{"dataset":{"slug":"coordinate-and-time-systems","title":"Coordinate & Time Systems","description":"The astrometry and time foundation — the coordinate systems (RA/Dec, equatorial, galactic, ecliptic, horizontal, supergalactic) and the astrometric effects (precession, nutation, aberration, refraction, light-time, Earth orientation) added by the Coordinates, Time & Reference Systems program. Only well-established, standard definitions; nothing fabricated.","version":"1.0.0","lastGenerated":"2026-06-29","license":"CC BY-SA 4.0","entityCount":14,"sources":["iau","usno"]},"entities":[{"id":"astrometric_effect:atmospheric-refraction","name":"Atmospheric Refraction","type":"astrometric_effect","domain":"science","description":"The bending of a light ray as it passes through the Earth's atmosphere, which lifts the apparent position of a celestial body above its true one — by about half a degree right at the horizon, so the Sun is fully refracted into view when geometrically it has already set. Refraction must be removed to turn an observed altitude into a true one.","entryPath":"/reference-systems/atmospheric-refraction"},{"id":"coordinate_system:declination","name":"Declination","type":"coordinate_system","domain":"science","description":"The celestial equivalent of latitude — the angle of a body north (positive) or south (negative) of the celestial equator, from +90° at the north celestial pole to −90° at the south. With right ascension it pins a star's position in the equatorial coordinate system.","entryPath":"/reference-systems/declination"},{"id":"astrometric_effect:earth-orientation-parameters","name":"Earth Orientation Parameters","type":"astrometric_effect","domain":"science","description":"The measured quantities that describe how the real, wobbling, irregularly rotating Earth is oriented in space relative to the celestial reference frame — the difference UT1−UTC, the position of the pole (polar motion), and small corrections to the precession-nutation model. Determined and published by the IERS, they are what make it possible to transform between celestial and terrestrial coordinates to full precision.","entryPath":"/reference-systems/earth-orientation-parameters"},{"id":"astrometric_effect:light-time-correction","name":"Light-Time Correction","type":"astrometric_effect","domain":"science","description":"The correction that accounts for the time light takes to travel from a moving Solar-System body to the observer: what is seen is where the body was when the light left it, not where it is now. Together with stellar aberration it makes up what is called planetary aberration, and it is essential for computing accurate apparent positions of planets, moons, and spacecraft.","entryPath":"/reference-systems/light-time-correction"},{"id":"astrometric_effect:nutation","name":"Nutation","type":"astrometric_effect","domain":"science","description":"The small, shorter-period nodding of the Earth's axis superimposed on the steady precessional wobble, caused mainly by the changing orientation of the Moon's orbit, with a dominant period of about 18.6 years. Nutation must be added to precession to compute a body's true position of date to arcsecond precision.","entryPath":"/reference-systems/nutation"},{"id":"astrometric_effect:precession","name":"Precession","type":"astrometric_effect","domain":"science","description":"The slow conical wobble of the Earth's rotation axis, driven by the gravitational pull of the Sun and Moon on the equatorial bulge, which carries the celestial poles and the equinoxes around the sky once in about 25,772 years. Precession is why equatorial coordinates drift with time and must be referred to a stated epoch, and why Polaris is only temporarily the pole star.","entryPath":"/reference-systems/precession"},{"id":"coordinate_system:right-ascension","name":"Right Ascension","type":"coordinate_system","domain":"science","description":"The celestial equivalent of longitude — the angle measured eastward along the celestial equator from the vernal equinox, conventionally expressed in hours, minutes, and seconds (24 hours around the sky). Together with declination it fixes a star's place in the equatorial system, and it connects directly to sidereal time.","entryPath":"/reference-systems/right-ascension"},{"id":"astrometric_effect:aberration-of-light","name":"The Aberration of Light","type":"astrometric_effect","domain":"science","description":"The small apparent shift of a star's position in the direction of the observer's motion, because light travels at a finite speed while the observer moves. The Earth's orbital motion makes every star describe a tiny yearly ellipse whose semi-major axis — the maximum displacement from the true position — is about 20.5 arcseconds; discovered by James Bradley in 1728, it was early direct evidence that the Earth orbits the Sun.","entryPath":"/reference-systems/aberration-of-light"},{"id":"coordinate_system:celestial-sphere","name":"The Celestial Sphere","type":"coordinate_system","domain":"science","description":"The imaginary sphere of arbitrarily large radius, centred on the observer, onto which all celestial bodies appear projected. It is the geometric stage on which every astronomical coordinate system is drawn — the sky treated as a two-dimensional surface whose points are fixed by pairs of angles.","entryPath":"/reference-systems/celestial-sphere"},{"id":"coordinate_system:ecliptic-coordinate-system","name":"The Ecliptic Coordinate System","type":"coordinate_system","domain":"science","description":"A coordinate system based on the ecliptic — the plane of the Earth's orbit and the Sun's apparent yearly path — measuring ecliptic longitude eastward from the vernal equinox and ecliptic latitude perpendicular to it. It is the natural frame for the Solar System, where the planets and the Moon stay close to the ecliptic.","entryPath":"/reference-systems/ecliptic-coordinate-system"},{"id":"coordinate_system:equatorial-coordinate-system","name":"The Equatorial Coordinate System","type":"coordinate_system","domain":"science","description":"The standard astronomical coordinate system, projecting the Earth's equator and poles onto the sky and fixing positions by right ascension and declination. Because it is tied to the slowly precessing equator and equinox, an equatorial position must be qualified by a reference frame and epoch, such as ICRS or J2000.","entryPath":"/reference-systems/equatorial-coordinate-system"},{"id":"coordinate_system:galactic-coordinate-system","name":"The Galactic Coordinate System","type":"coordinate_system","domain":"science","description":"A coordinate system aligned with the Milky Way, measuring galactic longitude from the direction of the Galactic Centre along the galactic plane and galactic latitude above or below it. It is the natural frame for describing the structure of our Galaxy — spiral arms, the disk, and the distribution of stars and gas.","entryPath":"/reference-systems/galactic-coordinate-system"},{"id":"coordinate_system:horizontal-coordinate-system","name":"The Horizontal Coordinate System","type":"coordinate_system","domain":"science","description":"The observer-centred system that describes where a body appears in the local sky: altitude above the horizon and azimuth around it. Simple and intuitive for pointing a telescope, horizontal coordinates change continuously as the Earth turns and depend on the observer's location and the moment of observation.","entryPath":"/reference-systems/horizontal-coordinate-system"},{"id":"coordinate_system:supergalactic-coordinate-system","name":"The Supergalactic Coordinate System","type":"coordinate_system","domain":"science","description":"A coordinate system whose equator follows the flattened plane in which the nearby galaxies and clusters are concentrated — the supergalactic plane of the Local Supercluster. It is used to describe the large-scale distribution of galaxies in our cosmic neighbourhood.","entryPath":"/reference-systems/supergalactic-coordinate-system"}]}