{{short description|Using stars to measure Earth}}

{{More citations|date=June 2025}}

'''Geodetic astronomy''' or '''astronomical geodesy''' ('''astro-geodesy''') is the application of [[astronomy|astronomical]] methods into [[geodetic network]]s and other technical projects of [[geodesy]].

== Applications ==

The most important applications are: * Establishment of [[datum (geodesy)|geodetic datum]] systems (e.g. [[ED50]]) or at expeditions * [[apparent places]] of stars, and their [[proper motion]]s * precise [[celestial navigation|astronomical navigation]] * astro-geodetic [[geoid]] determination * modelling the rock [[density|densities]] of the topography and of [[geological]] layers in the [[Subsurface (geology)|subsurface]] * Monitoring of the [[Earth rotation]] and polar wandering * Contribution to the [[time system]] of physics and [[geosciences]]

== Measuring techniques ==

Important measuring techniques are: * [[Latitude determination]] and [[longitude determination]], by [[theodolite]]s, tacheometers, [[astrolabe]]s or [[zenith camera]]s * [[Time]] and [[star position]]s by observation of [[star transit]]s, e.g. by [[meridian circle]]s (visual, photographic or [[Charge-coupled device|CCD]]) * [[Azimuth]] determination ** for the exact orientation of [[geodetic network]]s ** for mutual [[Transformation (mathematics)|transformation]]s between terrestrial and space methods ** for improved accuracy by means of "[[Laplace point]]s" at special fixed points * [[Vertical deflection determination]] and their use ** in [[geoid determination]] ** in mathematical [[Reduction (mathematics)|reduction]] of very precise networks ** for geophysical and [[geological]] purposes (see above) * Modern [[space|spatial]] methods ** [[VLBI]] with radio sources ([[quasar]]s) ** [[Astrometry]] of stars by scanning satellites like [[Hipparcos]] or the future [[Gaia probe|Gaia]].

The [[accuracy]] of these methods depends on the [[measuring instrument|instrument]] and its spectral wavelength, the measuring or scanning method, the time amount (versus economy), the [[atmosphere|atmospheric]] situation, the stability of the surface resp. the satellite, on mechanical and [[temperature]] effects to the instrument, on the experience and skill of the [[observation|observer]], and on the accuracy of the physical-mathematical [[mathematical model|models]]. Changing weather or atmospheric conditions near the observation site can negatively affect [[atmospheric refraction]] in the [[zenith|zenithal direction]], referred to as ''anomalous'' or ''zenithal refraction''; anomalous refraction is considered to be the primary source of error in geodetic astronomy deflection data.<ref>{{cite journal |last1=Hirt |first1=Christian |last2=Bürki |first2=Beat |url=https://ddfe.curtin.edu.au/models/ERTM2160/pdf/Hirt_Buerki2006_status_geodetic_astronomy.pdf |title=Status of Geodetic Astronomy at the Beginning of the 21st Century |journal=Geodäsie und Geoinformatik der Universität Hannover |issue=258 |pages=81–99 |date=2006 |access-date=1 June 2025}}</ref>

Therefore, the accuracy reaches from 60" (navigation, ~1 mile) to 0,001" and better (a few cm; satellites, VLBI), e.g.: * [[angle]]s ([[vertical direction|vertical]] deflections and [[azimuth]]s) ±1" up to 0,1" * geoid determination & height systems ca. 5&nbsp;cm up to 0,2&nbsp;cm * [[astrometry|astronomical]] lat/long and star positions ±1" up to 0,01" * [[HIPPARCOS]] star positions ±0,001" * [[VLBI]] quasar positions and [[Earth's rotation]] poles 0,001 to 0,0001" (cm...mm)

{{anchor|Astrogeodetic leveling|Leveling}}'''Astrogeodetic [[leveling]]''' is a local [[geoid determination]] method based on [[vertical deflection]] measurements.<ref>{{cite web |url=https://www.ngs.noaa.gov/research/geopotential-datums/evaluation-dov.shtml |title=Geoid Evaluation |work=[[National Geodetic Survey]] |publisher=[[NOAA]] |date=2024 |access-date=1 June 2025}}</ref> Given a starting value at one point, determining the [[geoid undulation]]s for an area becomes a matter for simple [[integral|integration]] of vertical deflection, as it represents the horizontal [[spatial gradient]] of the geoid undulation.<ref>{{cite journal |last1=Jekeli |first1=Christopher |last2=Kwon |first2=Jay H. |url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2001JB001626 |title=Geoid profile determination by direct integration of GPS inertial navigation system vector gravimetry |journal=[[Solid Earth (journal)|Solid Earth]] |volume=107 |issue=B10 |pages=ETG 3-1–ETC 3-10 |date=October 2002 |doi=10.1029/2001JB001626|url-access=subscription }}</ref><ref>{{cite web |url=https://unterm.un.org/unterm2/en/view/UNHQ/08D03A32639D4A5385256B410065E30F |title=Deflection of the vertical |work=[[UNTERM]] |publisher=[[United Nations]] |access-date=1 June 2025}}</ref>

== See also ==

* [[Arc measurement]] — determining the curvature of Earth's surface by comparing astronomical observations to distance measurements * [[Celestial navigation]] — determining a ship's position using astronomical observations * [[Satellite geodesy]] * [[Spherical astronomy]] * [[Space geodesy]]ET * [[Stellar triangulation]] * [[Triangulation (surveying)]] * [[Zenith camera]]

== References ==

{{reflist}}

== External links ==

* {{cite web |last1=Thomson |first1=D. B. |title=Introduction to Geodetic Astronomy |url=https://www2.unb.ca/gge/Pubs/LN49.pdf |publisher=Department of Geodesy and Geomatics Engineering University of New Brunswick}} * {{cite web |title=Geodetic Astronomy at NGS: Past and Present |url=https://geodesy.noaa.gov/web/science_edu/webinar_series/geodetic-astronomy.shtml |website=geodesy.noaa.gov |publisher=NOAA |language=EN-US}}

{{portal bar|Astronomy|Stars|Spaceflight|Outer space|Solar System}}{{geodesy navbox|state=uncollapsed}}{{authority control}}

[[category:astronomical sub-disciplines]] [[category:geodesy|astronomy]]