celestial-coordinates

# Celestial Coordinates A coordinate system tells you where something is. On Earth we use latitude and longitude. In the sky we need analogous systems, but with a complication: the sky rotates. An object's position depends on where the observer is standing, when they look, and which frame of reference they choose. This skill catalogs four celestial coordinate systems, the transformations between them, the corrections that matter (precession, nutation, aberration, refraction), and practical procedures for locating objects with a planisphere, a star chart, or a telescope's setting circles. **Agent affinity:** caroline-herschel (observational practice), hubble (catalog cross-reference) **Concept IDs:** astro-constellation-navigation, astro-planisphere-use, astro-earth-moon-sun-geometry ## The Four Coordinate Systems at a Glance | System | Reference plane | Origin | Coordinates | Best for | |---|---|---|---|---| | Horizon | Observer's horizon | South (or North) | Altitude, Azimuth | Pointing a telescope right now | | Equatorial | Celestial equator | Vernal equinox | Declination, Right Ascension | Catalog positions, long-term records | | Ecliptic | Ecliptic plane | Vernal equinox | Ecliptic latitude, longitude | Solar system, planetary motion | | Galactic | Galactic plane | Galactic center | Galactic latitude, longitude | Milky Way structure, stellar populations | No single system is "best" — each is convenient for a particular class of problem. Professionals move freely bet...