December 3, 1891] 



NATURE 



117 



OUR ASTRONOMICAL COLUMN. 



Motion of Stars in the Line of Sight. — In a paper read 

 before the Royal Society in January 1890, Prof. Lockyer 

 described a new method of observing spectra of stars and 

 nebulas which did away with errors due to the collimator of 

 the spectroscope not being exactly in the optic axis of the 

 telescope owing to the flexure of the telescope tube. It 

 consisted in using a siderostat to reflect the light of the body 

 under observation to a vertical object-glass, whence it was 

 converged on the slit of a fixed spectroscope. By this means 

 perfect stability can be secured. This method has been utilized 

 by M. Deslandres, of Paris Observatory, for the photographic 

 determination of the displacements of lines in stellar spectra due 

 to motion in the line of sight {Comptes rendus, November 23). 

 Comparison spark spectra are taken above and below the 

 spectrum of the star, and the difference of position of the lines 

 common to the star and these spectra afterwards measured. 

 The elements used for comparison are iron, calcium, and 

 hydrogen, and the best results have been obtained with the first 

 of the three. The lines in a spectrum of Sirius, taken on 

 March 3, 1 89 1, in this manner, exhibited a displacement which 

 corresponded to a velocity of recession relative to the earth of 19 

 kilometres per second. But as the earth's motion towards Sirius 

 at the time of observation was 20*2 kilometres per second, the 

 approach of the star to the sun was 1 "2 kilometres per second. 

 The results indicate that considerable advantage is to be gained 

 by the use of the siderostat in the study of the radial motions 

 of stars. 



The Variation of Latitude. — Some determinations of 

 the latitude of Cambridge, U.S., made in 1884-85 exhibited a 

 progressive variation, from which, however, no inference was 

 drawn at the time. The stars observed were contained between 

 - 5" and +5° of declination, but a subsequent discussion based 

 on more northerly stars ( + 5° to + 50°) gave an exactly 

 corresponding variation in latitude. Mr. S. C. Chandler, in the 

 Astronomical Joiirnal, No. 248, gives the results of a recent 

 examination of his values, and from the curve connecting the 

 residuals finds the minimum latitude to have been on September 

 I, 1884, and the maximum latitude on May I, 1885, with a 

 range of about o"7. 



Photography of the Eclipsed Moon.— During the lunar 

 eclipse of November 15, M. Courty, of Bordeaux Observatory, 

 took four photographs of the moon after it had entered the 

 earth's shadow. The exposure given was about two minutes, 

 and the disk of the moon could be easily traced on the negatives, 

 and on some positives presented with a note by M. Rayet to the 

 Paris Academy on November 23. M. Janssen remarked that by 

 photographing the eclipsed moon and the full moon on the same 

 plate, and determining the times of exposure necessary to obtain 

 both images of equal density, a good idea of the relation of the 

 light intensity in the two cases may be obtained. 



PROPOSALS FOR A SCHEME OF CO-OPERA- 

 TIVE OBSERVATION OF THE SO-CALLED 

 LUMINOUS CLOUDS. 



CINCE 1885 curious cloud formations have been seen on 

 "^ sum.mer nights in both the northern and southern hemi- 

 spheres, in evident connection with those phenomena which 

 followed the great volcanic eruption at Krakatab. The intense 

 brightness of these formations, considering the position of the 

 sun, denoted that they were situated very far above the earth's 

 surface. Probably these clouds consisted of erupted particles 

 thrown to a very great height and there illuminated on summer 

 nights by the sun. 



These cloud-like formations, commonly called luminous 

 clouds, are extremely interesting, both on account of the extra- 

 ordinary height at which they have for years been moving above 

 the surface of the earth (more than 80 kilometres) and of the 

 movements themselves. A very important point about these 

 clouds is that they are — so far as we yet know — visible in each 

 hemisphere only in the summer. It is the more important 

 that these phenomena should be carefully and widely observed, 

 since it is believed that they are gradually breaking up, so that 

 probably in a very few years no distinct traces of them may 

 remain (see also O. Jesse on so-called luminous clouds, in the 

 journal Himmel und Erde, vol. i. p. 263). 



