October 20, 1892] 



NA TV RE 



189 



and many makers' plates would be found much more satis- 

 factory. 



I shall esteem it a favour if you will allow this letter to appear 

 in your journal. Enclosing my card, I subscribe myself, 



October 17. Prevkntion. 



INVITATION TO OBSERVE THE LUMINOUS 

 NIGHT CLOUDS.^ 



SINCE the year 1885 a very remarkable phenomenon 

 has been noticed in the sky in our latitudes, which 

 well deserves to exciie the interest of astronomers and 

 geophysicists. The following is the substance of what 

 has so far become known regarding the so-called lumi- 

 nous night-clouds. 



In the latitude of Berlin the phenomenon shows itself 

 only during a comparatively short period of the year — 

 from May 23 to August 11. While in the first years it 

 was seen pretty frequently even before midnight, it has, 

 during the last four ) ears, rarely appeared except after 

 midnight. The phenomenon appears in the form of 

 cirrus-clouds, which come out bright on the twilight sky. 

 This especially distinguishes them from the ordinary 

 cirrus-clouds, which, with the depths of the sun in which 

 the luminous clouds are seen at present, come out dark 

 on the light twilight sky. The colour of the pheno- 

 menon is generally a bluish white, which becomes yel- 

 lowish and reddish in close proximity to the horizon. 



Often repeated photographs which have been taken 

 simultaneously at various points in the neighbourhood of 

 Berlin, show that the altitude of the luminous clouds is 

 constant and exceedingly great — 82 kilometres. In con- 

 sequence of this great altitude, they receive light from 

 the sun standing below the horizon, which makes them 

 appear light on the twilight-sky. They are visible only 

 so long as the sun shines on them ; as soon as the sha- 

 dow of the earth passes over them they become invisible. 

 As a rule they begin in the morning, shortly before twi- 

 light, and they disappear as soon as the sun stands 

 higher than 8° to 10" below the horizon. 



Of late years these clouds have been seldom seen. 

 Within the period above stated, they occurred this year 

 only about ten times, while in the first years they were 

 very frequent. Their appearance is subject to great 

 changes ; while they frequently exist only in a few little 

 luminous stripes or patches, at times they appear in 

 greater accumulations and with a more intense light. 

 Especially in the last days of the period, from August 2 

 until 6, their light seems to be considerable in our lati- 

 tudes. Generally they are observed in the proximity of 

 the horizon — over that part of it under which the sun is. 



Frequent observations of the movements of the pheno- 

 mena, which, after midnight, are always from the direc- 

 tion of N.E. ± 40°, render it probable that the movements 

 are caused principally through the resisting medium of 

 the mundane space. In accordance with this is the fact 

 that in the half-year after its appearance in this country, 

 the phenomenon has been observed repeatedly in the 

 southern latitudes of 53" by the meteorological observer, 

 Mr, Stubenrauch, in Punta Arenas, as well as several 

 times by ship-captains. 



Other observations confirm the assumption of an 

 annual wandering of this kind. For instance, in Graham's- 

 town under 33° S. lat. the phenomenon was observed on 

 October 27, 1890,-' and in Haverford under 40"" N. lat. ; 

 according to written information it was observed on 

 May 17, 1892. These dates, taken in association with the 

 time of the appearance in this country, directly indicate 

 a wandering of the phenomenon from N. to S. and 

 back. 



The luminous night-clouds decrease year after year 

 in respect to the frequency of their appearance as well as 



' Scientific journals are requested to reproduce this article. 

 = Compare /^str. Nachr., No. 3008. 



to their extent and to their intensity of light. The phe- 

 nomenon therefore will entirely disappear within a few 

 years. It seems, however, that during the next two years 

 observations will still be possible, which may give us 

 information regarding several questions of extraordinary 

 importance. 



Measurements of the apparent altitude of the upper 

 limits of the luminous clouds, mainly in the time in 

 which the upper limit of the twilight-segment has the 

 comparatively small altitude of, say, 1° to 10'', would be 

 of great value. Such measurements will serve to decide 

 the question whether the altitude of the clouds varies 

 under different geographical latitudes, providmg that the 

 measurements always refer to such points as lie within 

 the upper limits of the clouds, produced by the shadow 

 of the earth. 



During the last few years the whole twilight-segment 

 has been comparatively seldom filled out by the luminous 

 night-clouds, and it may therefore frequently remain 

 doubtful whether the highest point of the phenomenon 

 really lies in the limit of the earth- shadow. In order to 

 make sure that the measurements are adapted'^ to 

 their purpose, they must be repeated as often as possible 

 in intervals of a few minutes. In the evening this limit 

 is generally recognized by the fact that within it parts of 

 the phenomenon disappear from above, while towards 

 morning new parts always become visible within the 

 limit upwards. The distance of the zenith of the upper 

 limit of the luminous clouds in the vertical of the sun for 

 the latitude of Berlin, presuming that the phenomenon 

 stretches over the whole of the twilight-segment, may be 

 seen from the following statement : — 



Zenith distance 

 of the upper- 

 most limit. 



80 



83 



Depth of the 

 sun below 

 the horizon. 



12 O 

 I2-S 

 130 

 13-5 



I4'u 



Moreover, as by means of a telescope the upper limit 

 of the phenomenon is generally seen a little higher than 

 with the naked eye, it is desirable that the telescope 

 should always be adjusted to the limit-line seen with the 

 naked eye. A comparison of the appearance seen with 

 the naked eye with the one seen through the telescope, 

 will enable the observer to discover easily the line 

 corresponding to the one seen with the naked eye. The 

 exactitude of these measurements must be about 3' to 6', 

 with respect to the azimuth and to the altitude, while the 

 time should be exact within two to four seconds. 



The employment of photographic apparatus is of 

 advantage for the indication of the place, as well as of 

 the movements, of the phenomenon. But only those 

 kinds of apparatus are suitable in which the proportion of 

 the diameter of the opening to the focal distance is at 

 least I : 4 or greater. If the proportion is smaller, the 

 duration of lighting will last too long, and conse- 

 quently, on account of the quick changings of the 

 phenomenon, the details will get lost. With an 

 apparatus of which the proportion of the aperture- 

 diameter to the focal distance is i : 3, the duration of 

 lighting for the various depths of the sun below the 

 horizon, on condition that the phenomenon is light 

 in some degree, is as follows : — 



Depth of the Sun below 

 the Horizon. 



Duration of 

 Lighting. 



s. 



16 



21 

 . 27 



• 35 

 . 48 



• 72 

 . 122 



NO. 1 199, VOL. 46] 



