sun time vs clock time it is due to i) the plane of the Equator is not the same as the plane of the Earth's orbit around the sun, but is offset from it by the angle of obliquity, and ii) the orbit of the Earth around the sun is an ellipse and not a circle, and the apparent motion of the sun is thus not exactly equal throughout the year. Positive values when a sundial appears faster compared to a clock showing local mean time; negative when slower sundials uk oarval wiki usno navy mil archive codeproject in c# wikipedia