# Geodetic astronomy

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Using stars to measure Earth

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**Geodetic astronomy** or **astronomical geodesy** (**astro-geodesy**) is the application of [astronomical](/source/Astronomy) methods into [geodetic networks](/source/Geodetic_network) and other technical projects of [geodesy](/source/Geodesy).

## Applications

The most important applications are:

- Establishment of [geodetic datum](/source/Datum_(geodesy)) systems (e.g. [ED50](/source/ED50)) or at expeditions

- [apparent places](/source/Apparent_places) of stars, and their [proper motions](/source/Proper_motion)

- precise [astronomical navigation](/source/Celestial_navigation)

- astro-geodetic [geoid](/source/Geoid) determination

- modelling the rock [densities](/source/Density) of the topography and of [geological](/source/Geological) layers in the [subsurface](/source/Subsurface_(geology))

- Monitoring of the [Earth rotation](/source/Earth_rotation) and polar wandering

- Contribution to the [time system](https://en.wikipedia.org/w/index.php?title=Time_system&action=edit&redlink=1) of physics and [geosciences](/source/Geosciences)

## Measuring techniques

Important measuring techniques are:

- [Latitude determination](/source/Latitude_determination) and [longitude determination](/source/Longitude_determination), by [theodolites](/source/Theodolite), tacheometers, [astrolabes](/source/Astrolabe) or [zenith cameras](/source/Zenith_camera)

- [Time](/source/Time) and [star positions](/source/Star_position) by observation of [star transits](/source/Star_transit), e.g. by [meridian circles](/source/Meridian_circle) (visual, photographic or [CCD](/source/Charge-coupled_device))

- [Azimuth](/source/Azimuth) determination - for the exact orientation of [geodetic networks](/source/Geodetic_network) - for mutual [transformations](/source/Transformation_(mathematics)) between terrestrial and space methods - for improved accuracy by means of "[Laplace points](https://en.wikipedia.org/w/index.php?title=Laplace_point&action=edit&redlink=1)" at special fixed points

- [Vertical deflection determination](/source/Vertical_deflection_determination) and their use - in [geoid determination](/source/Geoid_determination) - in mathematical [reduction](/source/Reduction_(mathematics)) of very precise networks - for geophysical and [geological](/source/Geological) purposes (see above)

- Modern [spatial](/source/Space) methods - [VLBI](/source/VLBI) with radio sources ([quasars](/source/Quasar)) - [Astrometry](/source/Astrometry) of stars by scanning satellites like [Hipparcos](/source/Hipparcos) or the future [Gaia](/source/Gaia_probe).

The [accuracy](/source/Accuracy) of these methods depends on the [instrument](/source/Measuring_instrument) and its spectral wavelength, the measuring or scanning method, the time amount (versus economy), the [atmospheric](/source/Atmosphere) situation, the stability of the surface resp. the satellite, on mechanical and [temperature](/source/Temperature) effects to the instrument, on the experience and skill of the [observer](/source/Observation), and on the accuracy of the physical-mathematical [models](/source/Mathematical_model). Changing weather or atmospheric conditions near the observation site can negatively affect [atmospheric refraction](/source/Atmospheric_refraction) in the [zenithal direction](/source/Zenith), referred to as *anomalous* or *zenithal refraction*; anomalous refraction is considered to be the primary source of error in geodetic astronomy deflection data.[1]

Therefore, the accuracy reaches from 60" (navigation, ~1 mile) to 0,001" and better (a few cm; satellites, VLBI), e.g.:

- [angles](/source/Angle) ([vertical](/source/Vertical_direction) deflections and [azimuths](/source/Azimuth)) ±1" up to 0,1"

- geoid determination & height systems ca. 5 cm up to 0,2 cm

- [astronomical](/source/Astrometry) lat/long and star positions ±1" up to 0,01"

- [HIPPARCOS](/source/HIPPARCOS) star positions ±0,001"

- [VLBI](/source/VLBI) quasar positions and [Earth's rotation](/source/Earth's_rotation) poles 0,001 to 0,0001" (cm...mm)

