June 12, 1665.] 



♦ KNOWJLEDG 



509 





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THE RUDDY ECLIPSED MOON. 



[1743] — As I cannot spare the time to discuss all the points 

 raised by ilr. Proctor on this subject, I will only explain niy own 

 theory. I am surprised to find that he ever imagined for a moment 

 that I supposed so monstrous an absurdity as a moon that is ruddy 

 " with primreval heat," especially as he knows that I have expressed 

 my scepticism concerning the existence of any sach ])rim;rval 

 heat, and the deductions concerning dying suns, cooling planets, 

 youthful, middle-aged, and senile worlds that are based upon it. 

 I even regard the nebular hypothesis, concerning which our astro- 

 nomers of to-day are so cock-sure, as a very pretty mathematical 

 dream, and nothing better. I do not deny it — such denial would be 

 as unphilosophical as affirming it. Mj' thesis is that human know- 

 ledge has fairly-defined limits ; bej-ond these is the region of specu- 

 lative folly — the blessed abode of transcendental mathematicians. 



If a red-hot orb were "launched into space," as the fine writers 

 have it, my conclusion is that, with a composition like that of the 

 earth, its stirface would cool down to a black heat in a week or less. 

 The red heat, which I regard as a necessary consequence of the 

 exposure of a tufaceons surf,ace to unchecked solar radiation, must 

 be of the most superficial character, and must be radiated away 

 just as rapidly as absorbed. 



As I explained in my second note on the subject (Gentleman' s 

 Magazine, Xovember, 1884), the visibility of such red heat by us 

 would depend upon the condition of our atmosphere. The twilight 

 glows of 1884 indicated just the condition for intercepting and 

 scattering the obscure red rays, and therefore the non-appearance 

 of the ruddy glow on October 4th of that year is just what my 

 theory demanded ; while, on the other hand, the theory which Mr. 

 Proctor defends demanded an unusual display of ruddiness at that 

 period which " the poor thing," as Mr. Proctor says, failed to 

 display. The same with the eclipse of March 30 (see Knowledge 

 of last week, page 4SG). Instead of this failure telling against my 

 explanation, and in favour of ruddy illumination by the solar rays 

 modified by passing through our atmosjihere, it affords a "crucial 

 instance " in confirmation of my theory and refutation of his. 



Prof. Langley's researches, as I read them, show reason for at- 

 tributing a still higher temperature to the full moon than was 

 previously justifiable. He has shown that our atmosphere cuts ofl' 

 more of the solar radiations than was formerly supposed, therefore 

 the difference in favour of an airless globe must be proportionally 

 greater. 



Jlr. Proctor makes much of the difference between the pencil 

 of solar rays and the axis of that pencil, assuming, at the same 

 time, that I do not understand this difference. I certainly do not 

 understand how such difference can have any practical bearing on 

 the subject, seeing that the diverging pencils of the solar rays have 

 become practically parallel pencils before reaching the earth, and 

 the difference between the axes of a parallel pencil and the pencil 

 itself is zero — such a pencil is its own axis. It is quite true that 

 my elementary optical stndies are rather ancient — more than forty 

 years old — and I do not know what treatment these pencils may 

 have received at Cambridge in the meantime ; but when I then 

 dealt with them in the construction of optical instruments, they 

 behaved as practically parallel by converging to the principal foci 

 of our lenses and mirrors. If I am wrong, perhaps Mr. Proctof 

 will set me right, by stating the amount of angular difference 

 between the course of the axis of a pencil of solar rays as received 



on the earth, and that of t lie rays themselves on all sides of it — 

 i.e. tho amount of half their divergence. 



1 used the term focus according to its old-fashioned definition — 

 viz., as " the point where tho refracted rays or thoir prolongations 

 meet," or " tho point at or near which tho niysof light are collected 

 by a lens or mirror," and regarded tho opaque eartli with its sur- 

 rounding atmosphere as a spherical spot-lens with a very lai'go spot, 

 and very low and v.iriable index of refniction. Sir John Ilerschel 

 does tho same, but ho used tho words " convex lona of gradually- 

 decreasing density." Such a lens has many foci, and illuminates 

 an object placed anywhere between their limits accordingly. This 

 is the case whether tho pencils of rays refracted by it are parallel, 

 converging, or, within certain limits, diverging. 



