THE SUN AND SOLAR PHENOMENA. 



51 



latitude at different seasons of the year ; but much depends upon the atmosphere being 

 free from clouds and fogs, or surcharged with them, as to its perceptible length in all 

 places and at all times. Astronomically speaking, in the latitude of Greenwich, there is 

 no night from the 22d of May to the 21st of July, but twilight from sunset to sunrise. 

 It reaches its minimum three weeks after the autumnal equinox, and three weeks before 

 the vernal equinox, having a duration of one hour and fifty minutes. At midwinter it is 

 longer by rather more than a quarter of an hour than at the former periods, though this 

 is not often perceptible, but rather the contrary, owing to the greater prevalence of clouds 

 and mists at the winter solstice, intercepting, absorbing, and reflecting away from the 

 earth the rays of light that otherwise would visit it. The duration is least at the equator 

 and greatest at the poles ; but at the former there are two twilights every twenty-four 

 hours, and at the latter only two in the year. At the north pole the sun is below the 

 horizon for six months. But from the autumnal equinox to the 12th of November, and 

 from the 29th of January to the vernal equinox, the solar depression is within eighteen 

 degrees. Consequently through the whole of these intervals there is twilight, which 

 reduces the extent of the absolute night to about two months and a half. If this region 

 is thus for a long interval deprived of the solar presence, it is compensated by a 

 lengthened possession of it at an opposite season of the year, for from the 20th of March 

 to the 23d of September, the sun is constantly above the horizon. The annexed view 

 exhibits the sun as seen at midnight at the North Cape of Europe, which is within 



Sun at Midnight at the North Cape. 



eighteen degrees of the pole an appearance novel in the extreme to the dwellers in 

 more southern latitudes. In the locality upon the earth's surface in which we are 



