#celestial sphere

• Almucantar — A small circle on the celestial sphere parallel to the horizon, passing through all points that share the same angular altitude above it; an imaginary line used in positional astronomy and classic celestial navigation to measure and compare the vertical coordinates of celestial bodies.
• Almucantar — Small circles on the celestial sphere parallel to the horizon, used to measure and determine equal altitudes of celestial bodies above the horizon line. This geometric and astronomical concept is fundamental in classical navigation, allowing the precise localization of an observer's position by determining the intersections of angular coordinates of stars.
• Almucantar — A terminological variant for almucantar, designating a small circle on the celestial sphere parallel to the horizon where all points share the same altitude; the term is also used to name a special astronomical instrument (a modified meridian transit circle) designed to measure time and stellar coordinates as they cross the same parallel of altitude.
• Antapex — An imaginary point on the celestial sphere, located in the constellation Columba, away from which the solar system moves in its global galactic motion relative to nearby stars. This astronomical structural element is located exactly one hundred and eighty degrees away, being diametrically opposed to the solar apex.