56 



INTENSITY OF SUN'S HEAT AND LIGHT. 



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 indicated, the limit of refractional light is when the sun is 

 34' below the horizon, or m = 34'; civil twilight, when m = lh° ; and common or 

 astronomical twilight when m = 17°. Thus we shall find, 







Annual Duration. 









Sunlight. 

 2(217). 



Refractional 

 Light. 



Civil 

 Twilight. 



Astronomic 

 Twilight. 



Darkness. 

 i(2#). 



North Pole. 

 Lat. 40°. 

 Equator. 



186d. llh. 



183d. 8h. 

 182d. 15h. 



2d. 22h. 

 Id. 14h. 

 Id. 5h. 



38d. 15h. 

 21d. 6h. 

 15d. 21h. 



94d. 16h. 

 49d. 2h. 

 36d. Ih. 



84d. 3h. 

 132d. 20h. 

 146d. 14h. 



From this table, it appears that the annual length of darkness diminishes from 

 the equator to the pole; while the duration of twilight 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 to atmo- 

 spheric refraction. The second, fifth, and sixth columns correspond to the formula 

 2 (2 H ) + 2 (2 r) + 2 (2 N) = 365 d 6\ 



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

 midnight to noon, of Sunlight, Astronomic Twilight, and Darkness are exhibited 

 to the eye in the accompanying Plate V, 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 

 twilight 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 60°, 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 year, darkness there, will be represented by an elliptic segment ; 

 the longest nights and shortest clays being at mid-winter. In approaching the 

 highest latitudes, the lines which form the limits continually change their inclina- 

 tion, till at the Pole, they become perpendicular to their position at the Equator. 



The present Section contains formulas and tables for determining both the diurnal 

 and the yearly limits of twilight, with tabular examples 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 laAvs of continuance. The duration of moonlight which is unattended 

 by sensible heat, has not been discussed. From this source, the reign of night is 

 still further diminished, till in this latitude, the remaining duration of total darkness 

 after twilight and moonlight, can scarcely exceed three months in the year. The 

 interval towards the close of astronomic or common twilight, corresponds to what 

 is commonly termed, in the country, "early candle-light," when the glimmering 



