Ixxvi INTRODUCTION. 



TABLE 94. 



Table 94. Duration of sunshine at different latitudes for different values of 

 the sun's declination. 



Let Z be the zenith, and NH the hori- 

 zon of a place in the northern hemi- 

 sphere. 

 P the pole; 



QEQ' the celestial equator; 

 RR' the parallel described by the sun on 



any given day; 

 5" the position of the sun when its upper 



limb appears on the horizon; 

 PN the latitude of the place, 0. 

 ST the sun's declination, 5. 

 PS the sun's polar distance, 90° — 8. 

 ZS the sun's zenith distance, z. 

 ZPS the hour angle of the sun from meridian, /. 



r the mean horizontal refraction = 34' approximately. 

 5 the mean solar semi-diameter = 16' " 



s = 90° + r + s = 90° 50' 

 In the spherical traingle ZPS, the hour angle ZPS may be computed 

 from the values of the three known sides by the formula 



sin * ZPS 



\/^ 



(ZS + PZ - PS) sin i {ZS + PS- PZ) 



sin PZ sin PS 



or 



sin ^ / = 



sm 



+ 5 - 0) sin Hs - 5 +<A) 



cos COS b 



The hour angle /, converted into mean solar time and multiplied by 2 

 is the duration of sunshine. 



Table 94 has been computed for this volume by Prof. Wm. Libbey, Jr. 

 It is a table of double entry with arguments 5 and 0. For north latitudes 

 northerly declination is considered positive and southerly declination as 

 negative. The table may be used for south latitudes by considering south- 

 erly declination as positive and northerly declination as negative. 



The top argument is the latitude, given for every 5° from 0° to 40°, for 

 every 2° from 40° to 60°, and for every degree from 60° to 80°. 



The side argument is the sun's declination for every 20' from 5' 23° 27' 

 to N 23° 27'. 



The duration of sunshine is given in hours and minutes. 



To find the duration of sunshine for a given day at a place whose lati- 

 tude is known, find the declination of the sun at mean noon for that day 

 in the Nautical Almanac, and enter the table with the latitude and declina- 

 tion as arguments. 



