62 



KNOWLEDGE 



[Makch 1, 1899. 



elongated in an east and west direction. Now Miidler 

 drew and described the two as exactly alike in every 

 respect, and Beer and Miidler examined this pair more 

 than three hundred times between 1829 and 1837, without 

 noticing any change. Schroeter, on the other hand, whose 

 observations preceded those of Miidler by more than a 

 quarter of a century, drew the western crater as the larger. 

 Webb, in 1855, found this inequality had been reversed. 

 Its appearance as he then recorded it corresponds with 

 that which the photograph shows and which it retains to 



the present day. 



♦ 



THE PLANET EROS (D Q, 433). 



A CAREFUL search has been made by Mrs. Fleming 

 upon the Harvard plates for early photographs of 

 Witt's Planet (433) D Q. Mr. S, C. Chandler 

 has courteously furnished ephemerides based upon 

 the best available material, and has devoted much 

 time to correcting the elements and computing the positions 

 corresponding to the times at which certain photographs 

 were taken, as is more fully explained in the Astronomical 

 Journal, No. 452. 



In making this search the following method of procedure 

 has been adopted. Mr. Chandler, by means of the 

 elements published in the Astronomical Journal, No. 451, 

 computed ephemerides for the oppositions of 1894 and 

 1896. It appeared that the observations then available 

 were insufficient to determine the position in 1894. An 

 error of one second in the mean daily motion in the orbit 

 would change the right ascension of the object in 1894 by 

 about half- an -hour. Moreover, the value of the daily 

 motion differed by several seconds not only in the early 

 ephemerides of this planet, but in those dependent on a 

 large number of visual observations. Although plates 

 were examined by Mrs. Fleming, covering a region of 

 about one thousand three hundred square degrees, the 

 planet was not found. Plates taken in 1896 were next 

 examined, as it was thought that the smaller errors of the 

 ephemeris would compensate for the extreme faintness of 

 the planet. This examination proved to be especially 

 laborious and fatiguing to the eyes. It was feared that 

 the object might be too faint to appear upon the plates, 

 and accordingly the faintest objects were carefully 

 scrutinized. 



Each plate was examined by superposing it upon another 

 plate of the same region taken with the same instrument. 

 Two adjacent images then appeared of each star, while 

 the planet, if present, would appear only upon the upper 

 plate. Numerous suspicious objects were thus found, 

 including several images of the planets Flora (8) and 

 Nysa (44), and two new variable stars, whose approximate 

 positions for 1900 are in R. A. 8h. 33-9m., Dec. +50'= 29 , 

 and R. A. 18h. 38-7m., Dec. -38° 62', were discovered. 

 The star -f29"551 fails to appear on ten plates which 

 show other faint Durchmusterung stars, and two stars at 

 E. A. 9h. ll-4m., Dec. -10° 43', and R. A. 9h. ll-5m.. 

 Pec. —10° 16', appear upon the Durchmusterung charts 

 and upon the photographic plates, but are not given in the 

 Durchmusterung catalogue. 



At last a faiut image was found on a plate taken on 

 .Tune 5th, 1896, and confirmed by other plates taken on 

 June 4th and June 5th. A plate taken on April 6th 

 covered the region of the planet, which was readily found 

 by means of its computed position. Mr. Chandler, from 

 positions of these images, was enabled to furnish a corrected 

 ephemeris for 1894, by means of which the planet was 

 readily detected on several plates. 



The following elements have been computed by Mr. 



Chandler, by combining the observations of 1898 with 

 those derived from the photographs taken December 19th 

 and 27th, 1893, February 16th, 1894, April 6th, 1896, and 

 June 4th and 5th, 1896 :— 



Elements. 

 Epoch 1898, Aug. 81-5, Gr.M.T. fx. = 2015-2826" 



M = 221° 85' 45-6" log a = 0-1637876 



w = 177° 37' 560" ) Period = 643' lOd. 



a= 303° 31' 57-1" [■ 1898-0. 



i = 10° 50' 11-8" ) 



4, = 12° 52' 9-8" 

 December 26th, 1898. 



The above method of search for Eros (433) has been 

 continued. The ephemeris has been extended by Mr. 

 Chandler, as required, and images of the planet have been 

 found by the writer on thirteen plates, three plates being 

 taken at Arequipa, the others at Cambridge. 



The earliest of these photographs, taken October 28th, 



1893, is important, since, with that taken on May 19th, 



1894, the anomaly through which the planet was observed 

 in 1893-1894 becomes 162°. 



The orbit of the planet could be well determined from 

 the observations in 1896 alone, using for the first place the 

 position of April 6th, for the second the three positions on 

 June 4th and June 5th, and for the third the photographs 

 taken on June 29th and 30th. 



Some important conclusions may be derived from this 

 investigation. All the photographs on which the planet 

 has been found were taken with doublets. If they had 

 been taken with lenses of the usual form, with a field two 

 degrees in diameter, all of the images would have fallen 

 outside of the plates. In view of the difficulties found in 

 photographing this object with an ordinary lens at Green- 

 wich and Oxford (Observatory XXI., 429), it is doubtful 

 whether we should have obtained many images of it here 

 with such a lens, even if it had been in regions photographed. 

 The number of plates on which the planet appears probably 

 fairly represents the number we have of all other similar 

 objects, whether already discovered or not. This planet 

 is bright during only a small portion of time. During the 

 last eleven years it has been brighter than the ninth 

 magnitude, photographically, for only two months, or about 

 a seventieth part of the entire time. There may be other 

 similar objects, even brighter, as yet undiscovered. Nova 

 AurigiB was as bright as the fifth magnitude for six weeks 

 before it was discovered. Had Eros attained the sixth 

 magnitude instead of the eighth, it should have appeared 

 on plates taken with the transit photometer. In this case, 

 we should have had an image of it on every clear night on 

 which it culminated after dark. Fairly good positions 

 could have been obtained from these images, since the 

 focal length of the telescope is about two feet, and the 

 exposures are so short that the images are always circular. 

 We have now a similar instrument in Arequipa, so that in 

 general two images should be obtained every night. 



January 16th, 1899. Edwaed C. Pickering. 



Utttcv. 



[The Editors do not hold themselves responsible for the opiniona or 

 statements of correspondents.] 



LONGITUDE OF THE EARTH'S PERIHELION. 



To the Editors of Knowledge. 



Sirs, — I should esteem it a favour if one of your readers 



or contributors would be good enough to explain the 



apparent discrepancy between the longitude of the earth's 



perihelion, as given in the appendix to Sir R. Ball's 



