Solar time is based on the idea that, when the sun reaches its highest point in the sky, it is noon.
The length of a solar day varies throughout the year for two reasons. First, the Earth's orbit is an ellipse, not a circle, so the Earth moves faster when it is nearest the Sun and slower when it is farthest from the Sun (see Kepler's laws of planetary motion). Second, due to Earth's axial tilt, the Sun does not move along Earth's celestial equator but usually moves at an angle to it during the year, thus it moves fast or slow depending on whether it is far from or close to the equator (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).
Mean solar time is artificial clock time adjusted via observations of the diurnal rotation of the fixed stars to agree with average apparent solar time. The length of a mean solar day is a constant 24 hours throughout the year even though the amount of daylight within it may vary. An apparent solar day may differ from a mean solar day (of 86,400 s) by as much as nearly 22 s shorter to nearly 29 s longer. Because many of these long or short days occur in succession, the difference builds up to as much as nearly 17 minutes early or a little over 14 minutes late. The difference between apparent solar time and mean solar time is called the equation of time.
Many methods have been used to simulate mean solar time throughout history. The earliest were clepsydras or water clocks, used for almost four millennia from as early as the mid second millennium BC until the early second millennium. Before the mid first millennium BC they were only adjusted to agree with the apparent solar day, thus were no better than the shadow cast by a gnomon (a vertical pole), except that they could be used at night.
Nevertheless, it has always been known that the Sun moves eastward relative to the fixed stars along the ecliptic. Thus since at least the mid first millennium BC the diurnal rotation of the fixed stars has been used to determine mean solar time, against which clocks were compared to determine their error rate. Babylonian astronomers knew of the equation of time and were correcting for it as well as the different rotation rate of stars, sidereal time, to obtain a mean solar time much more accurate than their water clocks. This ideal mean solar time has been used ever since then to describe the motions of the planets, Moon, and Sun.
Mechanical clocks did not achieve the accuracy of Earth's 'star clock' until the beginning of the twentieth century. Even though today's atomic clocks have a much more constant rate than the Earth, its star clock is still used to determine mean solar time. Nowadays, selected stars are photographed at appropriate atomic times as they transit the local zenith determined via telescopes using a pool of mercury as a mirror (photographic zenith tubes). The error between the photographed transit and the calculated transit is used to determine whether a leap second is needed to keep Coordinated Universal Time (UTC) within 0.9 seconds of Greenwich Mean Time, mean solar time at Greenwich, England.