Look at Precision of Common Formulae Computing Solar Declination Angle

Solar declination angle is one of the most important geometrical characteristics of earth-sun system. In this study, we applied 13 of recent years formulae to compute solar declination angle for 5 selective years of 1970, 1980, 1986, 1989, and 2001. Algorithms of Michalsky (1988) and Walraven (1978), Bourges (1985) formulae have been found as most precise ones respectively. Michalsky (1988) as most precise one of those three shows maximum difference of 0.003 degree on 3 rd March 1980. Well known Spencer and WMO (1984) formulae rank 5th and 6th. These two are not recommended for very precise computation of solar coordinates, comprehensive study of solar radiation and applied purposes of solar energy. Spencer formula that uses Fourier expansion shows an error of about 1 degree. Cooper (1969) and Pisimanis (1987) formulae show highest error and rank 12th and 13th.