W. Mattieu Williams. 



[1744] — I see that Mr. Mattien Williams has suggested in your 

 columns a theoi'y with regard to tho variation from one eclipse to 

 another in tho amount of tho light and iu tho colour of tho light 

 which illuminates tho dark body of the moon when it is immersed 

 in the earth's shadow. It seems to me that tlie variation referred 

 to is not due, as is usually assumed, to the presonco or absence of 

 clouds in tho earth's atmosphere, but to differences of absorption 

 in a higher region than that in which clouds usu.ally float. We shall 

 probably be much over tho mark if we assume that tho height of tho 

 cloud-layer which could obstruct tho transmitted rays of the sun is 

 as much as two miles. At such a height tho atmospheric refraction 

 would very appreciably beud the sun's rays into tho earth's geomo- 

 trical shadow. But the observed shadow into which tho moon 

 plunges always appears larger than the geometrical sliatlow which 

 would be cast by a body as large as the earth not surrounded by 

 an atmosphere. Miidler estimated this increase of diameter as 

 l-54th (see tho " Astronomischo Nachrichten," Vol. .XV., p. 29.), 

 and the English "Nautical .Vlmanac" uses for tho purpose of pre- 

 dictions the fraction 1-GOth as representing tho increase in the 

 diameter of the observed shadow over the geometrical sh.adow. 



More than one factor has to be taken into consideration iu 

 accounting for this increase, but it is evident that the lowest trans- 

 mitted rays which con-espond to the edge of the shadow region into 

 which the moon plunges cannot be greatly refracted inwards by 

 the earth's atmosphere. 



It will be seen that refraction tends to decrease the area of the 

 shadow. Absorption of liglit in the earth's atmosphere tends to 

 dim down and tint with colour the inner zones of shadow and 

 penuuibra, and absorption of the solar light in the sun's atmo- 

 sphere tends to increase the area of the observed shadow ; for it 

 decreases the brightness of the sun's light as you proceed from the 

 centre of the disc towards the limb, so that the light derived from 

 a crescent of the sun's disc is less than the light which would be 

 given if the sun were uniformly bright. 



The edge of the shadow thrown upon tho moon is not perfectly 

 sharp, but the transition from considerable brightness to compara- 

 tive darkness does not occupy a minute of arc. It is evident that 

 this region of sudden transition, which lies outside the geometrical 

 shadow, must be illuminated by a solar crescent of sensible thick- 

 ness, and that the greater part of the light which illuminates it 

 must be transmitted through the earth's atmosphere at a 

 height where the density of the air causes but little refraction. 

 Celestial objects seen upon the horizon are raised upwards 

 by refraction more than half a degree, consequently solar 

 light which grazed the earth's surface at the sea-level woidd be 

 refracted more than ono degree within the geometrical shadow; 

 but the geometrical shadow thrown at the distance of the moon 

 is never two degrees in diameter — or to speak more correctly, the 

 section at the moon's distance of the geometrical shadow thrown 

 by the earth, never subtends two degrees as seen from the earth. 

 Consequently, the whole shadow at the moon's distance would be 

 illutninated if the light transmitted through the earth's atmo- 

 sphere at the sea-level gave a perceptible illumination. 



No doubt, the effects perceived during a lunar eclipse are greatly 

 modified by contrast of light, but what the eye perceives is a some- 

 what sudden transition from bright light to darkness, and within 

 the dark region on some occasions a nearly uniform reddish disc, 

 and on other occasions a nearly uniform dark or bluish disc. 



When the moon is centrally eclipsed, its limb is never much more 

 than half a degree from tho edge of the geometrical sliadow or the 

 region where the transition from light to darkness is so rapid, and 

 its central portions are only about three-quarters of a degree from 

 this region. Near to the centre of the moon at central eclipse the 

 whole limb of tho sun would be visible around the earth, but at 

 other places the chief part of the illumination must be derived 

 from light transmitted through the earth's atmosphere with the 

 minimum of deflection, it is therefore evident that the chief part 

 of the light which illuminates tho lunar surface must have suffered 

 a deflection of less than three-quarters of a degree in passing 



