April 2, 1894.] 



KNOWLEDGE. 



87 



was of precisely that hue with which we are familiar in 



our terrestrial morning and evening skies, and prompts 

 me to ask the question whether it is a phenomenon often 

 observed in regard to sunrising on the moon, and whether 

 or not it may be held to be evidence of a lunar atmosphere 

 possessing a certain amount of absorptive power similar 

 to that of our own air. 



I may say that, not being familiar with such an 

 appearance, 1 at once attributed it to instrumental dis- 

 persion, and took considerable pains to ascertain whether 

 the colour could arise from such a source, but, after trying 

 several powers both upon Clavius and other parts of the 

 moon, I was unable to connect the colour with any such 

 cause : and whether the eye-piece was in proper focus or 

 not, the red colour still remained conspicuous in the 

 imperfect image so produced. 



The atmospheric conditions, I might add, on that 

 occasion were, from the unusual amount of detail and 



telescope would be more recognizable upon them than on 

 a bright object upon a bright background. Sunset and sun- 

 rise tints are certainly not generally recognizable upon the 

 moon's terminator, and it is doubtful whether there are 

 any half-tints, or region of crepusculum, such as would 

 probably be seen upon the earth in the regions of sunset 

 and dawn. The parts of the moon just beyond the parts 

 lit up by the first rays of the rising sun would be 

 illuminated by the light of the corona and chromosphere, 

 but I have not been able to trace any coronal or chromo- 

 spheric lines in the light from the dark region just 

 beyond the terminator when examined with a spectroscope. 

 M. Thollon and Mr. Lockyer have similarly searched for 

 evidence of coronal light at the lunar terminator, but have 

 failed to find it. It seems, therefore, improbable that the 

 ruddy tints observed by Mr. Maclver could have been due 

 to illumination by the red light of the chromosphere. If 

 the little craters shone with a red-hot glare we should 



expect to see them glowing 

 on the dark side of the 

 moon before the lunar sunrise, 

 and possibly retaining their 

 red colour after the sun had 

 risen. 



The only suggestion that 

 occurs to me is that Mr. 

 Maclver' s eyes may be very 

 sensitive to strong light, and 

 that he sees coloured images 

 for some time after looking 

 at a bright object. The 

 brighter parts of the moon 

 are very dazzling when looked 

 at in the telescope without a 

 dark wedge to subdue their 

 light, and they are relatively 

 very bright compared with 

 objects on the lunar termi- 

 nator. — A. C. Ranyard.] 



Clavius, from a dr,awing by Gr. K. Gilbert. 



steadiness perceptible, apparently better suited for observa- 

 tion of the moon than it has usually been my fortune to 

 find. I am, Sir, yours truly, 



Charles MacIver. 



[I am unable to suggest any optical illusion which 

 would account for Mr. Maclver's observation. He seems 

 to have taken ample precautions to test whether the 

 brilliant red colour was due to the eye-piece used. 



Sunrise upon Clavius is a magnificent spectacle, which 

 has been watched with wonder and delight by many 

 generations of astronomers ; Beer and Miidler, Webb and 

 Neison, all speak of it with enthusiasm. At first the sun 

 lights up the western wall of the great crater, which is 

 one hundred and forty-two miles in diameter. As the sun 

 rises it presents the appearance of a great bay of darkness 

 penetrating into the illuminated portion of the moon ; 

 then some small bright points appear in the bay — these 

 are the summits of the rings of craterlets on the floor of 

 the great crater ; as the sun continues to rise, the craterlets 

 appear as bright rings, like golden atolls in a sea of ink ; 

 at last the sunlight reaches the floor of the great 

 crater, and the long shadow of the western wall of the 

 crater slowly recedes tiU the whole of the crater floor is lit up. 



Mr. Maclver's observation seems to have been made at 

 the time when the sun's hght had reached the ridges of the 

 inner craters ; they then appear as intensely brilliant rings 

 on a dark background, and any chromatic defect of the 



To the Editor of Knowledge, 



Sir, — Mr. Lynn on page 7 calls 1899 the " last year of 

 the present century." I should call it the last but one. 

 The comet due therein, he says, has "only hitherto been 

 seen in the winter of 1865." But I submit it was seen in 

 the summer of 1.366, and recorded so that its perihehon 

 has been reckoned the 21st of October, or four hundred 

 and ninety-nine years seventy-two days before the last 

 perihelion. This divided by fifteen gives 33-281, instead 

 of the 33-18 of Oppolzer's reckoning from 1866 only. 

 Apart from all perturbations by Jupiter and Saturn, we 

 might therefore expect it early in May, 1899. That is 

 the least likely time of year for it to be seen, we being on 

 the opposite side of the sun. The eflects of Jupiter and 

 Saturn seem eminently worth computing in this case. 



Yours truly, 



E. L. Gaebett. 



[Mr. Garbett is obviously right about the year 1899 not 

 being the last year of this century ; but it is a common 

 oversight, into which I inadvertently fell. No great rehance 

 can be placed on an orbit determined from observations 

 made in 1366, whilst Le Verrier's theory that the Leonid 

 meteors were introduced into our system by Uranus 

 cannot be proved, though it remains probable that their 

 introduction took place at a comparatively recent date. 

 Neither their period, nor that of the small comet which 

 moves in nearly the same orbit, is known very exactly. 