Photographic results of the researches of O. Jesse are given in 



Part xl. of the Transactions of the Berlin Academy of Science 

 for 1890, and Part xxvi. for 1891. It is very desirable that such 

 photographs should be taken in as many different localities as 

 possible, because from them we get the surest basis for con- 

 sideration of the situation and movements of the clouds. But 

 valuable aid may be given by the co-operation of numerous 

 observers in various regions of the earth without the aid of any 

 apparatus. 



The principal points upon which stress is to be laid in this 

 inquiry are : — 



(i) By what method can the so-called luminous clouds be 

 most surely distinguished from others, especially from the 

 ordinary cirrus cloud ? 



Clouds or cloud-like formations which after sunset and before 

 sunrise stand out brightly from the dark ground of the heavens, 

 no earthly or unearthly sources of light being present on the 

 horizon, can only produce this effect by means of their own light 

 or else by light which they receive directly or indirectly from the 

 sun or moon below the horizon. 



Cloud-like formations which shine at night by their own light 

 have doubtless been formerly observed above the surface of the 

 earth. To these formations belong not only thunder and 

 lightning clouds, but also some polar light and meteoric 

 phenomena. 



But the so-called luminous clouds do not belong to the 

 various species of self-luminous clouds, for finer measurements 

 of their light are wanting, besides which the fact that they are 

 only seen within the zone of twilight proves that the sun below 

 the horizon is the principal source of their light. 



It is well known that there are clouds within this twilight 

 zone which resemble high mountain peaks, and which in the 

 first stages of twilight shine in the light of the sun, though the 

 latter is below the horizon of the observer. It is easy to deter- 

 mine the relation between the position of the sun below the 

 horizon, and the height of those layers of atmosphere which 

 receive the sun's light and reflect it. 



But the laws which govern the whole course of twilight are 

 modified when the distribution of the sunlight-reflecting particles 

 in the atmosphere is altered to any great extent. If, for instance, 

 numerous minute atoms produced by volcanic eruption or by the 

 breaking up of meteoric bodies find their way into those heights 

 above the earth's surface in which usually the gaseous elements 

 of the atmosphere are present in a very scattered form, it may 

 happen that such a layer, which reflects the sunlight very 

 strongly, may curiously alter the course of the twilight. 



So long after sunset as the masses of air beneath such a layer 

 receive direct light from the sun and reflect it, the observer will 

 not distinguish any deviation from the usual course of twilight. 

 But as soon as the further sinking of the setting sun gradually 

 deprives the lower layers of air of the direct light, the higher 

 layer of dust still receiving light from the sun stands out in 

 astonishing brightness, the particles of dust having strong 

 reflecting power, thus giving to the close of twilight the curious 

 effect of the sudden appearance of shining clouds on the broad 

 surface of the heavens. 



The phenomena of the luminous clouds corresponded when 

 first perceived to the above description. At present they are 

 no longer so strong or so extensive, but only form thin whitish- 

 blue shining veils, similar in form to the so-called cirrus or 

 feather clouds, occupying but a comparatively small part of the 

 I floor of the heavens inside the twilight segment, and in our zone 

 mostly near the horizon. Probably, the layers are now so 

 thin that very near and exactly above us they can no longer be 

 seen. 



From the above considerations it is clear in what way these 

 clouds diffier from those situated nearer to us, and especially 

 from the cirrus clouds floating scarcely more than 13 kilometres 

 above the earth's surface. All these lower clouds appear in the 

 later twilight grey and shadowy on a light ground, because the 

 layers of atmosphere above them are the chief source of the 

 remaining twilight. The luminous clouds differ too in shape 

 and structure from the other kinds of clouds. 



We must guard, however, against the error of mistaking 

 cirrus for luminous clouds, when, in exceptional cases, the 

 former look very bright, in consequence of receiving light either 

 directly or indirectly from the moon or other sources. In this 

 case, the question is decided by the relatively high degree of 

 stability in position and form of the very high and distant 

 luminous clouds, as ordinary clouds lie lower and nearer, and 

 i show much more rapid changes of position. 



NO. II53, VOL. 45] 



