noun

• (astronomy, archaic or historical) A small circle on the celestial sphere, parallel to the horizon, that is used in astronomy and navigation to show altitude of a star or any other heavenly body.
  "Almucantars, or parallels of altitude, are circles parallel to the horizon, or whose poles are the zenith and nadir. All the points of any one almucantar are at equal altitudes above the horizon."
• (astronomy, archaic or historical) An instrument for observing the heavenly bodies as they cross such a circle.
  "A wide field of work appears open in the application of photography to meridian instruments, or to the almucantar. The sensitive plate should be substituted for the reticule, and the position marked by a graver attached to the tail-piece of the telescope."