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Solar time

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	Topics Solar time Topic Home Solar times are measures of the apparent position of the SunSun \|+ The Sun \|+\|-\| colspan="2" align="center" \| \|-... on the celestial sphereCelestial sphere In astronomy and navigation, the celestial sphere is an imaginary rotating sphere of "gigantic radius", concentric and coaxi... . They are not actually the physical timeTime Two distinct views exist on the meaning of time.... , but rather hour angleHour angle In astronomy, an object's hour angle is defined as the difference between the current local sidereal time and the right as... s, that is, angleAngle An angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle.... s expressed in time units. They are also local times in the sense that they depend on the longitudeLongitude Longitude, sometimes denoted by the Greek letter ? , describes the location of a place on Earth east or west of a north-sout... of the observer. Apparent solar time Apparent solar time or true solar time is the hour angle of the Sun. It is based on the apparent solar day, which is the interval between two successive returns of the Sun to the local meridianMeridian (astronomy) In the sky, a meridian is an imaginary great circle on the celestial sphere that passes through the zenith and the north poi... . Note that the solar day starts at noonNoon Noon is the time exactly halfway through the day, written 12:00 in the 24-hour clock and 12:00 noon in the 12-hour clock.... , so apparent solar time 00:00 means noon and 12:00 means midnightMidnight Midnight, literally "the middle of the night", is a time arbitrarily designated to determine the end of a day and the beginn... . Solar time can be measured by a sundialSundial Summary A sundial measures time by the position of the sun.... . The length of a solar day varies throughout the year for two reasons. First, Earth's orbitORBit ORBit is a CORBA compliant Object Request Broker.... is an ellipseEllipse The search term "Elliptical" redirects to this page; for the exercise machine, see Elliptical trainer.... , not a circleCircle In Euclidean geometry, a circle is the set of all points in a plane at a fixed distance, called the radius, from a fixed poi... , 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 motionKepler's laws of planetary motion Johannes Kepler's primary contributions to astronomy/astrophysics were his three laws of planetary motion.... ). Second, due to Earth's axial tiltAxial tilt Axial tilt is an astronomical term regarding the inclination angle of a planet's rotational axis in relation to a perpendicu... , the Sun moves along a great circleGreat circle A great circle is a circle on the surface of a sphere that has the same circumference as the sphere, and divides the sphere ... (the eclipticEcliptic Overview The ecliptic is the apparent path the Sun traces out along the sky — independent of Earth's rotation — in the co... ) that is tilted to Earth's celestial equatorCelestial equator The celestial equator is a great circle on the imaginary celestial sphere, which is actually the plane of the terrestrial eq... . When the Sun crosses the equator at both equinoxEquinox An equinox in astronomy is the moment when the Sun can be observed to be directly above the equator.... es, it is moving at an angle to it, so the projection of this tilted motion onto the equator is slower than its mean motionMean motion Mean Motion, , is a measure of how fast a satellite progresses around its orbit.... ; when the Sun is farthest from the equator at both solsticeSolstice A solstice is either of the two times of the year when the sun is at its greatest distance from the equator: in technical words, w... s, it moves parallel to it and closer to the polar axis than the equatorEquator Summary The equator is an imaginary circle drawn around a planet at a distance halfway between the poles.... , so the projection of this parallel motion onto the equator is faster than its mean motion (see tropical yearTropical year 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... ). 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. Mean solar time Mean solar time is the hour angle of the mean Sun (see below). As the mean Sun is a mathematical construction only and cannot be physically observed, the mean solar time is computed from an artificial clock time adjusted via observations of the diurnal rotationDiurnal motion Diurnal motion is an astronomical term referring to the apparent daily motion of stars around the Earth, or more precisely a... of the fixed stars to agree with average apparent solar time. Though the amount of daylight varies significantly, the length of a mean solar day does not change on a seasonal basis. However, the length of a mean solar day increases at a rate of approximately 1.4 milliseconds each century. It was exactly 86,400 (i.e. 24 hours * 60 minutes/hour * 60 secondSecond The second is the name of a unit of time, and today refers to the International System of Units base unit of time.... s/minute) SI seconds in approximately 1820. Currently, the length of a mean solar day is approximately 86400.002 SI seconds. An apparent solar day may differ from a mean solar day 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. Since these periods are cyclical, they do not accumulate from year to year. The difference between apparent solar time and mean solar time is called the equation of timeEquation of time The equation of time is the difference, over the course of a year, between time as read from a sundial and a clock.... . The mean solar day also starts at noon, with 00:00 meaning noon and 12:00 meaning midnight. As this is inconvenient for civilian use, the civil timeCivil time Civil time is another name for mean solar time reckoned from midnight.... is defined as mean solar time minus 12 hours. The length of the mean solar day is increasing due to the tidal accelerationTidal acceleration Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite , and the planet that it orbits.... of the Moon by Earth, and the corresponding deceleration of the Earth by the Moon. The mean Sun is defined as follows. First, consider a fictitious Sun that moves along the ecliptic at a constant speed and occupies the same position as the real Sun when Earth passes through the perihelionApsis Summary In astronomy, an apsis is the point of greatest or least distance of the elliptical orbit of a celestial body from its cente... and also when it passes through the aphelionApsis In astronomy, an apsis is the point of greatest or least distance of the elliptical orbit of a celestial body from its cente... . Then, the mean sun is a second fictive Sun that moves along the celestial equator at constant speed and passing through the vernal point simultaneously with the first fictive sun. History Many methods have been used to simulate mean solar time throughout history. The earliest were clepsydras or water clockWater clock A water clock or clepsydra is any timekeeper operated by means of a regulated flow of liquid into or out from a vessel... s, used for almost four millennia from as early as the middle of the second millennium BC until the early second millennium2nd millennium In the Gregorian calendar, the 2nd millennium commenced on 1 January 1001, and ended at the end of 31 December 2000.... . 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 gnomonGnomon The gnomon is the part of a sundial which casts the shadow.... (a vertical pole), except that they could be used at night. Nevertheless, it has long 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. BabyloniaBabylonia Babylonia, named for its capital city, Babylon, was an ancient state in the south part of Mesopotamia , combining the territ... n astronomers knew of the equation of time and were correcting for it as well as the different rotation rate of stars, sidereal timeSidereal time Sidereal time is time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identic... , 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 20th century. Even though today's atomic clockAtomic clock An atomic clock is a type of clock that uses an atomic resonance frequency standard to feed its counter.... s have a much more constant rate than the Earth, its star clockStar clock A star clock is a method of using the stars to determine the time.... is still used to determine mean solar time. Since sometime in the late 20th century, 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 TimeCoordinated Universal Time Coordinated Universal Time is a high-precision atomic time standard.... (UTC) is used to determine whether a leap secondLeap second A leap second is an intercalary, one-second adjustment that keeps broadcast standards for time of day close to mean solar ti... is needed. See also Apparent sunApparent sun Summary Apparent sun is a concept proposed by Simon Newcomb to solve the problem of determining the right time of a year, by assumin... Local mean timeLocal mean time Local mean time is a form of solar time that corrects the variations of local apparent time.... External links : Apparent solar time in a digital display.