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Sidereal time was defined such that the March equinox would transit the meridian of the observatory at 0 hours local sidereal time. [7] Beginning during the 1970s, the radio astronomy methods very-long-baseline interferometry (VLBI) and pulsar timing overtook optical instruments for the most precise astrometry.
The local hour angle (LHA) of an object in the observer's sky is = or = + where LHA object is the local hour angle of the object, LST is the local sidereal time, is the object's right ascension, GST is Greenwich sidereal time and is the observer's longitude (positive east from the prime meridian). [3]
Standard time zones of the world. The number at the bottom of each zone specifies the number of hours to add to UTC to convert it to the local time. As international commerce increased, the need for an international standard of time measurement emerged. Several authors proposed a "universal" or "cosmic" time (see Time zone § Worldwide time zones).
Windows Clock (known as Clock & Alarms on Pocket PC 2000, [2] Alarms on Windows 8.1, and, until July 2022, Alarms & Clock on Windows 10) is a time management app for Microsoft Windows, with five key features: alarms, world clocks, timers, a stopwatch, and focus sessions. The features are listed on a sidebar.
As seen from above the Earth's north pole, a star's local hour angle (LHA) for an observer near New York. Also depicted are the star's right ascension and Greenwich hour angle (GHA), the local mean sidereal time (LMST) and Greenwich mean sidereal time (GMST). The symbol ♈︎ identifies the March equinox direction.
Local mean time (LMT) is a form of solar time that corrects the variations of local apparent time, forming a uniform time scale at a specific longitude. This measurement of time was used for everyday use during the 19th century before time zones were introduced beginning in the late 19th century; it still has some uses in astronomy and navigation.
Universal time tracks the Earth's rotation in time, which performs one revolution in about 24 hours. The Earth's rotation is uneven, so UT is not linear with respect to atomic time. It is practically proportional to the sidereal time, which is also a direct measure of Earth rotation. The excess revolution time is called length of day (LOD).
Huygens first used a clock to calculate the equation of time (the difference between the apparent solar time and the time given by a clock), publishing his results in 1665. The relationship enabled astronomers to use the stars to measure sidereal time, which provided an accurate method for setting clocks. The equation of time was engraved on ...