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Encyclopedia > Sidereal time

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. They differ by the precession of the vernal equinox relative to the stars. 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. ... Illumination of Earth by Sun on the day of equinox In astronomy, the vernal equinox (spring equinox, March equinox, or northward equinox) is the moment when the sun appears to cross the celestial equator, heading northward. ... The Pleiades star cluster A star is a massive body of plasma in outer space which is currently or has in the past produced energy through nuclear fusion. ... There are two types of precession: torque-free precession and torque-induced precession. ...


Solar time is measured by the apparent diurnal motion of the sun, and local noon in solar time is defined as the moment when the sun is at its highest point in the sky (exactly due south in the northern hemisphere and due north in the southern hemisphere). The time taken for the sun to return to its highest point is exactly 24 hours, or a solar day. Solar time is based on the idea that when the sun reaches its highest point in the sky, it is noon. ... Noon is the time exactly halfway through the day, written 12:00 in the 24-hour clock and 12:00 pm in the 12-hour clock. ... A compass rose with South highlighted South is most commonly a noun, adjective, or adverb indicating direction or geography. ... The Northern Hemisphere is the half of a planets surface (or celestial sphere) that is north of the equator (the word hemisphere literally means half ball). On Earth, the Northern Hemisphere contains most of the land and population. ... Compass rose with north highlighted and at top North is one of the four cardinal directions, specifically the direction that, in Western culture, is treated as the primary direction: north is used (explicitly or implicitly) to define all other directions; the (visual) top edges of maps usually correspond to the... The Southern Hemisphere is the half of a planets surface (or celestial sphere) that is south of the equator (the word hemisphere literally means half ball). On Earth it contains four continents (part of Africa, Australia, most of South America, and Antarctica) and four oceans (South Atlantic, Indian, Pacific... Solar time is based on the idea that, when the sun reaches its highest point in the sky, it is noon. ...


However, the stars appear to move in a slightly different way. During the course of one day, the earth has moved a short distance along its orbit around the sun, and so must rotate a small extra angular distance before the sun reaches its highest point. The stars, however, are so far away that the earth's movement along its orbit makes a generally negligible difference to their apparent direction (see, however parallax), and so they return to their highest point in slightly less than 24 hours. A mean sidereal day is about 23h 56m 4.1s in length. Due to variations in the rotation rate of the Earth, however, the rate of an ideal sidereal clock deviates from any simple multiple of a civil clock. In practice, the difference is kept track of by the difference UTCUT1, which is measured by radio telescopes and kept on file and available to the public at the IERS and at the United States Naval Observatory. 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. ... Parallax (Greek: παραλλαγή (parallagé) = alteration) is the change of angular position of two stationary points relative to each other as seen by an observer, due to the motion of said observer. ... On a prograde planet like the Earth, the sidereal day is shorter than the solar day. ... UTC also stands for the University of Tennessee at Chattanooga Coordinated Universal Time or UTC, also sometimes referred to as Zulu time, the basis for civil time, differs by an integral number of seconds from atomic time and a fractional number of seconds from UT1. ... Universal Time (UT) is a timescale based on the rotation of the Earth. ... The International Earth Rotation Service (IERS) is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Paramater (EOP) and International Celestial Reference System (ICRS) groups. ... Aerial view of USNO. The United States Naval Observatory (USNO) is one of the oldest scientific agencies in the United States. ...


Sidereal time is defined as the hour angle of the vernal equinox. When the meridian of the vernal equinox is directly overhead, local sidereal time is 00:00. Greenwich Sidereal time is the hour angle of the vernal equinox at the prime meridian at Greenwich, England; local values differ according to longitude. When one moves eastward 15° in longitude, sidereal time is larger by one hour (note that it wraps around at 24 hours). Differences are counted to the accuracy of measurement, however, not just in whole hours. Greenwich sidereal time and UT1 differ from each other by a constant rate (1.00273790935). Sidereal time is used at astronomical observatories because sidereal time makes it very easy to work out which astronomical objects will be observable at a given time. Objects are located in the night sky using right ascension and declination relative to the celestial equator (analogous to longitude and latitude on Earth), and when sidereal time is equal to an object's right ascension, the object will be at its highest point in the sky, or culmination, at which time it is best placed for observation, as atmospheric extinction is minimised. In astronomy, an objects hour angle (HA) is defined as the difference between the current local sidereal time (LST) and the right ascension () of the object: HAobject = LST - object Thus, the objects hour angle indicates how much sidereal time has passed since the object was on the local... Meridian is: Meridian (astronomy): an imaginary circle perpendicular to the horizon. ... The Prime Meridian, Greenwich The Prime Meridian is the meridian (line of longitude) passing through the Royal Greenwich Observatory, Greenwich, England; it is the meridian at which longitude is 0 degrees. ... Map of Earth showing curved lines of longitude Longitude, sometimes denoted by the Greek letter λ, describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. ... In ancient Greece and other early civilizations, astronomy consisted largely of astrometry, measuring positions of stars and planets in the sky. ... Observatory of Strasbourg An observatory is a location used for observing terrestrial and/or celestial events. ... See also Lists of astronomical objects Categories: Astronomical objects ... Right ascension (RA; symbol α: Greek letter alpha) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. ... In astronomy, declination (dec) is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. ... Latitude, denoted by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. ... Extinction is a term used in astronomy to describe the absorption of light from astronomical objects by matter between them and the observer. ...


See also

Coordinated Universal Time or UTC, also sometimes referred to as Zulu time, is an atomic realization of Universal Time(UT) or Greenwich Mean Time, the astronomical basis for civil time. ... Temps Atomique International (TAI) or International Atomic Time is a very accurate and stable time scale. ... Solar time is based on the idea that when the sun reaches its highest point in the sky, it is noon. ... This article is about terrestrial time; for other meanings of TT, see TT (disambiguation). ... 8:17 am, August 6, 1945, Japanese time. ... Universal Time (UT) is a timescale based on the rotation of the Earth. ...

External links

  • For more details, see the article on sidereal time from Jason Harris' Astroinfo, which comes along with KStars, a Desktop Planetarium for Linux/KDE.
  • See also KStars website.
  • Get a free Sidereal Clock for your desktop at SolarClock Website

  Results from FactBites:
 
Sidereal time - Wikipedia, the free encyclopedia (457 words)
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.
Solar time is measured by the apparent diurnal motion of the sun, and local noon in solar time is defined as the moment when the sun is at its highest point in the sky (exactly due south in the northern hemisphere and due north in the southern hemisphere).
Sidereal time is used at astronomical observatories because sidereal time makes it very easy to work out which astronomical objects will be observable at a given time.
MSN Encarta - Time (1388 words)
The concept that time is a fourth dimension—on a par with the three dimensions of space: length, width, and depth—is one of the foundations of modern physics.
Sidereal time is also based on Earth’s rotation, but uses the apparent motion of the “fixed” stars across the sky as Earth rotates as the basis for time determination.
Sidereal time is based on the apparent motion of the distant, “fixed” stars across the sky.
  More results at FactBites »

 
 

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