"The declination of the Sun (δ) is the angle between the rays of the sun and the plane of the earth equator. Since the angle between the earth axis and the plane of the earth orbit is nearly constant, δ varies with the seasons and its period is one year, that is the time needed by the earth to complete its revolution around the sun. When the projection of the earth axis on the plane of the earth orbit is on the same line linking the earth and the sun, the angle between the rays of the sun and the plane of the earth equator is maximum and its value is 23°27'. This happens at the solstices. Therefore δ = +23°27' at the northern hemisphere summer solstice and δ = -23°27' at the northern hemisphere winter solstice. When the projection of the earth axis on the plane of the earth orbit is perpendicular to line linking the earth and the sun, the angle between the rays of the sun and the plane of the earth equator is null. This happens at the equinoxes. Therefore δ is 0° at the equinoxes. Since the eccentricity of the earth orbit is quite low, it can be approximated to a circle, and δ is approximately given by the following expression: δ = -23.45° . cos[(360°/365).(N+10)] where cos operates on degrees; if cos operates on radians, 360° in the equation needs to be replaced with 2π and will still output δ in degree; N is Day of the Year, that is the number of days spent since January 1." Source and further information: http://en.wikipedia.org/wiki/Special:Search?search=%22declination+of+the+sun%22 Further information: Table of the Declination of the Sun http://www.wsanford.com/~wsanford/exo/sundials/DEC_Sun.html