METEOROLOGY. 



353 



At the pole the duration of twilight is easily found by noting in 

 the ephemeris the time at which the sun's declination south is equal 

 to the depression of the crepusculum circle below the horizon ; this 

 instant and the equinox being its limits of duration. As before indi- 

 cated, the limit of refractional light is when the sun is 34' below the 

 horizon ; civil twilight when it is 7^° ; and common or astronomical 

 twilight when it is 17°. Thus we shall find — 



Annual Duration. 



From this table it appears that the annual length of darkness 

 diminishes from the equator to the pole, while the duration of twi- 

 light increases from about one month on the equator to three months 

 at the Pole. In this latitude about thirty-eight hours of daylight, 

 at the sun's rising and setting, are annually due solely to atmospheric 

 refraction. The second, fifth, and sixth columns represent ilQo'^ Q^. 



In further illustration of this subject, the duration from noon to 

 midnight, or from midnight to noon, of sunlight, astronomic twi- 

 light, and darkness are exhibited to the eye in the accompanying 

 plate for every day in the year on different latitudes. On the equator 

 it will be seen that twilight has its least value and is almost uniform 

 through the year. In the latitude of 40°, the limiting curves of twi- 

 light bend upward in an arch-like form. The upper curve at the 

 same time recedes from the lower, and encroaches upon the duration 

 of darkness, till, as shown for latitude G0°, twilight lasts through the 

 whole night in summer. If the first and last extremities of the 

 curves at January and December be united to complete the circuit of 

 a j'^ar, darkness there will be represented by an elliptic segment, the 

 longest nights and shortest days being at mid-winter. In approach- 

 ing the highest latitudes, the lines which form the limits continually 

 change their inclination, till, at the pole, they become perpendicular 

 to their position at the equator. 



The present section contains formul?e and tables for determining 

 both the diurnal and the yearly limits of twilight for A, D, 1853, 

 computed for 34', 7° 30', and 17°, depressions of the crepusculum 

 circle below the horizon, the reasons for which have before been stated. 

 Although these phenomena are varied by mists and clouds, and by 

 the atmospheric temperature and density, still the assumption of mean 

 depressions has been necessary in order to obtain a general view of 

 their laws of continuance. The duration of moonlight, which is un- 

 attended by sensible heat, has not been discussed. From this source 

 the reign of night is still further diminished, till, in this latitude, the 



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