Celestial navigation

Celestial navigation determines position or direction by measuring the apparent positions of celestial bodies and comparing them with predicted or measured values.

Sextant: An optical instrument that measures the angle between two visible objects, commonly a celestial body and the horizon. It measures the altitude of a celestial body above the horizon.
Celestial sphere: An imaginary sphere centered on Earth onto which celestial objects are projected for describing their apparent positions. It provides the coordinate framework used to describe observed and predicted sky positions.
Polynesian navigation: Traditional Pacific wayfinding that uses stars, winds, waves, and other environmental signs. It shows how skilled star-based wayfinding developed without European instruments.
Marine navigation: The planning and control of a vessel's movement from one place to another. Celestial fixes have long provided position estimates during ocean passages.
Global Positioning System: A satellite-based radio-navigation system that computes position and time from signals broadcast by orbiting satellites. It automates positioning but depends on receiving satellite signals.
Sight reduction: The calculation that converts a celestial observation into a predicted altitude and azimuth for a chosen Earth position. It supplies the predicted altitude needed to turn a sight into a position line.
Celestial coordinates: A system of angular coordinates used to specify the apparent positions of objects on the celestial sphere. Almanac positions use celestial coordinates that connect a body to an observation time.
Astrolabe: An instrument for solving problems involving the positions of celestial bodies and the local horizon. Its maritime forms helped sailors measure celestial altitude before the sextant.
Noon sight: A navigational observation of the Sun near its highest daily altitude. The Sun's meridian altitude provides a direct estimate of latitude.
Inertial navigation system: A navigation system that estimates motion and position from accelerometers and gyroscopes without external references. It needs no view of the sky, but accumulated sensor error causes drift.

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