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Encyclopedia > Solar time

Solar time is based on the idea that when the sun reaches its highest point in the sky, it is noon.

Apparent solar time 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 be measured by a sundial. The Sun is the star at the centre of our Solar system. ... Meridian is: Meridian (astronomy): an imaginary circle perpendicular to the horizon. ... Wall sundial Wall sundial in Warsaws Old Town A sundial measures time by the position of the sun. ...

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 usually appear to move along Earth's celestial equator but usually appears to move at an angle to it during the year; thus, the sun appears to move either fast or slow depending on whether it appears to be 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). A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... Earth, also known as the Earth, Terra, and (mostly in the 19th century) Tellus, is the third-closest planet to the Sun. ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... In mathematics, an ellipse (from the Greek for absence) is a plane algebraic curve where the sum of the distances from any point on the curve to two fixed points is constant. ... Johannes Keplers primary contributions to astronomy/astrophysics were his three laws of planetary motion. ... Axial tilt is an astronomical term regarding the inclination angle of a planets rotational axis in relation to its orbital plane. ... The celestial equator is a great circle on the imaginary celestial sphere, which could be constructed by inflating the Earths equator until it intersects with said sphere. ... A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ... March is the third month of the year in the Gregorian Calendar and one of seven Gregorian months with the length of 31 days. ... September is the ninth month of the year in the Gregorian Calendar and one of four Gregorian months with 30 days. ... June is the sixth month of the year in the Gregorian Calendar, with a length of 30 days The month is named after the Roman goddess Juno (mythology), wife of Jupiter and equivalent to the Greek goddess Hera. ... December is the twelfth and last month of the year in the Gregorian Calendar and one of seven Gregorian months with the length of 31 days. ...

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 seconds) by as much as nearly 22 seconds shorter to nearly 29 seconds 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. A wall clock A clock (from the Latin cloca, bell) is an instrument for measuring time. ... Diurnal motion is an astronomical term referring to the apparent daily motion of stars in orbit around the Earth, caused by the Earths rotation around its axis. ... During the course of the year, the time as read from a sundial can run ahead of clock time by as much as 16 min 33 s (around October 31–November 1) or fall behind by as much as 14 min 6 s (around February 11–12). ...

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 middle of the second millennium BC until the early second millennium. Before the middle of the first millennium BC, the water clocks 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. A water clock or clepsydra is a device for measuring time by letting water regularly flow out of a container usually by a tiny aperture. ... (3rd millennium BC – 2nd millennium BC – 1st millennium BC – other millennia) Events Second dynasty of Babylon First Bantu migrations from west Africa The Cushites drive the original inhabitants from Ethiopia, and establish trade relations with Egypt. ... (1st millennium – 2nd millennium – 3rd millennium – other millennia) Events The Black Death Mongol Empires in Asia The Renaissance in Europe The Protestant Reformation The agricultural and industrial revolutions The rise of nationalism and the nation state European discovery of the Americas and Australia and their colonization European colonization and decolonization... (2nd millennium BC – 1st millennium BC – 1st millennium AD – other millennia) Events The Iron Age began in Western Europe Egypt declined as a major power The Tanakh was written Buddhism was founded Cyrus the Great conquered Babylon and created the Persian Empire (6th century BC) Sparta and Athens fought the... The gnomon is the part of a sundial which casts the shadow. ...

Nevertheless, it has always been known that the sun moves eastward relative to the fixed stars along the ecliptic. Thus since the middle of the 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. The plane of the ecliptic is well seen in this picture from the 1994 lunar prospecting Clementine spacecraft. ... Babylonia was an ancient state in Mesopotamia (in modern Iraq), combining the territories of Sumer and Akkad. ... Sidereal time is time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical with, the motion of stars. ...

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. Since sometime in the late 1900s, Earth's rotation has been defined relative to an ensemble of extra-galactic radio sources and then converted to mean solar time by an adopted ratio. The difference between this calculated mean solar time and Coordinated Universal Time (UTC) is used to determine whether a leap second is needed. (19th century - 20th century - 21st century - more centuries) Decades: 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s The 20th century lasted from 1901 to 2000 in the Gregorian calendar (often from (1900 to 1999 in common usage). ... Atomic clock Chip-Scale Atomic Clock Unveiled by NIST An atomic clock is a type of clock that uses an atomic resonance frequency standard as its counter. ... Coordinated Universal Time or UTC, also sometimes referred to as Zulu time or Z, is an atomic realization of Universal Time (UT) or Greenwich Mean Time, the astronomical basis for civil time. ... A leap second is a one-second adjustment to civil time in order to keep it close to the mean solar time. ...

See also Day (language) A day (symbol: d) is a unit of time. ... Local mean time is a form of solar time that corrects the variations of local apparent time. ... 8:17 am, August 6, 1945, Japanese time. ...

Results from FactBites:

 AllRefer.com - solar time (Astronomy, General) - Encyclopedia (354 words) Because the earth moves with varying speed in its orbit at different times of the year and because the plane of the earth's equator is inclined to its orbital plane, the length of the solar day is different depending on the time of year. Such time, called mean solar time, may be thought of as being measured relative to an imaginary sun (the mean sun) that lies in the earth's equatorial plane and about which the earth orbits with constant speed. Mean solar time is the basis for civil time and standard time.
 Solar time - Wikipedia, the free encyclopedia (618 words) Apparent solar time is based on the apparent solar day, which is the interval between two successive returns of the Sun to the local meridian. 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. 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.
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