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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 .
EOT, the time difference between apparent solar time and mean solar time; GHA, the Greenwich Hour Angle of the apparent (actual) Sun; GMHA = Universal Time − Offset, the Greenwich Mean Hour Angle of the mean (fictitious) Sun. Here time and angle are quantities that are related by factors such as: 2 π radians = 360° = 1 day = 24 hours.
Right ascension is usually measured in sidereal hours, minutes and seconds instead of degrees, a result of the method of measuring right ascensions by timing the passage of objects across the meridian as the Earth rotates. There are 360° / 24 h = 15° in one hour of right ascension, and 24 h of right ascension around the entire ...
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]
Subdivisions of the day include the hour (1/24 of a day), which is further subdivided into minutes and seconds. The second is the international standard unit (SI unit) for science. Celestial sphere-based: as in sidereal time, where the apparent movement of the stars and constellations across the sky is used to calculate the length of a year.
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 ...
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.
[1] [7] By comparing the corrected lunar distance with the tabulated values, the navigator finds the Greenwich time for that observation. Knowing Greenwich time and local time, the navigator can work out longitude. [1] Local time can be determined from a sextant observation of the altitude of the Sun or a star. [8] [9] Then the longitude ...