adjective Relating to the midline of any approximately spherical object.
adjective of a bond, lying approximately in the plane of a ring, perpendicular to its axis
noun A kind of telescope mounted so as to have two axes of motion at right angles to each other, one of them parallel to the axis of the Earth, and each carrying a graduated circle, one for measuring declination, and the other right ascension, or the hour angle, so that the telescope may be directed, even in the daytime, to any star or other object whose right ascension and declination are known.