228 



KNOWLEDGE 



[October, 1902. 



sticks of a fan from the point whore the sun has just gone 

 down. Tiu'v are fornioil m a very simihir manner to the 

 rays of light which we sometimes se<! iu raoisture-hiden 

 air, when the sun itself is hidden behind a dense eloud, 

 and which children are accustomed to speak of as " the 

 sun drinking." In the latter case, these bands of light 

 are due to the sun shining out from between irregularities 

 of the clouds and lighting up the laden air, which 

 shines in the path of the rays just as the particles of dust 

 do — " the motes iu a sunbeam " — when light is admitted 

 through a small ajierture into a darkened room. In the 

 case of the crei)useular rays, mountain ridges or banks 

 of cloud may serve to jiartially intercept from the upper 

 atmosphere the liglit of the sun which has set, throwing 

 their shadows in long lines upwards, whilst the sunset 

 glow shines out in the intervals between the clouds or in 

 the depressions between the mountain peaks, and is seen 

 by us as a radiating system of broad pink streamers. Here 

 in England it is rare that these streamers can be traced 

 very far towards the zenith ; they are usually lost as they 

 pass overhead, though sometimes their termination may 

 be seen as counter rays iu the east. Col. E. E. Markwick,* 

 however, records that he has seen the rays in South Africa 

 night after night extending from horizon to horizon ; the 

 radiation from the west and convergence again towards 

 the east being, of course, an effect of perspective. In the 

 case of the Xrakatoa sunsets. Prof. Eicco remarked on 

 several occasions these crepuscular rays as being con- 

 spicuous in the primary glow, but they were not seen in 

 the secondary, a further proof that the former was due to 

 direct, and the latter to reflected sunlight. 



If we look across water towards the setting sun, we see 

 a broad track of light extending from below the sun's 

 place towards us, due to the reflection of the sun from the 

 surface of the waves. It occasionally happens that an 

 analogous reflection is produced in the air from the under 

 surface of ice-films floating horizontally. In this case a 

 vertical shaft of light is produced, rising from the sun as a 

 base, which is known as a " Sun- Pillar," and of which a very 

 fine example was seen on March tJtli, 1902, over the greater 

 part of southern England. The difference between the 

 formation of such a sun-pillar and the Krakatoa afterglows 

 depends partly upon the greater elevation of the stratum 

 giving rise to the glows, and partly because of the smaller 

 size of the particles forming it. The ordinary atmospheric 

 particles are very small compared with the wave-length of 

 light, and hence scatter especially the shorter rays, those 

 producing the sensation of blue. The particles of the 

 Krakatoa dust were large in compai-ison with these, and 

 hence scattered rather the rays of long wave-length — the 

 red rays. The composite effect of the glows therefore 

 resulted from the interaction of these two different orders 

 of particles upon the rays passing through them. And 

 over and above the richer and more complicated sunset 

 colours that were thus produced, there was the reflection 

 of the sunset glow itself by a stratum of highly reflective 

 dust particles at a great elevation. 



" Thus we may probably conclude that the haze which 

 followed the eru])tion of Xrakatoa, and produced the 

 twilight glows, was composed mainly of very fine dust, 

 and that this dust at a great altitude reflected the light of 

 the setting or rising sun after diffraction through the 

 stratum and diffraction and absorjition by the lower atmo- 

 si)here, and secondarily again reflected this reflected light. "f 

 The heavens are the province of the astronomer, the 

 atmosphere of the meteorologist, and all the various 

 phenomena that have been referred to above belong to the 



* Knowledok, 1901, April, p. 88. 



t Hon. RoUo Russell : " The Eruption of Krakatoa," p. 19.5. 



atmosphere, and hence meteorology may be said to have 

 a claim upon them. But they are distinguished from 

 I)urely meteorological objects — such as the various orders 

 of clouds, rainbows, haloes, parhelia and the like — in that 

 they are connected directly with the earth's rotation, and 

 with its position relatively to the sun. They are phenomena 

 of the earth as a planet rather than of the earth considered 

 as a world complete in itself, and from this point of view 

 may be considered as belonging to astronomy. But they 

 are referred to here, not so much on this account as from 

 the illustration they afford of the value of the habit of 

 exact observation. " Science," it has been often said, " is 

 measurement " ; it certainly depends upon the record of 

 phenomena in numerical form. The difference in value is 

 immense between the most vivid and picturesque description 

 of a flight of a meteor and the half-dozen numliers which 

 give the time of its appearance, its brightness, and duration, 

 and the precise position of its path. It is onh' the latter 

 which are of permanent value, and it is only from the habit 

 of registering the obvious facts connected with a given 

 phenomenon that the faculty is developed of recognizing 

 other points needing numerical expression and record. 

 Thus in the case of the beautiful sun-pillar of March tJth, 

 1902, though many vivid descriptions were written of it, 

 so far as I know only one observer noted what should have 

 been apparent to all, namely that the pillar moved an 

 appreciable distance in azimuth, following the unseen sun 

 in its northward movement below the horizon. So in the 

 case of the afterglows it was only those observers who 

 had made specific observations of ordinary sunsets, and 

 who knew accurately their characteristic features, who 

 could state definitely and with precision that the sunsets 

 after Krakatoa were abnormal, and define wherein their 

 peculiarities consisted. Striking as they were, it was by 

 no means everyone who saw anything in them out of the 

 common, and even so experienced an artist as the late Mr. 

 John Brett, p.b.a.s., failed to recognise wherein they 

 differed from an ordinary sunset, or indeed that they 

 differed from it at all. Then when their abnormal 

 character was recognised, the careful timing of the sinking 

 below the horizon of the primary and secondary glows was 

 of the first importance as giving the means for computing 

 the height of the reflecting stratum. In the case of the 

 appearance of such glows on future occasions, or of the 

 crepuscular rays, or of a sun-pillar, it is very desirable 

 that not only a precise note of the times of the phenomenon 

 should be taken, but that also the angular extent of the 

 various rays should not be merely roughly estimated but 

 actually measured. A long, light, but stiff' rod carrying 

 a sliding cross-piece — a sort of tangent staff, in fact, not 

 unlike that used by Chaudresakhara — would be of much 

 service in determining the height and breadth of a sun- 

 jiillar, the angle which a crepuscular beam made with the 

 horizon, the height of tlie brightest part of an afterglow, 

 and so on. Such an instrument might, indeed, seem rough, 

 but its accuracy and precision would be found to be 

 beyond comparison greater than that of mere estimation. 



THE ECLIPSE OF THE MOON, OCTOBER 17 

 AND HOW TO PROJECT IT. 



By W. Shackleton, f.r.a.s. 



From few 2)roblenis in astronomy can such satisfaction 

 be derived as that of the projection of a lunar eclipse. 

 Moreover, it is so sim]>le that few need fear to attempt it, 

 and thereby be abk' to delineate the phase of an eclipse at 

 any moment, whilst it is instructive in that actual use is made 

 of "terms which are frequently no more than definitions. 



