Astrolabe

An astrolabe is a handheld instrument that models the celestial sphere and measures celestial positions and altitudes. It supports timekeeping, navigation, and astronomical calculations.

Planispheric astrolabe: A flat, rotating projection of the celestial sphere used to represent stars and the horizon on an astrolabe. Its stereographic projection makes the instrument’s rotating star map possible.
Altitude (astronomy): The angular height of a celestial object above an observer’s horizon. Measuring altitude is a central operation performed with an astrolabe.
Hipparchus: A Greek astronomer of the second century BCE known for star catalogues, trigonometry, and work on precession. Ancient stereographic astronomy associated with his era supplied a mathematical foundation for the instrument.
Quadrant (instrument): An instrument with a quarter-circle scale used to measure or construct angles. A quadrant measures altitude more directly but does not model the rotating celestial sphere.
Stereographic projection: A map projection that represents a sphere on a plane by projecting points from one pole onto the opposite hemisphere. Astrolabes use this projection to flatten the celestial sphere onto a plate.
Timekeeping: The measurement and organization of time using clocks, calendars, or astronomical observations. Sky positions on an astrolabe can be converted into local solar or sidereal time.
Theon of Alexandria: A fourth-century CE Greek mathematician and astronomer who wrote commentaries on Ptolemy and Euclid. His writings preserve early evidence about stereographic projection and astrolabe construction.
Sextant: An optical instrument that measures the angular distance between two visible objects, especially for navigation. The sextant replaced many nautical altitude measurements with more precise angular comparisons.
Rete (astrolabe): The openwork rotating disk of an astrolabe, marked with pointers for selected stars and the ecliptic. It overlays a movable star map on the fixed coordinate plate.
Latitude determination: The estimation of geographic latitude from observations of celestial bodies or other measurements. The altitude of the celestial pole or a known star can reveal an observer’s latitude.

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