The altitude of the sun can be computed from astronomical triangle (Figure 26.3) using the relation

h = sin^-1 (sin f sin d + cos f cos d cos LHA) --------------------- Equation (27.1)

The altitude of the sun can also be obtained from the observation of the sun by taking the vertical angle measurement along with horizontal angle. In this case, the observations need to be updated with the following corrections:

(a) Correction for refraction: Mean value of refraction (r₀) based on observed altitude is available from star almanac. A table for modification factor (f), depending on the temperature and pressure during observation, is also available in star almanac. Thus, Correction for refraction from star almanac is C_r = r_o f. Other wise, a can be computed using C_r = -58^" cos a¢. It is always subtractive from the observed altitude.

(b) Correction for parallax: A correction strictly needs to be applied for the Sun's parallax, which will suffice to be considered as 0.1¢ for all altitudes less than 70°. Otherwise, it may be computed using

C_p = +8.8² cos a¢

The correction for parallax is to be added always to the observed altitude.

(c) Correction for semi-diameter

a = a¢ ±