Hour circle
= Hour circle
{wiki}
An <hour circle> is a <great circle> through both reference <celestial poles> and a chosen direction. Its two halves have <right ascensions> differing by $12^{\rm h}$; choose the half containing the direction when measuring <declination>. The observer's <celestial meridian> is the particular <hour circle> through the <zenith>. https://www.astro.uvic.ca/~tatum/celmechs/celm6.pdf[Jeremy Tatum: Section 6.3, hour circles and declination circles]{ref}.