Equation for computing Sunset … Sunset = 18 H +Equation of time + Chara +Refraction Correction. Chara, the difference between Right Ascension , VIshu Vat Vritteeya Sphuta and Oblique Ascension, Kranti Vritteeya Sphuta , can be calculated by the trig formula , Sin C = Tan Latitude * tan Angle of solar declination. A of S D can be got by using the formula, Sin J = SIn L sin w.