Equation of time

difference of timeeccentricity of the Earth's orbitEquation of Time Tablesherenonuniform rate of changeseasonal differences and anomalies
The equation of time describes the discrepancy between two kinds of solar time.wikipedia
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Sundial

sundialssun dialsun clock
Apparent solar time can be obtained by measurement of the current position (hour angle) of the Sun, as indicated (with limited accuracy) by a sundial.
The amount of correction is described by the equation of time.

Analemma

earliest and latest sunrise and sunsetanalemaAnalemma calendar
The equation of time is the east or west component of the analemma, a curve representing the angular offset of the Sun from its mean position on the celestial sphere as viewed from Earth.
The east–west component results from the nonuniform rate of change of the Sun's right ascension, governed by combined effects of Earth's axial tilt and orbital eccentricity.

Ecliptic

ecliptical orbitsecliptic planeplane of the ecliptic
This is a consequence of the tilt of the Earth's rotational axis with respect to the plane of its orbit, or equivalently, the tilt of the ecliptic (the path the Sun appears to take in the celestial sphere) with respect to the celestial equator.
The variation of orbital speed accounts for part of the equation of time.

Equation clock

equation
Some clocks, called equation clocks, included an internal mechanism to perform this "correction".
An equation clock is a mechanical clock which includes a mechanism that simulates the equation of time, so that the user can read or calculate solar time, as would be shown by a sundial.

Longitude

WestlongitudinalE
These calculated the mean time, albeit local to a point of longitude.
The details are more complex than described here: see the articles on Universal Time and on the equation of time for more details.

Universal joint

Cardan jointuniversal jointscardan
Robert Hooke (1635–1703), who mathematically analyzed the universal joint, was the first to note that the geometry and mathematical description of the (non-secular) equation of time and the universal joint were identical, and proposed the use of a universal joint in the construction of a "mechanical sundial".
In fact, the component of the equation of time which accounts for the tilt of the equatorial plane relative to the ecliptic is entirely analogous to the mathematical description of the universal joint.

Time zone

Sri Lanka Standard Time Zonetime zoneslocal time
Civil time is the local mean time for a meridian that often passes near the center of the time zone, and may possibly be further altered by daylight saving time.
Apparent and mean solar time can differ by up to around 15 minutes (as described by the equation of time) because of the elliptical shape of the Earth's orbit around the Sun (eccentricity) and the tilt of the Earth's axis (obliquity).

Orbital eccentricity

eccentricityeccentriceccentricities
The equation of time is constant only for a planet with zero axial tilt and zero orbital eccentricity.

Solar time

mean solar timesolar dayapparent solar time
The equation of time describes the discrepancy between two kinds of solar time.
As explained in the equation of time article, this is due to the eccentricity of the Earth's orbit (i.e. the Earth's orbit is not perfectly circular, meaning that the Earth-Sun distance varies throughout the year), and the fact that the Earth's axis is not perpendicular to the plane of its orbit (the so-called obliquity of the ecliptic).

Equinox

autumnal equinoxautumn equinoxequinoxes
It is a minimum at the equinoxes, when the Sun's apparent motion is more sloped and yields more change in declination, leaving less for the component in right ascension, which is the only component that affects the duration of the solar day.

Heliostat

siderostatheliostatsSiderostats
Machines such as solar trackers and heliostats have to move in ways that are influenced by the equation of time.
The setting of the drive clock can also be occasionally adjusted to compensate for changes in the Equation of Time.

Equation of the center

Equation of the center – Analytical expansionsequation of the centre
is called the equation of the center; the expression written here is a second-order approximation in
It was specified and used by Kepler, as that variable quantity determined by calculation which must be added or subtracted from the mean motion to obtain the true motion. In astronomy, the term equation of time has a similar meaning.

Diurnal motion

diurnalapparent motionappears to move
The two times that differ are the apparent solar time, which directly tracks the diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun with uniform motion.

Sun

solarSolThe Sun
The two times that differ are the apparent solar time, which directly tracks the diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun with uniform motion.

Mean

mean valueaveragepopulation mean
The two times that differ are the apparent solar time, which directly tracks the diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun with uniform motion.

Hour angle

Greenwich hour anglelocal hour angleSidereal hour angle
Apparent solar time can be obtained by measurement of the current position (hour angle) of the Sun, as indicated (with limited accuracy) by a sundial.

Celestial sphere

celestialcelestial hemispherehemisphere
The equation of time is the east or west component of the analemma, a curve representing the angular offset of the Sun from its mean position on the celestial sphere as viewed from Earth. This is a consequence of the tilt of the Earth's rotational axis with respect to the plane of its orbit, or equivalently, the tilt of the ecliptic (the path the Sun appears to take in the celestial sphere) with respect to the celestial equator.

Observatory

astronomical observatoryobservatoriesastronomical observatories
The equation of time values for each day of the year, compiled by astronomical observatories, were widely listed in almanacs and ephemerides.

Almanac

almanacsalmanachparapegma
The equation of time values for each day of the year, compiled by astronomical observatories, were widely listed in almanacs and ephemerides.

Ephemeris

ephemeridesAstronomical Ephemerisastronomical table
The equation of time values for each day of the year, compiled by astronomical observatories, were widely listed in almanacs and ephemerides.

Minute

minmmins.
Apparent time, and the sundial, can be ahead (fast) by as much as 16 min 33 s (around 3 November), or behind (slow) by as much as 14 min 6 s (around 11 February).

Second

ssecmegasecond
Apparent time, and the sundial, can be ahead (fast) by as much as 16 min 33 s (around 3 November), or behind (slow) by as much as 14 min 6 s (around 11 February).

Tropical year

solar yearmean tropical yearsolar years
Ignoring very slow changes in the Earth's orbit and rotation, these events are repeated at the same times every tropical year.

Constant function

constantconstant mapconstant mapping
The equation of time is constant only for a planet with zero axial tilt and zero orbital eccentricity.

Mars

MartianCoordinatesplanet Mars
On Mars the difference between sundial time and clock time can be as much as 50 minutes, due to the considerably greater eccentricity of its orbit.