234 



KNOWLEDGE 



[October 1, 1897. 



falls away from one objection to accepting the otherwise 

 almost inevitable inference that true calcium in a state of 

 chemical integrity, although of the utmost physical tenuity, 

 is a main ingredient of the solar chromosphere and pro- 

 minences. It is not unlikely that sodium, magnesium, 

 and perhaps other elements, are invisibly associated with 

 it, calcium retaining luminosity long after the other 

 elements have become extinct. Its persistence in light 

 emission may be in part explained by the concentration 

 of vibrative energy, just verging towards total cessation, 

 in the violet twin-lines, the special import of which to 

 astrophysics has unfolded further with each advance in 

 knowledge. 



SOME NEW VIEWS AS TO THE PLANET VENUS. 



By CAiniXB Flammaeiox, F.R.A.S., 



Author of " Astronomic Populaire," " Les Etoiles," " Les 



Terres du del" " La Planete Mars,'' etc. 



THIS neighbour of our own world, which it equals 

 in size, and nearly equals in weight and density, 

 is the nearest to us of all the heavenly bodies, 

 with the exception of the moon. In its circuit 

 around the sun it frequently comes a great deal 

 nearer to us than our other neighbour. Mars — in fact, to a 

 distance of only forty millions of kilometres, whilst Mars 

 never comes nearer than sixty-four millions of kilometres. 

 Its apparent diameter then attains sixty seconds of arc — 

 that is to say, one-thirtieth of the diameter of the moon — 

 V. so that a telescope of a magnifying power not exceeding 

 thirty diameters will show us Venus as being apparently 

 of the same size as the moon viewed with the naked eye. An 

 optical instrument of a magnifying power of three hundred 

 diameters will, therefore, make the diameter of this planet 

 appear to be ten times larger than that of the moon, while 

 a glass magnifying six-hundred-fold will make it appear to 

 be twenty times larger. 



These are dimensions excellently adapted for telescopic 

 research, and, in the case of Mars, the results of observation 

 have been truly marveUous for diameters generally far less 

 than these, for Mars is both farther distant and sfnaller 

 than Venus. Yet our knowledge of the planet Venus is 

 incomparably less advanced than it is in regard to Mars. 

 In fact, we hardly know anything at all about the former 

 as yet, and are, in this respect, groping along at a mere 

 snail's pace. 



The reason of this fact lies in the diiBculty of observation. 

 In the first place, since Venus is circling around the sun 

 in an orbit interior to ours, the epochs of its nearest 

 approach to us are those in which it passes between the 

 sun and our planet ; its illuminated hemisphere being 

 naturally, in all cases, turned towards the sun. The result 

 of this to us is that it exhibits phases like those of the 

 moon. The nearer Venus approaches the earth the larger 

 becomes its diameter, but at the same time its crescent 

 becomes narrower in proportion, and we see less of its 

 surface. And, conversely, the farther the planet recedes, 

 its disc becomes more rounded, but at the same time much 

 smaller. Now herein lies the observer's trouble. We 

 never see Venus fully illuminated, except under the worst 

 conditions, for then it is on the side of the sun further 

 from the earth, and is reduced to its minimum dimensions, 

 being then, moreover, practically inobservable on account 

 of its proximity to the sun and of its great distance from 

 the earth. 



Another circumstance which interferes no less with 

 successful observation is that the planet is surrounded by 

 an immense atmosphere, about twice as dense as. and far 

 more elevated than, om-s. The absorption of solar light 



by this atmosphere is considerable. The illumination of 

 the sky in the daytime on the earth gives us some idea of 

 the enormous amount of light diffused by our air, and 

 reflected by its molecules. Now this atmosphere is im- 

 pregnated with light, the blue rays of which are reflected 

 to us, the other colours of the solar spectrum bsing 

 absorbed. The results of photometric, actinometric, and 

 calorimetric experiments concur in showing that our 

 atmosphere absorbs about one-third of the solar rays that 

 reach the planet ; two-thirds only come right down to the 

 sui'face of the earth even in the case of a zenithal sun. 

 Were an observer, placed on the moon, say, to examine 

 the terrestrial disc within the field of view of a telescope, 

 he would fail to see it with anything like the distinctness 

 with which we descry the full moon. The central region 

 of the disc would have lost two-thirds of its brilliancy, as 

 the atmosphere would act as a veil and reduce the light 

 received by the surface from the sun by one-third ; and it 

 would diminish it by an additional one-third in the reflection 

 from the surface to the eye of the observer. Then, as 

 absorption would increase with the obliquity, and with the 

 depth of the atmospheric layer — that is to say, with the 

 distance from the centre of the disc — such absorption 

 would become so considerable that most certainly the 

 geographical configurations would be entirely effaced for 

 at least one-half of the distance between the centre and 

 limb of the disc. 



Now, perfectly concordant observations have shown that 

 the atmosphere of Venus is about twice as dense as, and 

 of considerably greater extent than, that of our earth, so 

 that even if we were able to see the planet when in 

 opposition to — i.e., fully illuminated by — thesim, we could 

 distinguish scarcely anything of its surface, though the 

 skies over Venus were ever so clear. 



But we have just seen that owing to the position of the 

 planet we can never observe its full face ; besides, there is 

 nothing to show that its atmosphere is free from clouds. On 

 the contrary, spectral analysis has shown water vapour to be 

 present in it ; indeed, its close proximity to the sun cannot 

 but be favourable to great evaporation. Besides, this 

 planet, when examined through the telescope, always 



Midy 



•luoidsj 

 Fio. 1. — Cassini's Original Sketclies of Venus, KiSfi and IGfiT. 



displays a dazzling brightness in perfect accord with the 

 presence of a layer of clouds perpetually present in the 

 higher regions of the planet's atmosphere, and which 

 reflects back to us the greater part of the sunlight. 



In view of these several facts, it would seem to be 

 extremely difficult to detect the actual surface of Venus 

 from the earth. Observation, moreo\er, points to the 

 same conclusion. 



No astronomer has ever yet tried to draw a telescopic 

 picture of Venus without noticing the difficulty, peculiar 

 to this planet, of being quite certain of any of its aspects. 



• See B'dlelin de la SociefJ Astronomiqne de France for 1897, 

 pp. 134 to 137 ; also Astronomie Populaire, Les Terres du Ciel, etc. 



