April 1911. 



K\o^^•L^D^,E. 



161 



account for the brilliancv of \'e!iiis and this point 

 has been brought forward as an objection to the 

 cloud theory. Since it was necessan,- to give some 

 ahernative explanation, it has been suggested that 

 the light is reflected from minute particles of dust 

 suspended in the atmosphere, but this idea does not 

 appear altogether satisfactory and it is possible to 

 find another reason for the unusual brilliancy of the 

 planet. The mean temperature of the air on \'enus 

 must be considerably higher than that of the 

 terrestrial atmosphere, and should be taken into 

 consideration. This would necessarily involve the 

 attainment of a higher altitude and diminished 

 pressure for the formation of clouds, which would, 

 therefore, be probably thinner than ours, and would 

 be covered by a film of cirrus extending almost 

 to the limits of the atmosphere. It is well 

 known that an attenuated substance is capable of 

 reflecting more light than when condensed, and the 

 rarefied condition of the clouds might thus account 

 for the abnormal albedo of the planet. The peculiar 

 appearance of the terminator, caused by the 

 prolongation and occasional detachment of the cusps, 

 can also be most easily explained hv the theorv of 

 cloud reflection. 



.\nother peculiarit\' has been noticed more than 

 once when \'enus has been examined under suitable 

 conditions. The dark portion of the sphere has 

 been found to emit a faint luminosit\' similar to that 

 observed on the new moon. We know this latter 

 spectacle to be caused by the reflection of light from 

 the earth on the dark portion of the lunar surface. 

 In the case of \'enus. there is no one bodv capable 

 of transmitting the necessar}- illumination, and its 

 cause has always been rather a mvstery to astro- 

 nomers. Now this phenomenon, which for want of 

 a better name may be termed '' starshine," has been 

 advanced as an argument in favour of a rotation of two 

 hundred and twenty-five davs. It has been pointed 

 out earlier in this article that under these conditions 

 if there were water on the planet it must all have 

 been deposited on the dark side in the form of ice : 

 and it is suggested b\" supporters of this rotational 

 theory that the ghostlv appearance sometimes 

 observed is caused by the light from the Earth 

 and stars mirrored on the ice. .\ moment's con- 

 sideration will, however, show the impossibilitv of this 

 idea. It is generally admitted that the whole sphere 

 is surrounded by a dense atmosphere, and this would 

 be quite sufficient to prevent the greater portion of 

 starlight from reaching the surface, even in the 

 absence of clouds. A\'hen one considers that the 

 sunward side, where no ice can exist, is capable of 

 transmitting over 90 per cent, of the light received, 

 it is hardly necessarv to invent an ice mirror to 

 account for the " starshine "" on the dark portion 

 of the planet. 



The evidence of the spectroscope shows that water 

 vapour is probably present on \'enus and thus this 

 instrument adds its testimon\- to the likelihood of a 

 cloud-laden atmosphere. At the same time the spec- 

 troscope gives no evidence of rapid axial rotation. 



It is well known that a rotating body, which emits or 

 reflects light, displaces the lines of the spectrum. 

 This gives the latter a twisted appearance, owing to 

 one limb of the sphere approachiiig the observer, 

 whilst the other recedes, and thus altering the relative 

 wave-lengths of the light. The amount of this alter- 

 ation for a given speed of rotation is well known, and 

 as it is i)ossible to obtain by these means the angular 

 velocities of the Sun, or any body whose diameter is 

 known, and compare them with the results obtained 

 by visual observation, the accuracy of this method 

 has been indisputablv proved. Naturally, the lower 

 the velocity happens to be, the greater is the difficulty 

 experienced in its measurement by these means, but 

 in the case of a body having a diameter of seven 

 thousand seven hundred miles, any motion involving 

 a rotational period of less than two months might be 

 detected. 



As the spectroscope has failed to show any shift 

 in the spectral lines, this presents an almost insuper- 

 able obstacle to a period anything like so short as 

 twentv-four hours. We have also seen that a 

 rotation, which would cause the planet to turn the 

 same face always to the Sun, is incompatible with the 

 presence of water. We are thus led to conclude 

 that the planet's da\' is between two months and 

 two hundred and twenty-five of our days in 

 length. 



This is rendered more probable since the slow 

 axial rotation which this hypothesis necessarily 

 involves would be almost impossible to detect from 

 observation. Schiaparelli himself admitted that the 

 planet's da\" m.ight be any period between six and 

 nine months in length, and it was, therefore, con- 

 cluded somewhat prematureh- that if \'enus had 

 no visible rotation it had already reached the stage 

 when the same face was always turned to the Sun. 

 If this has happened there must have been an inter- 

 mediate stage when the planet's day was shorter 

 than its \ear, but still not short enough to enable us 

 to detect anv motion, niid there is no evidence to 

 shoic that this sta}>e has yet been passed. \\'ith 

 the exception of Mercurv all the other principal 

 planets appear to have rotations of twenty-four 

 hours or under, and if their periods are eventualh- 

 lengthened until they are synchronous with their 

 respective orbital revolutions, the time required for 

 this process will be extended as their velocities 

 decrease, owing to the consequent reduction of tidal 

 friction. This friction w ill also be further diminished 

 since time will cause an increasing rigidity of each 

 planet on account of its gradual loss of heat, and in 

 the case of the inner planets through the escape of 

 water vapour by molecular activity. Consequently, 

 the final stage of rotational independence will be 

 prolonged for a considerable period before it is 

 completely controlled by the sun, and this renders it 

 likelv that \'enus still has a somewhat shorter day 

 than its \"ear. Moreover, the only friction capable 

 of acting on \'enus is that generated by the solar 

 tides, and although, as their capacity for retardation 

 varies inversely as the sixth power of the distance. 



