On a prograde planet like the Earth, the sidereal day is shorter than the solar day. Direct motion is the motion of a Planetary body in a direction similar to that of other bodies within its system and is sometimes called prograde motion. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Sidereal time is a measure of the position of the Earth in its rotation around its axis or time measured by the apparent Diurnal motion of the Vernal equinox At time 1, the Sun and a certain distant star are both overhead. The Sun (Sol is the Star at the center of the Solar System. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 = one sidereal day). But it is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day). Or more simply, 1-2 is a complete rotation of the Earth, but because the revolution around the Sun affects the angle the sun hits a position on the Earth, 1-3 is how long it takes noon to return

Solar times are measures of the apparent position of the Sun on the celestial sphere. Rotation period Earth's rotation period relative to the Sun (its mean solar day is 86400 Seconds of mean solar time In Geometry and Trigonometry, an angle (in full plane angle) is the figure formed by two rays sharing a common Endpoint, called Noon (also midday) is the hour of 1200 in an observer's local time zone or more loosely a time near the middle of the day when workers in many countries take a meal break The Sun (Sol is the Star at the center of the Solar System. In Astronomy and Navigation, the celestial sphere is an imaginary rotating Sphere of "gigantic Radius " They are not actually the physical time, but rather hour angles, that is, angles expressed in time units. For other uses see Time (disambiguation Time is a component of a measuring system used to sequence events to compare the durations of In Astronomy, the hour angle is one of the coordinates used in the Equatorial coordinate system for describing the position of a point on the Celestial sphere In Geometry and Trigonometry, an angle (in full plane angle) is the figure formed by two rays sharing a common Endpoint, called They are also local times in the sense that they depend on the longitude of the observer. Longitude (ˈlɒndʒɪˌtjuːd or ˈlɒŋgɪˌtjuːd symbolized by the Greek character Lambda (λ is the east-west Geographic coordinate measurement

## 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 meridian. This article is about the astronomical concept For other uses of the word see Meridian. Note that the solar day starts at noon, so apparent solar time 00:00 means noon and 12:00 means midnight. Noon (also midday) is the hour of 1200 in an observer's local time zone or more loosely a time near the middle of the day when workers in many countries take a meal break Midnight is literally "the middle of the night" In most systems it is when one day ends and the next begins when the date changes Solar time can be measured by a sundial. A sundial is a device that measures time by the position of the Sun.

The length of a solar day varies throughout the year for two reasons. 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). In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star In Mathematics, an ellipse (from the Greek ἔλλειψις literally absence) is a Conic section, the locus of points in a Circles are simple Shapes of Euclidean geometry consisting of those points in a plane which are at a constant Distance, called the In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from In Astronomy, Kepler's Laws of Planetary Motion are three mathematical laws that describe the motion of Planets in the Solar System. Second, due to Earth's axial tilt, the Sun moves along a great circle (the ecliptic) that is tilted to Earth's celestial equator. In Astronomy, axial tilt is the Inclination angle of a planet's rotational axis in relation to its orbital plane. A great circle is a Circle on the surface of a Sphere that has the same circumference as the sphere dividing the sphere into two equal Hemispheres. The ecliptic is the apparent path that the Sun traces out in the sky during the year The celestial equator is a Great circle on the imaginary Celestial sphere, in the same plane as the Earth 's Equator. When the Sun crosses the equator at both equinoxes, it is moving at an angle to it, 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, it moves parallel to it and closer to the polar axis than the equator, so the projection of this parallel motion onto the equator is faster than its mean motion (see tropical year). An equinox is the event of the Sun passing over the Earth's equator in its annual cycle Mean motion, n\\! is a measure of how fast a Satellite progresses around its Orbit. Solstices occur twice a year when the tilt of the Earth's axis is most oriented toward or away from the Sun, causing the Sun to reach its northernmost and southernmost extremes The equator (sometimes referred to colloquially as "the Line") is the intersection of the Earth 's surface with the plane perpendicular to the A tropical year (also known as a solar year) is the length of time that the Sun takes to return to the same position in the cycle of seasons as seen from Earth 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 rotation of the fixed stars to agree with average apparent solar time. Diurnal motion is an astronomical term referring to the apparent daily motion of Stars around the Earth, or more precisely around the two 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 seconds/minute) SI seconds in approximately 1820. The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units Year 1820 ( MDCCCXX) was a Leap year starting on Saturday (link will display the full calendar of the Gregorian calendar (or a Leap year Currently, the length of a mean solar day is approximately 86400. 002 SI seconds. [1] 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 time. The equation of time is the difference over the course of a year between time as read from a Sundial and time as read from a Clock, measured in an ideal situation 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 time is defined as mean solar time minus 12 hours. In modern usage civil time refers to statutory time scales designated by civilian authorities or to local time indicated by clocks

The length of the mean solar day is increasing due to the tidal acceleration of the Moon by Earth, and the corresponding deceleration of the Earth by the Moon. Tidal acceleration is an effect of the Tidal forces between an orbiting Natural satellite ( i

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 perihelion and also when it passes through the aphelion. In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from 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. An equinox is the event of the Sun passing over the Earth's equator in its annual cycle [2]

## History

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. A water clock or clepsydra ( Greek kleptein to steal; hydro water) is any timekeeper operated by means of a regulated flow of liquid into (inflow The 2nd millennium BC marks the transition from the Middle to the Late Bronze Age. The second millennium is a period of time that commenced on January 1, 1001, and ended on December 31, 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 gnomon (a vertical pole), except that they could be used at night. The 1st millennium BC encompasses the Iron Age and sees the rise of successive empires The gnomon is the part of a Sundial that casts the Shadow. Gnomon (γνώμων is an Ancient Greek word meaning "indicator" "one who

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. 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. Babylonia was an Amorite state in lower Mesopotamia (modern southern Iraq) with Babylon as its capital Sidereal time is a measure of the position of the Earth in its rotation around its axis or time measured by the apparent Diurnal motion of the Vernal equinox 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 clocks have a much more constant rate than the Earth, its star clock is still used to determine mean solar time. An atomic clock is a type of Clock that uses an Atomic resonance Frequency standard as its timekeeping element A star clock is a method of using the Stars to determine the 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 Time (UTC) is used to determine whether a leap second is needed. A leap second is a one- Second adjustment that keeps broadcast standards for time of day close to mean solar time.