468 



NATURE 



[September 15, 1892 



THE PLANET VENUS 



ME. L. TROUVELOT has published a most impor- 

 • tant and extensive paper on some observations 

 of the planets Venus and Mercury, which for many years 

 past have been occupying his attention. The physical 

 features of the other planets have been treated on previous 

 occasions in like manner, and have extended our know- 

 ledge very considerably, so that the reader of this work 

 will be sure to find something really new in the great 

 number of observations that are here recorded. Up to 

 the month of April, 1882, the observations were made at 

 Cambridge, United States ; but since then Meudon has 

 been the seat of operations ; the air at the latter place did 

 not prove so pure as that in the States, and the horizon 

 not being so open, the number of observations of course 

 was somewhat reduced. 



In the work which we have before us the author 

 divides this subject up into nine sections, and we cannot 

 do better than treat of each of them in turn, commencing 

 with the visibility of Venus to the naked eye in full day- 

 light. The best way is, he says, to use the tele-cope as a 

 pointer, directing it to her by means of the circles ; by 

 then looking along the telescope tube he has been able to 

 see her at every point of her orbit, when her angular dis- 

 tance from the sun towards inferior conjunction was not less 

 than 10°, and also towards superior conjunction when she 

 was not less than 5°. Her visibility depended to some 

 extent on the phase she represented, for it is known that 

 the eye can distinguish more easily a disc small and 

 distant than a comparatively larger and nearer crescent. 

 At Cambridge it seems to have been more or less the 

 rule, while at Meudon it was the exception, to see Venus 

 in the daytime, the atmospheric conditions at the latter 

 place being comparatively very bad. 



With regard to the general aspect of Venus nothing 

 very striking has been noticed ; the part of the limb 

 turned towards the sun, as recorded by other observers, 

 always appeared more brilliant than the more central 

 portions extending towards the terminator. Sometimes 

 the limb was not so bright as usual, being observed to be 

 " dull and without brilHancy,'' one very noticeable time 

 occurring on April 15, 1878. 



Under favourable conditions, whitish and greyish spots 

 can be seen on the surface of Venus, which, at any time, 

 are very difficult to observe. These different-tinted spots 

 give, according to M. Trouvelot, indications of being at 

 different levels. The whitish spots, situated near the 

 terminator, produce on it slight deformations, and seem 

 to so alter it as to suggest that these spots are at a higher 

 level than the other parts. The greyish spots, on the 

 other hand, when situated in about the same positions, 

 also deform the terminator to a small extent, but in an 

 opposite way to those just mentioned, suggesting that 

 these spots lie at a lower level than the parts near them. 

 These two kinds of spots have another pecuUarity which 

 has been particularly noticed, and that is their size ; the 

 white ones seem to assume a round or slightly oval form, 

 and are nearly always small, but the grey spots are gene- 

 rally of an elongated shape, and are of very large propor- 

 tions, forming sometimes straight bands. The interval 

 between the appearance and disappearance of these spots 

 is not long ; in their formation they are analogous, as 

 M. Trouvelot says, " avec ces taches diffuses des couches 

 nuageuses continues de notre atmosphere precMent les 

 pluies, et qu'un simple jeu de lumiere fait naitre ou dis- 

 parattre." Their contours are always very vague, the 

 whites being a little less brilliant, and the greys a little 

 less dark. 



One of the largest spots that has been diligently ob- 

 served was that which appeared on the 3rd September, 

 1876. Its size, as will be seen from the figure, was, com- 

 paratively speaking, enormous, occupying nearly a third 

 of the illuminated visible surface. At its north and south 



NO. II 94. VOL. 46] 



extremities it was separated from the terminator by a 

 large white band, the north one being considerably larger 

 than the southern one. Up to the loth of the same 

 month this spot was still visible, but after that date na 

 trace of it at all could be found. Curiously enough, on 

 February 13th, 1 891, another large grey spot (Fig. 2), 

 bordered with white, made its appearance, and was very 

 similar to the one we have just mentioned, both with re- 

 gard to its position and form — indeed, the resemblance 

 was so striking that the spots were considered the same. 



Fig. I.— Showing the Isrge spot on Septei 



Why it should have disappeared so soon in 1876, and 

 become visible again in 189 1, is a mystery which is hard 

 to fathom. 



Perhaps one of the most interesting features visible on 

 the surface of Venus are the two snow caps (Figs. 3 and 

 4) at the extremities of her poles. These spots, as M. 

 Trouvelot says, surpass in brilliancy and importance all 

 that he has ever observed. In 1877, on November 

 13th, a white spot was seen at the north limit of Venus i 

 its brilliancy attracted considerable attention, resembling 



Fig. 2. — Large spot visible on February 



very much those situated on Mars. On the following 

 day, another spot, also very striking and of the same 

 character, diametrically opposed, was observed. The 

 question then arose as to the cause of these spots, and 

 we may here quote an entry that was written in the ob- 

 server's book on the 17th of the same month : — "Est-ce 

 que V^nus aurait des taches blanches semblables aux 

 taches polairesde Mars?" The seeing of these spots was by 

 no means a difficult task, and it seemed certain that if 



