August. 1912. 



knowlp:dge. 



305 



object was found to be lather less than one-third of the 

 planet's diameter in size, and presenting a similar phase. He 

 saw it several times during about an hour, but never after- 

 wards found it. On four evenings during May, 1761, M. 

 Montaigne, of Limoges, saw what seemed to be the satellite, 

 alwajs presenting the same phase as the planet, and one 

 ijuarter of its diameter, but in altered positions. During 

 March, 1764, Riidkier, Horrebow and others with a refractor 

 at Copenhagen, and Montbarron at Auxerre with a reflector 

 repeatedly observed this object. Subsecjnently, Lambert 

 collected the whole of the observations and calculated its 

 orbit, publishing the calculations in Bode's Jliarbuch for 

 1777. To allow the supposed moon, however, to complete 

 its circuit in ll** 5'' 13"' at a distance of two hundred and 

 fifty- five thousand miles, as calculated, the mass or weight of 

 Venus would have need to be increased tenfold. Doubtless 

 the supposed satellite was a " ghost " in the eyepiece, yet it 

 must be admitted that Short's use of at least three eyepieces 

 is very puzzling. Since 1754, however, the "ghost" seems to 

 have made no further apparitions. 



Perhaps the greatest paradox has been the rotation period, 



period was twenty-three days. Other observers, such as 

 Terby. Perrotin. and Lowell confirmed Schiaparelli, and the 

 general conclusion of such has been that the rotation period 

 is probably identical with the period, in other words is two 

 hundred and twenty-five days. Nienslen and Stuyvaert, at 

 Brussels, observing in 1881 and IS'JO. supported the short 

 rotation period, as also did Trouvelot. Brenner gave the 

 period as 23'' 57"" 36', and latterly McHarg, from his own 

 observations and all available material, finds the period to 

 be 23*" 28'" 13" -595, and constructs a map of its surface, 

 which we reproduce. On the other hand, Mascari came to 

 the conclusion that the period was slow. 



Long ago De V'ico noticed that the observers who were best 

 able to see the markings on Venus were those who had the 

 greatest difficulty in seeing minute companions to bright stars. 

 The writer sees the markings with ease in a good air, yet at 

 the same time the details lack the sharpness of many of the 

 Martian markings. ."M the same time the motion from rota- 

 tion could be watched in half-an-hour as readily as could that 

 of Mars. The only conclusion that can be arrived at is that 

 some observers do see the surface of the planet modified by 



'\A 



i:Me difcetr ane sen 



Sea. At].j 



S o u t li 

 A m e !■ i c a 



Ko-rea 



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America '''n Imli 



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Figure 331. Map of Venus. 



Ceylo 



/■J' J. Mcl/ar^, M.A. 



some observers finding it to be less than a day, whilst others 

 have thought it was nearly or quite equal to Venus' orbital 

 period of two hundred and twenty-five days. The real 

 trouble seems to be due to the different sensitiveness of 

 different eyes to certain light rays. Cassini, who appears to 

 have paid special attention to bright spots, wrote : " The space 

 on these" (Mars and Jupiterl " I could attentively observe for 

 a whole night, when the planets were in opposition to the Sun ; 

 I could see them return to the same situation, and consider 

 their motion during some hours, and judge whether they were 

 the same spots or not, and what time they took in turning 

 round : but it was not the same with the spots of Venus, for 

 they can be observed only for so short a time, that it is 

 much more difficult to know with certainty when they return 

 into the same situation. I can, however, supposing that the 

 bright spot which I observed on Venus and particularly this 

 year was the same, say that she finishes her motion, whether 

 of rotation or libration, in less than a day ; so that, in twenty- 

 three davs nearly, the spot comes into the same situation on 

 nearlv the same hour of the day. though not without some 

 irregularity." In 1667. on April 20th, he observed the motion 

 of the bright spot during the period of his observation. The 

 period according to Cassini, the younger, w^as 23 21'". In 

 1726-27 Bianchini with a' two and a half inch refractor, no 

 less than sixty-six feet in length, came to the conclusion that 

 the rotation occupied 24'' 8". the error doubtless arising from 

 his small means and the short time he could follow the 

 object of his study. Schroeter. from eight observations of 

 a fixed point on the surface, obtained a rotation period of 

 23" 21'" 7•98^ At the Vatican Observatory, Rome. 

 De Vico and his helpers, 1839-41, found a period of 

 23" 21"" 22', a result obtained after an enormous amount 

 of work. In 1890, Schiaparelli, after discussing all 

 available material, came to the conclusion that rotation was 

 slow, not less than six or more than nine months. He seenis 

 to have in some way misread Cassini, and supposed that his 



an atmospheric envelope, whilst others fail altogether. Can 

 this be due to a form of colour blindness? the markings 

 being of a tint to which some eyes are susceptible and others 

 not. This fact, if accepted, would explain a good many of the 

 mysteries of observational astronomy. Mascari's observa- 

 tions can possibly be explained in another way. If the little 

 map be examined it will be noted that large dark markings are 

 situated one hundred and eighty degrees apart, whilst midway 

 between them on either side only very delicate details occur. 

 Mascari seems to have thought the dark markings were both 

 the same, and when the intermediate portions were presented 

 he was unable to see anything. 



Perhaps the greatest mystery is yielded by the spectroscope. 

 In 1900, Belopolsky, at Moscow, found the lines were curved 

 at the limb to an extent only to be explained by a quick 

 rotation. This seemed to give a definite answer. However, 

 in 1903 the spectroscope at Flagstaff Observatory, in the 

 hands of Slipher, gave absolutely contradictory results. The 

 news was given in the number of The Observatory for August. 

 1911, that the Russian observer has continued his researches, 

 and that the results of 1903, 1908. and 1911 confirm those 

 previously obtained. Moreover, the instrument has been 

 verified on Mars, the rate of whose rotation is known. " He 

 found for its equatorial velocity 0-354 kilometres per second, 

 instead of 0-254 kilometres. The value found for Venus, 0-38 

 kilometres, corresponds to a period of rotation of 1-44 days." 



There is another mystery respecting Venus, to which only 

 allusion must be made— the visibility of its unillummated 

 surface near inferior conjunction. Some observers have 

 recorded that the disc appears dark on a brighter background. 

 Others, however, maintain that it appears brighter than the 

 background upon which it is seen. The former can easily be 

 explained, but it is strange how a dark body can appear 

 brighter than a somewhat illuminated background. Even 

 an explanation of auroral lighting is very inadequate when we 

 consider the brightness of the background upon which it is seen. 