**Astrogeodetic [leveling](/source/Leveling)** is a local [geoid determination](/source/Geoid_determination) method based on [vertical deflection](/source/Vertical_deflection) measurements.[2] Given a starting value at one point, determining the [geoid undulations](/source/Geoid_undulation) for an area becomes a matter for simple [integration](/source/Integral) of vertical deflection, as it represents the horizontal [spatial gradient](/source/Spatial_gradient) of the geoid undulation.[3][4]

## See also

- [Arc measurement](/source/Arc_measurement) — determining the curvature of Earth's surface by comparing astronomical observations to distance measurements

- [Celestial navigation](/source/Celestial_navigation) — determining a ship's position using astronomical observations

- [Satellite geodesy](/source/Satellite_geodesy)

- [Spherical astronomy](/source/Spherical_astronomy)

- [Space geodesy](/source/Space_geodesy)ET

- [Stellar triangulation](/source/Stellar_triangulation)

- [Triangulation (surveying)](/source/Triangulation_(surveying))

- [Zenith camera](/source/Zenith_camera)

## References

1. **[^](#cite_ref-1)** Hirt, Christian; Bürki, Beat (2006). ["Status of Geodetic Astronomy at the Beginning of the 21st Century"](https://ddfe.curtin.edu.au/models/ERTM2160/pdf/Hirt_Buerki2006_status_geodetic_astronomy.pdf) (PDF). *Geodäsie und Geoinformatik der Universität Hannover* (258): 81–99. Retrieved 1 June 2025.

1. **[^](#cite_ref-2)** ["Geoid Evaluation"](https://www.ngs.noaa.gov/research/geopotential-datums/evaluation-dov.shtml). *[National Geodetic Survey](/source/National_Geodetic_Survey)*. [NOAA](/source/NOAA). 2024. Retrieved 1 June 2025.

1. **[^](#cite_ref-3)** Jekeli, Christopher; Kwon, Jay H. (October 2002). ["Geoid profile determination by direct integration of GPS inertial navigation system vector gravimetry"](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2001JB001626). *[Solid Earth](/source/Solid_Earth_(journal))*. **107** (B10): ETG 3-1–ETC 3-10. [doi](/source/Doi_(identifier)):[10.1029/2001JB001626](https://doi.org/10.1029%2F2001JB001626).

1. **[^](#cite_ref-4)** ["Deflection of the vertical"](https://unterm.un.org/unterm2/en/view/UNHQ/08D03A32639D4A5385256B410065E30F). *[UNTERM](/source/UNTERM)*. [United Nations](/source/United_Nations). Retrieved 1 June 2025.

## External links

- Thomson, D. B. ["Introduction to Geodetic Astronomy"](https://www2.unb.ca/gge/Pubs/LN49.pdf) (PDF). Department of Geodesy and Geomatics Engineering University of New Brunswick.

- ["Geodetic Astronomy at NGS: Past and Present"](https://geodesy.noaa.gov/web/science_edu/webinar_series/geodetic-astronomy.shtml). *geodesy.noaa.gov*. NOAA.

[Portals](https://en.wikipedia.org/wiki/Wikipedia:Contents/Portals):
- [Astronomy](https://en.wikipedia.org/wiki/Portal:Astronomy)
- [Stars](https://en.wikipedia.org/wiki/Portal:Stars)
- [Spaceflight](https://en.wikipedia.org/wiki/Portal:Spaceflight)
- [Outer space](https://en.wikipedia.org/wiki/Portal:Outer_space)
- [Solar System](https://en.wikipedia.org/wiki/Portal:Solar_System)

v t e Geodesy Overview Geodesists History Timeline Subfields Cartography Computer cartography Web mapping Earth's orbit Geodetic astronomy Geomatics Gravity of Earth Navigation Photogrammetry Remote Sensing Geopositioning Virtual globe Physical phenomena Chandler wobble Coriolis effect Earth's energy budget Earth's gravity field Geodynamo Gravity of Earth Plate tectonics Precession of the equinoxes Tide Related disciplines Astronomy Geology Geophysics Mathematics Physics Category Commons

Authority control databases International GND National Czech Republic 2 Other NARA Encyclopedia of Modern Ukraine Yale LUX

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Adapted from the Wikipedia article [Geodetic astronomy](https://en.wikipedia.org/wiki/Geodetic_astronomy) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Geodetic_astronomy?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.