NA TURE 



469 



THURSDAY, SEPTEMBER 28, 1876 



THE INTRA-MERCURIAL PLANET OR 

 PLANETS 



THE question of the existence of one or more planetary 

 bodies revolving within the orbit of Mercury is again 

 revived by Weber's observation of a round black spot 

 just within the sun's eastern limb, on the afternoon of 

 April 4 in the present year which had not been visible on 

 the same morning, and early on the following day had 

 disappeared. The position at 2J' only from this limb is 

 one, where an ordinary spot would not be expected to 

 exhibit a circular outline ; and a round black disk, in 

 such a position more especially, must instantly attract the 

 attention of a practised observer. On April 4 clouds 

 unfortunately prevented lengthened observation, and in 

 Weber's notice there is no reference to any perceptible 

 motion during the short time the spot could be watched. 



This observation resembles others already upon record, 

 made by persons equally worthy of credit, which it is 

 hardly possible to explain except on the hypothesis that 

 one or more planetary bodies exist with mean distance 

 less than Mercury, the rate of motion where motion has 

 been detected by the most reliable observers, not being 

 consistent with greater distance from the sun. While it 

 is certain that comets with perihelia within the earth's 

 orbit have transited the solar disk, and notwithstanding 

 such transits may have been more frequent than is gene- 

 rally supposed, the appearance of the spots now in ques- 

 tion seems, at least in several of the best authenticated 

 cases, to negative any idea of their being due to the 

 passage of comets across the sun, near their nodes. At 

 the same time there are several instances where the form 

 of the spots would perhaps accord better with the assump- 

 tion of a cometary transit, unless we can admit that the 

 deviation from circular contour is attributable to an 

 optical cause. 



It may be remembered that the attention of astronomers 

 was first seriously directed to the possible existence of a 

 planet or planets interior to the orbit of Mercury, by M. 

 Leverrier's announcement that the motion of the peri- 

 helion of this planet was not explained by known causes 

 of perturbation, but that an excess of 38 seconds in the 

 century must be admitted beyond the value derived from 

 theory, to produce an agreement between calculation and 

 observation in the discussion of the long series of observed 

 transits across the sun's disk. The unexplained motion 

 of the line of apsides might, as M. Leverrier remarked, 

 be due to the existence of a single interior planet of a 

 mass which would depend upon its mean distance. With 

 a distance of 0-17 (period of revolution, 2 5 "6 days) the 

 mass would be precisely equal to that of Mercury, and it 

 would vary inversely with the distance. Or it might be 

 due to a group of small planets circulating within the 

 orbit of Mercury. 



Having before us the whole of the recorded observa- 

 tions of the presence of suspicious spots upon the sun's 

 disk, we shall soon discover that they hardly admit of 

 explanation on the hypothesis of a single planet, even if 

 we assume a small inclination of the orbit of this planet 

 to the ecliptic, a condition which, while it would greatly 

 extend the transit-limits, must at the same time render 

 Vol. XIV.— No. 361 



the transits so frequent that it is in a high degree im- 

 probable the planet could have so long escaped certain 

 detection. Some few of the observations, as just remarked, 

 we may perhaps refer to comets in transit ; it remains to 

 endeavour to ascertain from observations not thus ex- 

 plained what period or periods will best represent them, 

 with the view to being warned of the probable times of 

 future transits. 



This subject has engaged the attention of M. Leverrier 

 during the last few weeks, or since he became cognisant 

 of Weber's observation last April, the notification of which 

 was long delayed. It appears that the observations of 

 Stark and Steinheibel, 1820, February 12, Lescarbault, 

 1859, March 26, and that of Weber, may refer to the 

 same planetary body if the revolution be supposed 

 28'oo77 days ; this being the sidereal revolution with 

 respect to the node, the synodical period would be 3033 

 days ; the corresponding mean distance from the sun is 

 o'i8, and the maximum elongation \o\ degrees. Such a 

 planet would again be in conjunction with the sun on 

 October 2nd or 3rd of the present year ; and if Lescar- 

 baulc's observation affords any approximation to the posi- 

 tion of the line of nodes would pass across the sun's disk, 

 and for this reason M. Leverrier has directed attention to 

 the importance of a close watch upon the same, during 

 these days, such watch, if possible, to extend to distant 

 meridians, so as to insure pretty continuous observation 

 through the forty-eight hours, Paris time. He has already 

 advised American observatories through Prof Henry, 

 Secretary of the Smithsonian Institution, and it is to be 

 hoped the chance of making an important discovery at 

 this time, may be made known to observers in the East. 

 It will be seen that the aid of the telegraph is indispen- 

 sable, in order to secure complete evidence of the existence 

 or non-existence of the hypothetical planet this autumn. 



Other observations may be reconciled with a period of 

 similar length, but the planet to which they may be sup- 

 posed to refer cannot be identical with the above. Thus 

 if Mr. Lummis's sketch of the path of the small round 

 black spot, which he remarked upon the sun at Man- 

 chester on the morning of March 20, 1862, is reliable in 

 the hurried and otherwise disadvantageous circumstances 

 under which it was made, the ascending node was almost 

 diametrically opposite to that of Lescarbault's planet, 

 elements which have been attributed to MM. Valz and 

 Rddau, and exhibiting a near agreement in the position of 

 the line of nodes, being certainly erroneous. Again, one 

 of the most interesting observations bearing upon the 

 existence of an intra-mercurial planet is that made about 

 the end of June or beginning of July 1847 in this country, 

 which can hardly be supposed to refer to either of the 

 objects seen by Lescarbault and Lummis respectively. 

 The exact date of this observation is unfortunately lost 

 beyond recovery. 



Mr. B. Scott, the City Chamberlain, observing the sun's 

 disk near London, a short time before sunset late in June 

 or on one of the first days in July, remarked upon it a 

 perfectly circular black disk, and was so confident of the 

 unusual character of the spot that he was on the point of 

 making known his observation through one of the London 

 daily journals on the evening of the same day, when unfor- 

 tunately an astronomical friend, under the impression that 

 an ordinary spot had been observed, dissuaded Mr. Scott 



