hour circleArticle Free Pass hour circle, in astronomy, any
great circle (similar to longitude) on the celestial sphere that
passes through the celestial poles—i.e., is perpendicular to the
celestial equator. The declination of a celestial object is
measured along its hour circle. The hour circle that at any
moment is passing through an observer’s zenith is his meridian.
The term also refers to a scale on the mounting of an equatorial
telescope, indicating the hour angle at which the telescope is
aimed. What made you want to look up "hour circle"? Please share
what surprised you most...