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My question is seen in the title: What is the right ascension overhead at Greenwich at noon on 21 march

I'm a little bit confused since the right ascension is set by the vernal equinox and don't understand how this is related to Greenwich.

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At the vernal equinox, the sun crosses the celestial equator, and by definition, it has an RA of 0h (at the current epoch). At mid-day on that day, the sun will be more or less due south, and as the line of zero RA forms a great circle, the RA "overhead" will also be zero.

The sun won't be exactly zero RA except at the moment of the equinox, and also the length of the solar day varies throughout the year, which results in the time at which the sun is due south is not quite 12 noon by clock time.

All the above is independent of whether or not you are observing from Greenwich.

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  • $\begingroup$ This is essentially correct; the value of the Equation of Time (en.wikipedia.org/wiki/Equation_of_time) on the vernal equinox is about 7m, so the RA at noon Greenwich Mean Time is closer to 23h53m, although, as you note, the exact time depends on when the equinox occurs. $\endgroup$ – user21 Jun 21 '17 at 4:59

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