414 



NA TURE 



[March i, 1894 



Trouvelot in 1878, at a time when only a very fine crescent 

 was visible. (The bulging at the south-south-east portion 

 of the crescent was olaserved, and is not a defect in the 

 drawing.) 



Of all the planets, Venus approaches us the nearest, 

 her minimum distance amounting sometimes to approxi- 

 mately five million miles, that is, about five times nearer 

 than when she is furthest from us. Unfortunately, at these 

 times her illuminated disc is turned away from us, and all 

 we can do is to direct our attention to the small crescent 

 that remains before inferior conjunction is reached. 

 This accounts for the uncertain knowledge that we 

 pobsess with regard both to surface markings and the 

 period of rotation. The latter question is still a moot 

 point among astronomers, and it is interesting to note 

 the historical sequence in which these investigations 

 have been made. The first spots on the planet's disc 

 were noted by Dominique Cassini in October and June 

 of the years 1666 and 1667 respectively, and from them 

 he deduced a period of 23h. 21m. Bianchini, about 60 

 years afterwards (1726-27), came to quite a different 

 result, substituting 24 days 8 hours for that obtained 

 above. Jacques Cassini, discussing his father's observa- 

 tions and those made by Bianchini, concluded that a 

 period of 23h. 20m. satisfied both the old and new 

 observations, but that Bianchini's value would not agree 



Fig. 2. — Details of snow-caj)s January 19, 1S78 (Trouvelol), 



with that of his father. This value seems for some time 

 to have been accepted, and Schroeter's (1798-1799) and 

 De Vico's (1840-42) observations practically confirmed 

 it. Fig. 2 gives a view of the planet as seen on January 

 19, 1878, and shows the details in the polar spots some- 

 times available for "period of rotation" determinations. 

 Thus matters stood till that keen-eyed observer 

 Schiaparelli took the field. After a most careful study, 

 extending over many years, in which some single obser- 

 vations were made extending over eight consecutive 

 hours, he was led to make the statement that the rotation 

 of the planet is exceedingly slow, and probably takes place 

 in a period of 224 days 7 hours, the duration of the revo- 

 lution of Venus about the sun. At Nice, M. Perottin 

 has come to a similar view, expressing his opinion in the 

 following words : " Ne differe pas de la durce de la revo- 

 lution siderale soit 225 jours environ, de plus de 30 

 jours." These two observers, especially the former, thus 

 upset our whole belief in a short duration of the period, 

 but we are still again brought to consider the question from 

 observations emanating from another source. We refer to 

 those made by Prof. Trouvelot (see Nature, vol. xlvi. 

 p. 470), whose opinion is of great weight. The im- 

 portance of his work lies in the fact that it was carried 

 on at the same time as that of Schiaparelli " souvent 

 dans la mCme journce, sous un ciel egalement propice et 

 NO. I 270. VOL. 49] 



precisement sur la meme point de la planete." The 

 value ultimately deduced was 23h. 49m. 28s., which 

 again brings us back to a short period. In referring to 

 Schiaparelli's observations he says ; " La cause probable 

 de I'erreur de M. Schiaparelli semble resulter de ce fait que 

 les taches Ji et /■, qui ont servi de base a ses conclusions, 

 faisaient partie de la tache polaire meridionale qui, etant 

 situt^e centralement sur I'axe de rotation de la planete, 

 semble rester stationnaire, comme cela se voit sur la 



February 5, 2h. 



Fig. 3. 



"February 5. 50. 43m. 



tache polaire de Mars, quand elle se trouve reduite a de 

 faibles dimensions." He also refers to the general features 

 visible on the planet's surface as indications of a rapid 

 rotation, especially that of the rapid deformations of the 

 terminator and hours. 



Thus we are left with the choice of two periods, one 

 long and consisting of 224 days, the other short, of 24 

 hours nearly. We leave our readers to adopt that 

 which they think best, the balance of favour falling, in our 

 opinion, slightly towards the 24-hour side of the scale. 

 But just as Schiaparelli's observation of the doubling 

 of the canals of Mars was finally observed and universally 

 accepted, so perhaps time may prove his case as regards 

 this period of rotation. 



Some of the most recent work on the planet Venus 

 relates to the measurement of her diameter. Among a few 

 of the reduced measures the following may be given : — 

 Hartwig, with the Breslau heliometer, from forty-three 

 observations obtained a diameter of \'j"'6'j. The same 

 observer, from a reduction of the Oxford observations, 

 and also from Kaiser's observations with Airy's double- 

 image micrometer, obtained 17'' "5 82 and I7"'409 from 



Fig. 4. — Showing irregularity of terminator November 23, 1877 (Trouvelot^ 



thirty-three and thirty-four observations respectively. 

 Auwers from the transit of Venus measures deduced the 

 value i6"'8oi, while Ambronn ^, from thirty-four observa- 

 tions, measured the diameter as i7""jii. 



Among other interesting points to which we might 

 refer, are the planet's visibility in full daylight, the snow- 

 caps, the secondary light, the planet's form, &c. Each of 



■^ See As/r. A'ac/ir. No. 3204, p. igo. 



