Solar time is time kept or measured by the sun; and its basic division, the day, has been recognized and used since the dawn of history. The immediately visible sign of the passage of time by the sun, and the basis of its measurement, is the sun's apparent motion along the daily course that it appears to trace out in the sky from east to west. This apparent motion has been known for several centuries to be due to the daily rotation of the earth around its polar axis.
Two kinds of solar time, apparent solar time and mean solar time, are among the three kinds of time that were recognized and measured by astronomers up to the 1950s (the third traditional kind of time being sidereal time, time according to the apparent rotation of the stars). The measures of all these three kinds of time depend on the rotation of the earth. Nowadays both kinds of solar time, along with sidereal time, stand in contrast to newer kinds of time measurement, introduced from the 1950s onwards (starting with ephemeris time), which were designed to be independent of earth rotation.
Solar times can be measured in several ways, for example by the apparent position of the Sun on the celestial sphere. Such positions are not actually the physical time, but rather hour angles, that is, angles expressed in time units. They are also measures of local time in the sense that they depend on the longitude of the observer.
Apparent solar time
Apparent solar time or true solar time is given by the daily apparent motion of the true, or observed, Sun. It is based on the apparent solar day, which is the interval between two successive returns of the Sun to the local meridian. Solar time can also be measured (to a limited precision) by a sundial.
The length of a solar day varies throughout the year, and the accumulated effect of these variations (often known as the equation of time) produces seasonal deviations of up to 16 minutes from the mean. The effect has two main contributory causes. First, Earth's orbit is an ellipse, not a circle, so the Earth moves faster when it is nearest the Sun (perihelion) and slower when it is farthest from the Sun (aphelion) (see Kepler's laws of planetary motion). Second, due to Earth's axial tilt (often known as the obliquity of the ecliptic), the Sun moves along a great circle (the ecliptic) that is tilted to Earth's celestial equator. When the Sun crosses the equator at both equinoxes, the Sun is moving at an angle to the equator, so the projection of this tilted motion onto the equator is slower than its mean motion; when the Sun is farthest from the equator at both solstices, the Sun moves parallel to the equator, so the projection of this parallel motion onto the equator is faster than its mean motion (see tropical year). Consequently, apparent solar days are shorter in March (26–27) and September (12–13) than they are in June (18–19) or December (20–21). These dates are shifted from those of the equinoxes and solstices by the fast/slow Sun at Earth's perihelion/aphelion. (In addition to these two main effects there are others, due to lunar and planetary perturbations, which can produce a few more seconds in the equation of time.)
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