number shown in each of the corresponding zones of the 

 Bonn Durchmusterung and the Astronomische Gesellschaft 

 catalogues. Thus it is shown that the total in the Green- 

 wich section will be about 178,380, whilst for the same 

 region the B.D. contains only 25,184 stars. \ similar 

 table compares the number of stars shown on the Green- 

 wich chart plates in several zones with those contained 

 in the corresponding zones of the B.D. In the total area 

 °f .SSS-S square degrees the latter contains 2259 stars of 

 magnitude 90 and brighter, and 6542 altogether, whilst 

 for the different exposures given for the Greenwich plates 

 the following numbers are shown : — 



Exposure 

 Number of stars. 



20 sees. 

 12,019 



56,921 



6m. 



58, 193 



40m. 

 170,180 



Thus on the plates taken at Greenwich with forty 

 minutes' exposure there are 304-8 stars per square degree, 

 and about twenty-six times as many stars as are given in 

 the corresponding region in the B.D. The second Green- 

 wich volume of the Astrographic Catalogue is printed up 

 to the end of 84°, and will soon be ready for publication. 

 Twelve thousand photographic prints, reproducing on 

 double scale iqi plates in zones 65° to 70°, have been made 

 during the year, bringing the total number of plates re- 

 produced since the work began up to 401, or rather more 

 than one-third of those contained in the Greenwich section. 

 During the year under report the astrographic telescope 

 has been used to obtain duplicate plates for the chart to 

 replace previous ones which aie not entirely satisfactory for 

 reproduction purposes. 



Heliographic observations were carried out as usual, the 

 sun being photographed on 210 days. The solar activity 

 was very pronounced during 1905, the record for that year 

 being about double that for 1904. 



The magnetic observations were made as in former years, 

 and the principal results for the magnetic elements for 1905 

 were as follow : — 



Mean declination 



Mean horizontal force 



Mean dip (with 3-inch needle) 



16° 9' 9 West 

 j 4 0173 (in British units) 

 { I '8523 (in metric units) 



66° 55' 55" 



There were no days of " great " magnetic disturbance, and 

 only twelve days of lesser disturbance in 1905. 



The various meteorological observations were continuously 

 maintained throughout the year, the mean temperature 

 being o°.2 above, and the rainfall 1-2 1 inches below, their 

 respective averages for the fifty years 1841-1890. 



In the chronometer and time-service department the re- 

 port follows the usual lines, but the .-Astronomer Royal 

 remarks on the inferiority of the box chronometers and 

 the superiority of the watches submitted for tests during 

 the period covered by the report. Of fifty-nine chrono- 

 meters sent in, thirty-three were rejected because they 

 failed to attain the minimpm standard of constancy. This 



