Spherical astronomy

Spherical astronomy describes celestial positions and apparent motions using directions on the celestial sphere, spherical geometry, and coordinate systems.

Celestial sphere: An imaginary sphere centered on an observer, onto which celestial objects are projected to describe their directions. It provides the geometric surface on which spherical astronomy represents the sky.
Horizontal coordinate system: A local sky-coordinate system that locates objects by altitude above the horizon and azimuth around it. It describes positions directly in the observer’s horizon-based frame.
Rise, transit, and set: The events marking when a celestial object crosses the horizon, reaches the meridian, and crosses the horizon again. These events follow from an object’s coordinates and the observer’s location.
Hipparchus: A Greek astronomer of the second century BCE who developed quantitative methods for describing celestial positions and motions. His star catalog and work on precession helped establish positional astronomy.
Astrophysics: The study of the physical properties and processes of celestial objects and the universe. Spherical astronomy locates objects; astrophysics explains their physical nature.
Spherical trigonometry: The branch of geometry that relates angles and sides of triangles drawn on a sphere. Its formulas solve positional problems involving arcs on the celestial sphere.
Ecliptic coordinate system: A celestial coordinate system using the ecliptic as its reference plane and ecliptic longitude and latitude as coordinates. It aligns coordinates with the Sun’s apparent annual path and the planets’ near-plane of motion.
Meridian: The great circle passing through the celestial poles and the zenith of an observer’s location. An object’s meridian crossing determines its local transit and highest daily altitude.
Ptolemy: A second-century CE astronomer whose Almagest systematized Greek mathematical astronomy and celestial models. The Almagest preserved and organized geometric techniques used to calculate celestial positions.
Celestial mechanics: The study of the motion of celestial bodies under gravity using mathematical physics. It models dynamical causes of motion rather than primarily describing observed directions.

Linked from 11 pages