Definitions for "Equation of Time"
(ET) This is the correction, in minutes and seconds, to be applied to local time apparent time (sundial time) for deriving Local Mean Time (LMT), or Local Solar Time (LST).
The annual East-West swing of the location of the Sun which can be detected by noting the position of the Sun at the same time (such as noon) each day. This motion is caused by the libration (wobble) of the Earth and can be estimated by (Spencer, J. W. (1971). Fourier series representation of the position of the Sun. Search 2 (5), 172 ) :   ET = 229.18 * ( 0.000075 + 0.001868 cos D - 0.032077 sin D -0.014615 cos 2D - 0.040849 sin 2D ) where   D = nD ( 360° / 365 ) and nD is the number of the day (e.g., Feb. 1 makes nD = 32). NREL uses solar position algorithms that do not require the equation of time (Michalsky, J. J. (1988). The Astronomical Almanac's algorithm for approximate solar position (1950-2050). Solar Energy 40 (3), 227-235 ) .
The difference between mean solar time and true solar time when located on the reference longitude of the time zone considered.