THE PROBLEM OF THE ROTATION OF VENUS AND 



THE INFERENCE TO BE DRAWN FROM THE PROBABLE 



ATMOSPHERIC CONDITION OF THE PLANET. 



Bv B. G. HARRISON. l-.R.A.S. 



In connection with the problems of tidal friction 

 and planetar\- e\-oliition the question of the rotation 

 of \'enus is of the utmost importance. Unfortu- 

 nateh- its period is still a matter of considerable 

 uncertainty, but is t;enerall\- considered to occup\- 

 either about twenty-four hours, or else to be efjual 

 to the planet's orbital revolution of two hundred and 

 twenty-five days. Although most astronomers favour 

 the latter alternative, there are serious objections to 

 either, and it is (]uite possible there may be another 

 solution to the question. There now seems little 

 doubt that Mercury alwa\-s turns the same face to the 

 Sun, but the case is not nearly so certain with regard 

 to \'enus, either from a dynamical or obserxatorial 

 point of vie\\'. 



Indeed, all the early astronomers assigned U> the 

 planet a period of about twenty-four hours, but their 

 data seem to ha\'e been somewhat insufficient, owing 

 perhaps, to the time of (Ia\- at which their observa- 

 tions were carried out. The most faxouralile time 

 is between sunrise and sunset, as tlie planet is then 

 high in the heavens and its glare is not so pronounced, 

 on account of the brightness of the sky, while it is 

 also possible to observe any peculiar features that 

 are presented for several consecutive hours. Until 

 recently, however, all observations were carried t)ut 

 either shortly before sunrise iir just after sunset, and 

 consequently at much the same time each daw so 

 that if any markings were noticed on the planet in 

 the same position on successive da\-s, it ma}- have 

 been attributed to a revolution having been completed 

 in the interval. 



Until the observations of Schiaparelli. in bsyj, the 

 idea of a period of two hundred and t\\ent\-Hve da\s 

 does not seem to have been seriously considered, but 

 since then many eminent astronomers, including 

 Professor Lowell, have come to the same conclusion. 



If it could be established that Venus were cloud- 

 covered, it would render this latter altinnative \ery 

 improbable. For if the planet always turned the 

 same face to the Sun, unless its thermal conductivitv 

 is infinitely greater than tliat of the Earth, the 

 temperature on the dark side \\(_iuld be certainh- low 

 enough to cause precipitation of the moisture 

 contained in the hot winds blowing from the 

 sunward side. There would be a rapid and constant 

 circulation of air owing to the difference between the 

 temperatures of the two hemispheres, and evapor- 

 ation and condensation would be continuous. As 

 the supply of water could not be replenished from 

 the dark surface owing to its conversion into ice, 

 all water \\ould long ago have ceased to exist in 



li(]uid form, which would entail a conse(]uent absence 

 of cloud from the planet's atmosphere. 



It is, of course, possible that the surface of \'enus 

 still has a certain amount of intrinsic warmth, and in 

 that case the dark side ma\' be in a similar condition to 

 the De\'onian and Carboniferous jieriods here, as the 

 clouds \\ould.to a great extent, jnevent the accessor 

 escape of heat. Now, if we accept this as an 

 argument in favour of a da\' equal in length to two 

 hundred and twenty-five of t)urs, we must also bear 

 in mind the considerable difference in the relative 

 ages of the planets which the supposition would 

 necessarily involve. As the densit}' of \'enus is onU- 

 • N3 and its mass • 75 of that of the Earth, the former 

 would not onl\- ha\e generated less heat by con- 

 traction, luit would also radiate it more rapidly and 

 must, therefore, liaxe been evolved much more 

 recently to be still in a more primitive condition 

 than the Earth. The mimnunn period of terrestrial 

 rotation is unknown, but was possib]\' fix'e hours. 

 It. however, we assume that tlie Earth and X'enus 

 each had an original rotational \elocit\' proportional 

 to their masses, and that that of the former has 

 been diminished to its present speed by lunar and 

 solar friction, it is obx'ious that the more recent the 

 formation of X'euus is considered to be, the less 

 likely is it that it should have alread}' reached its 

 maximum rotational period owing to secular tidal 

 action. 



There is no doubt that a dense envelope of some 

 sort surrounds the planet. The appearance of a 

 luminous ring and the " black drop '" observed at 

 recent transits, are alone conclusi\e proof of this, 

 while the irregular and graduated appearance of the 

 terminator also furnishes us with further evidence of 

 its presence. Then, again, the extraordinaril)- high 

 albedo of \'enus almost prohibits the assumption 

 that it is the actual surface we see. Moreover, the 

 dark markings which ha\e been frequently observed 

 scarceh' exhibit the permanency one would expect in 

 the case of furrows on the face of the planet, and it 

 is quite possible they may be caused h\' con\ection 

 currents effecting openings in the clouds. In any 

 case, it is diificuh to understand how it w<iuld be 

 possible to see an\- detail on the surface itself when 

 one considers this is probably surrounded by a 

 mirror which reflects 92 per cent, of the light 

 re;cei\'ed. It is this \'er\ high reflecti\'e index which 

 presents some difficult\' to the theor\- of a i loudy 

 atmosphere. Until recently cloud was considered to 

 be the highest reflecti\'e surface of a planet. Now 

 as its albedo is only 72 it seems scarceh' enough to 



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