Take the 2-minute tour ×
Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. It's 100% free, no registration required.

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

share|improve this question

2 Answers 2

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.


share|improve this answer

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.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.