An hour circle is a great circle through both reference celestial poles and a chosen direction. Its two halves have right ascensions differing by ; choose the half containing the direction when measuring declination. The observer's celestial meridian is the particular hour circle through the zenith. [ref]
https://www.astro.uvic.ca/~tatum/celmechs/celm6.pdf. Jeremy Tatum: Section 6.3, hour circles and declination circles
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