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| horizontal coordinate system | |
|---|---|
| Name | Horizontal coordinate system |
| Caption | Altitude and azimuth on the local sky |
| Type | Topocentric celestial coordinate system |
| Epoch | Local apparent time |
| Axes | Altitude (elevation), Azimuth |
| Origin | Observer's location on Earth's surface |
| Used for | Locating celestial objects for observation, navigation, surveying |
horizontal coordinate system
The horizontal coordinate system is a topocentric celestial coordinate framework used to specify the apparent position of celestial objects relative to an observer at a particular place and time. It expresses positions as altitude (elevation) and azimuth referenced to the observer's local horizon and cardinal directions, linking practical observation by telescopes, sextants, and theodolites to celestial mechanics models developed by astronomers and navigators. Prominent observatories, navigation services, and survey institutions commonly convert between this system and equatorial or ecliptic frameworks for scheduling observations, guiding instruments, and producing star charts.
The horizontal coordinate framework ties the observer's location—such as the Mauna Kea Observatories, Greenwich Observatory, or Palomar Observatory—to the apparent sky by projecting the celestial sphere onto a plane tangent at the local horizon. It complements equatorial systems used by figures like Johannes Kepler and Isaac Newton and organizations like the International Astronomical Union and Jet Propulsion Laboratory by providing immediate, intuitive directions for observers at places including New York City, Sydney, and Cape Town. Instruments developed by innovators such as Tycho Brahe, John Flamsteed, and companies like Zeiss and Carl Zeiss AG often report pointing in horizontal coordinates for local alignment.
Altitude (also called elevation) measures the angular distance above the observer's horizon toward the zenith over positions like the Mount Wilson Observatory site; azimuth is the compass direction around the horizon, usually measured from true north via references such as the Prime Meridian at Greenwich. The zenith and nadir are the local vertical directions that link to works by Émilie du Châtelet on classical mechanics and to modern instruments produced by Leica Geosystems used in surveying projects like the Hoover Dam construction. The system depends on the observer's geographic latitude and longitude—coordinates comparable to entries in the datasets of National Geospatial-Intelligence Agency and mapping authorities like Ordnance Survey.
Converting between horizontal coordinates and equatorial coordinates (right ascension and declination) involves spherical trigonometry utilized by mathematicians such as Carl Friedrich Gauss and algorithms implemented by software from NASA, European Space Agency, and projects like Astropy. The conversion requires the observer's sidereal time, which ties to time standards maintained by institutions such as the International Bureau of Weights and Measures and observatories including USNO and Paris Observatory. Transformations incorporate rotation matrices used in spacecraft navigation by SpaceX and Boeing or in astrometry catalogs like those from Hipparcos and Gaia; practitioners reference classical formulae from Spherical trigonometry texts and modern libraries from GNU Scientific Library.
Apparent horizontal positions change continuously due to Earth's diurnal rotation, a phenomenon central to observations recorded at facilities such as Kitt Peak National Observatory and events like the Transit of Venus. Long-term shifts from precession and nutation—characterized in work by James Bradley and refined by the International Astronomical Union—alter the mapping between equatorial and horizontal systems over decades, affecting catalogs from Naval Observatory Vector Astrometry Subroutines and missions like Gaia. Atmospheric refraction near the horizon, studied by researchers at Royal Observatory, Edinburgh and institutions like MIT, further modifies apparent altitude as a function of local pressure and temperature.
In observational astronomy, the system guides targeting for telescopes at sites such as Arecibo Observatory (historically), Very Large Telescope, and amateur setups using mounts by Celestron and Meade Instruments. Celestial navigation—employed historically aboard vessels like HMS Beagle and by mariners referenced in United States Navy manuals—uses altitude and azimuth from sextant sightings and almanacs such as publications by United States Naval Observatory. In surveying and engineering, the horizontal framework appears in theodolite readings for projects managed by firms like Bechtel and agencies like USGS.
The horizontal system is inherently local: an object's coordinates differ between observers in Tokyo versus Buenos Aires and change with time, unlike global catalogs such as Messier catalogue or New General Catalogue entries. Light pollution from urban centers like Los Angeles and London reduces usability near the horizon; atmospheric extinction and refraction, analyzed at meteorological centers like National Oceanic and Atmospheric Administration, impose altitude-dependent errors. Magnetic declination complicates azimuth referencing when using magnetic compasses produced by companies like Fisher Scientific versus true north used by cartographic authorities such as National Geographic Society.
Comparisons often involve equatorial coordinates (right ascension and declination) standardized in catalogs from Hipparcos and Gaia, the ecliptic system used in works by Ptolemy and Johannes Kepler, and the galactic coordinate system applied in studies by Bertil Lindblad and missions like COBE. Transformations among these systems are central to astrometry efforts by International Celestial Reference Frame committees and operational pipelines at observatories including European Southern Observatory and Hubble Space Telescope.
Category:Astronomical coordinate systems