First, due to the eccentricity of Earth's orbit, 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). The length of a solar day varies through the year, and the accumulated effect produces seasonal deviations of up to 16 minutes from the mean. The equivalent on Mars is termed Mars local true solar time (LTST). Apparent solar time can be crudely measured by a sundial. It is based on the apparent solar day, the interval between two successive returns of the Sun to the local meridian.
Apparent solar time or true solar time is based on the apparent motion of the actual Sun. The apparent sun is the true sun as seen by an observer on Earth. (The third kind of traditional time reckoning is sidereal time, which is based on the apparent motions of stars other than the Sun.) By the 1950s it had become clear that Earth's rotation rate was not constant, so astronomers developed ephemeris time, a time-scale based on the positions of solar system bodies in their orbits. The two kinds of solar time ( apparent solar time and mean solar time) are among the three kinds of time reckoning that were employed by astronomers until the 1950s. Currently, a mean solar day is about 86,400.002 SI seconds. This is "mean solar time", which is still not perfectly constant from one century to the next but is close enough for most purposes. completing the same number of pendulum swings in each hour – cannot follow the actual Sun instead it follows an imaginary "mean Sun" that moves along the celestial equator at a constant rate that matches the real Sun's average rate over the year. The effect of this is that a clock running at a constant rate – e.g. This change is quantified by the equation of time, and is due to the eccentricity of Earth's orbit (as in, Earth's orbit is not perfectly circular, meaning that the Earth–Sun distance varies throughout the year), and the fact that Earth's axis is not perpendicular to the plane of its orbit (the so-called obliquity of the ecliptic). The problem is that in September the Sun takes less time (as measured by an accurate clock) to make an apparent revolution than it does in December 24 "hours" of solar time can be 21 seconds less or 29 seconds more than 24 hours of clock time. When the Sun has covered exactly 15 degrees (1/24 of a circle, both angles being measured in a plane perpendicular to Earth's axis), local apparent time is 13:00 exactly after 15 more degrees it will be 14:00 exactly. About 24 hours later the shadow will again point north–south, the Sun seeming to have covered a 360-degree arc around Earth's axis. That instant is local apparent noon, or 12:00 local apparent time. At one moment during the day, the shadow will point exactly north or south (or disappear when and if the Sun moves directly overhead).
The Earth's orbit around the Sun, showing its eccentricityĪ tall pole vertically fixed in the ground casts a shadow on any sunny day.
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