(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.

the hour angle of the true Sun minus the hour angle of the fictitious mean sun; alternatively, apparent solar time minus mean solar time.

the time difference between Local Apparent Time (apparent solar time) and mean solar time at the same location. Its value varies between extremes of about +14 minutes in February and –16 minutes in October. It arises because of the elliptical orbit of the Earth, and the tilt of the Earth's axis to the ecliptic. The preferred usage by diallists is: mean solar time = apparent solar time + EoT but this sign convention is by no means universal and the opposite sign is used in modern almanacs. Irrespective of the sign convention adopted, sundials will always appear slow compared to mean time in February, and fast in October/November.

time adjustment applied to local and mean solar time to account for certain irregularities in the daily rotation of the earth about its axis over the course of the year. [--] Zeitgleichung

The correction which must be applied to solar time in order to obtain mean solar time. See the tutorial on Time.

The difference in arc, translated into time, between the true sun and the fictitious mean sun.

an astronomical term accounting for changes in the time of solar noon for a given location over the course of a year. Earth's elliptical orbit and Kepler's law of equal areas in equal times are the culprits behind this phenomenon. Click here to see a plot of the equation of time vs. day of the year. For more information on this phenomenon, see this offsite Analemma page.

The difference between true solar time (determined by the Sun's position in the sky) and mean solar time (the time told by your watch). The two times can vary by as much as 16 minutes over the course of a year.

The difference in time between when the sun crosses a north-south line thru the zenith and twelve noon on a clock.

The amount of time used to compensate for difference between true solar time to the mean, or civil, solar time at any given time.

Indication of the difference, expressed in minutes, between conventional mean time and real solar time. This difference varies from -16 to +16 seconds between one day and the other.

The difference between true and mean time at any given moment. This difference arises due to Sun not always crossing the Meridian at 12 o' clock. The time by watch is regulated by mean solar time, which is constant in length, and is equal to the annual mean of the true solar days.

a relation that describes the difference in time between the meridian crossings of the mean Sun and the actual Sun.

Calculations that allow the translation between solar time and mean time.

The equation of time is the difference, over the course of a year, between time as read from a sundial and a clock. The sundial can be ahead (fast) by as much as 16 min 33 s (around November 3) or fall behind by as much as 14 min 6 s (around February 12). It results from an apparent irregular movement of the Sun caused by a combination of the obliquity of the Earth's rotation axis and the eccentricity of its orbit.