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If I know specific time, t0, how can I calculate RA of Greenwich?

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closed as off-topic by Walter, LDC3, TildalWave, Timtech, e-sushi Sep 28 '14 at 13:09

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I think you mean to ask, "What is the local sidereal time of Greenwhich?".

To calculate the GMST (Greenwich Mean Sidereal Time):

To an accuracy within 0.1 second per century, Greenwich (Mean) Sidereal Time (in hours and decimal parts of an hour) can be calculated as GMST = 18.697 374 558 + 24.065 709 824 419 08 * D , where D is the interval, in UT1 days including any fraction of a day, since 2000 January 1, at 12h UT (interval counted positive if forwards to a later time than the 2000 reference instant), and the result is freed from any integer multiples of 24 hours to reduce it to a value in the range 0–24.


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Right Ascension is a celestial coordinate, not a terrestrial one.

RA is used to locate a star on sky, and is measured between a fixed point in the sky (the crossing of the Ecliptic and the Celestial Equator) and the star's meridian.

It corresponds to terrestrial coordinate Latitude.

Thus, Greenwich has no Right Ascension.

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