5Q£ Tables 171—174 



DURATION OF DAYLIGHT, CIVIL TWILIGHT, AND ASTRONOMICAL 



TWILIGHT 



Daylight is defined as the interval between sunrise and sunset. The latter are con- 

 sidered to occur when the upper edge of the disk of the sun appears to be exactly on the 

 horizon with an unobstructed horizon and normal atmospheric refraction. It is assumed 

 that the upper edge of the sun appears on the horizon when the true center of the sun's 

 disk is SO' below the horizon, this corresponds to assuming a semidiameter of 16' and a 

 constant refraction of 34'. 



Civil twilight is defined as the interval between sunrise or sunset and the time when 

 the true position of the center of the sun is 6° below the horizon, at which time stars 

 and planets of the first magnitude are just visible and darkness forces the suspension 

 of normal outdoor activities. 



Astronomical twilight is defined as the interval between sunrise or sunset and the time 

 when the true position of the center of the sun is 18° below the horizon, at which time 

 stars of the sixth magnitude are visible near the zenith and generally there is no trace 

 on the horizon of the twilight glow. 



Tables 171-174 (including graphs) have been extracted from a publication of the 

 Nautical Almanac Office. 1 The data were computed for longitude 90° W. for the year 1966; 

 however, there will be no appreciable error in using these tables for other localities or 

 for other years during the remainder of the twentieth century in determining the duration 

 of daylight or twilight. 



For latitudes greater than 65° the data are given in graphical form. At these higher 

 latitudes the data become increasingly uncertain, small changes in atmospheric refrac- 

 tion can cause relatively large changes in the actual phenomena, as can small errors in 

 latitude, and the graphs give a clearer picture of the phenomena. Where the graphs are 

 difficult to read accurately the phenomenon itself is uncertain. These large uncertainties 

 are inevitable consequences of the physical circumstances and are not due to the inade- 

 quacy of the graphs. 



Tables 171-173 may be used for Southern Latitudes by entering the tables not with the 

 actual date but with a date about 6 months earlier or later as given in Table 174. 



For a historical summary of the various definitions of twilight and a description of 

 associated phenomena see Kimball. 1 



1 Tables of sunrise, sunset and twilight, Supplement to the American ephemeris, 1946, U. S. Naval 

 Observatory, Washington, 1945. 

 'Kimball, Herbert H., Month. Weath. Rev., vol. 44, pp. 614-620, 1916. 



SMITHSONIAN METEOROLOGICAL TABLES 



