August, 1903.] 



KNOWLEDGE. 



175 



THE DARK HEMISPHERE OF VENUS. 



By B. W. Lane. 



Although the object of this paper is to add one more 

 theory to the mauy which have been formed to account 

 for the visibility of this part of the planet Venus, I 

 think I may be excused if I first run over some 

 of the principal theories which have been previously put 

 forward. 



The phenomenon itself may be described as follows: — 

 Now and then the part of the planet not lit by sunlight 

 has been observed to shine with a faint light, exactly 

 similar to that emitted by the night side of the crescent 

 moon. This illumination, however, unlike that on our 

 satellite, which is constant, is intermittent, sometimes not 

 a])pearing for months together, or even longer, even 

 in the best telescopes, and at other times being 

 apparent at the first glance, and on rare occasions being 

 bright enough to have been once or twice visible in the 

 daytime. 



There is no satellite to illumine Venus in this manner ; 

 the earth is thousands of times too faint at that distance 

 to produce it ; the fact that the faintly-lit side always 

 appears smaller in radius than the bright portion, pre- 

 cludes the possibility of its being an optical delusion due 

 to the prolongation in the eye itself of the horns of the 

 crescent. It is therefore, apparently, as Webb calls it, an 

 " inexplicable phenomenon." 



All theories having for their basis the suppositional 

 existence of intelligent inhabitants on Venus may, I think, 

 be left over until everv other method has been found to 

 fail. 



The two theories which are at present most generally 

 considered to be most likely, explain the appearance : the 

 first by supposing aurorse of great brilliancy on the 

 planet, and the second by supposing the atmosphere 

 of Venus to be capable of refracting the sunlight 

 sufficiently to suffuse all the dark side of the planet with a 

 perpetual twilight. 



] u answer to the first of these theories it may be said 

 that in the first place we can scarcely fairly suppose auroras 

 of sutiicieut brightness on Venus, as Mr. Proctor points 

 out, and secondly, auroras are usually found to occur on 

 such occasions as when a sunspot crosses the solar 

 meridian, and, therefore, if this were the explanation, 

 illuminations should take place when the sun is spotted, 

 rather than at other times, yet no such coincidence has 

 l)een observed. 



Against the second, the objection may be urged that if 

 the dark side is lit up by refracted sunlight at one time 

 it should also be so lit up at another, and even supposing 

 such enormous changes of atmospheric conditions as are 

 necessary to account for the occasional nature of the 

 phenomenon, the terminator is just as sharp and 

 exactly defined on days when the dark side is visible 

 as when the dark side is invisible, whereas if the 

 above were the true explanation, at these times the light 

 and dark sides should fade almost imperceptibly into each 

 other. 



The theory which I here intend to put forward attributes 

 the apjiearance to a factor in the lighting of the sky which 

 has hitherto escaped recognition. I refer to meteor 

 swarms. 



Now it seems to me that the brilliant illuminations of 

 the sky caused by the Leonids in the old days before they 

 lost t-lieir way, should have been almost if not (juite 

 sutticient to make the night side of earth very faintly 

 visible from, say, Mars. Personally I never had the good 

 fortune to see the Leonids in 18(36, and still less so in 

 1833, but when I hear oa the authority of Humboldt and 



others that the swarm was sometimes so thick that at 

 times there were as many meteors visible at once as 

 ordinary stars, and when I compare this with the meteor 

 which I can see any night after ten minutes' watching, it 

 seems to me that the light given out, even after every 

 discount has been taken off for very natural exaggeration, 

 must have been very considerable. When I further read 

 that at some places in 1833 people were wakened up, 

 although the blinds were down, by the flickering light 

 thrown by these meteors, I begin to think that this light 

 must have been not only far stronger than moonlight, but 

 quite enough to make the dark side of our planet visible 

 to other members of our system. Persons who have seen 

 the Leonids thirty-five years ago will, of course, be able 

 to correct any mistaken impression I may have received, 

 out of their own experience. 



I think, however, it will be admitted that as there are 

 probably, nav, certainly, larger meteor swarms than the 

 Leonids wandering through space, swarms exceeding the 

 Leonids in density probably as much as the Leonids 

 exceed the ordinary swarms, it is practically certain that 

 these swarms if they collided with a planet having an 

 atmosphere such as Venus undoubtedly has, would be able 

 to light it up quite sufficiently to render it easily, if not 

 strikingly, visible. 



The question, then, is, are we justified in supposing that 

 Venus is lit in the same way and by the same means as 

 the earthly night skies have from time to time been 

 illuminated ? 



The average brightness of meteors entering the atmos- 

 phere of Venus will, by reason of their greater average 

 velocity, be about a-third greater than that of meteors 

 entering the earth's atmosphere. Therefore a swarm like 

 the Leonids acting on Venus — other conditions such as 

 inclination of orbit, etc., being equal — would produce about 

 four-thirds the illumination which it produces here. I 

 think, therefore, there need be little doubt that there are 

 meteor streams known to us capable of producing this 

 illumination, or something like it. 



The matter, however, does not rest here. The question 

 now is, how often do we meet such streams, and, above 

 all, how often do we meet streams which could produce 

 sufficient illumination to render the dark hemisphere 

 visible in the daytime ? Now if we write down six as the 

 number of showers which appear in a century on this earth, 

 and which are at all competent to render a planet visible at 

 any time, we shall be considerably over the mark. We 

 cannot, I think, expect that the number of swarms will, in 

 the ordinary way, be more than twice as great per given 

 unit of space, in the neighbourhood of Venus as near the 

 earth. Putting the figure, however, far higher in our 

 favour than we are really justified in doing, we find that 

 perhaps twenty times in a century Venus may be sufficiently 

 lit up by meteors to become visible. On quite two-thirds 

 of these occasions Venus will be either so near the sun, or 

 in so broad a gibbous phase, as to make the seeing of the 

 dark side an impossibility. We are thus reduceil to the 

 possibility that perhaps six times in a century the dark 

 side mav be observable. But we are expecting a great 

 deal more than we have a right to do if we suppose that 

 out of these six times it will be observed more than 

 three, whereas the actual number of times that the dark 

 side has been seen is more like thirty, if not far greater. 



We thus see that it is utterly impossible for meteor 

 streams of the same order as those with which we are 

 acquainted to produce an illumination anywhere near so 

 frequently as is actually observed. 



There is. however, an appearance sometimes seen in the 

 evening sky, and going under the name of the Zodiacal 

 Li"ht. Now whether this appearance is due to a vast 



