Geodetic astronomy
Encyclopedia
Geodetic astronomy is the application of astronomical
methods into networks and technical projects of geodesy
.
The most important topics are:
Important measuring techniques are:
The accuracy of these methods depends on the instrument
and its spectral wavelength, the measuring or scanning method, the time amount (versus economy), the 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 observer
, and on the accuracy of the physical-mathematical models
.
Therefore the accuracy reaches from 60" (navigation, ~1 mile) to 0,001" and better (a few cm; satellites, VLBI), e.g.:
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
methods into networks and technical projects of geodesy
Geodesy
Geodesy , also named geodetics, a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal...
.
The most important topics are:
- Establishment of geodetic datum systems (e.g. ED50ED50ED 50 is a geodetic datum which was defined after World War II for the international connection of geodetic networks....
) or at expeditions - apparent placesApparent placesThe wrong place of an object is the position in space as seen by the observer. Because of physical and/or geometrical effects it has a deviation from the "true position"....
of stars, and their proper motionProper motionThe proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
s - precise astronomical navigation
- astro-geodetic geoidGeoidThe geoid is that equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest , and extended through the continents . According to C.F...
determination and - modelling the rock densitiesDensityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of the topography and of geological layers in the subsurfaceSubsurfaceSubsurface is the seventh studio album by British progressive metal band Threshold. The album was released in August 2004, and received an Album of the Month award in several European music magazines.... - Satellite geodesySatellite geodesySatellite geodesy is the measurement of the form and dimensions of the Earth, the location of objects on its surface and the figure of the Earth's gravity field by means of artificial satellite techniques—geodesy by means of artificial satellites...
using the background of starStarA star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s (see also astrometryAstrometryAstrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...
and cosmic triangulation) - Monitoring of the Earth rotationEarth rotationEarth's rotation is the rotation of the solid Earth around its own axis. The Earth rotates towards the east. As viewed from the North Star Polaris, the Earth turns counter-clockwise.- Rotation period :...
and polar wandering - Contribution to the time system of physics and geosciences
Important measuring techniques are:
- LatitudeLatitudeIn geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
and longitudeLongitudeLongitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
determination by theodoliteTheodoliteA theodolite is a precision instrument for measuring angles in the horizontal and vertical planes. Theodolites are mainly used for surveying applications, and have been adapted for specialized purposes in fields like metrology and rocket launch technology...
s, tacheometers, astrolabeAstrolabeAn astrolabe is an elaborate inclinometer, historically used by astronomers, navigators, and astrologers. Its many uses include locating and predicting the positions of the Sun, Moon, planets, and stars, determining local time given local latitude and longitude, surveying, triangulation, and to...
s or zenith cameraZenith cameraZenith cameras are astronomic or geodetic instruments which are directed exactly to the zenith. The stars are recorded either by photographic plates or by CCD sensors.The main tasks in using these instruments are:...
s - timeTimeTime is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
and star positionStar positionStar position in the sky is defined by a pair of angles. These two angles - which refer to the celestial equator - are called declination and right ascension ....
s by observation of star transitStar transitA Star transit is the passage of a star through the eyepiece of an telescope.The precise observation of star transits is the basis of many methods in Astronomy and in Geodesy...
s, e.g. by meridian circleMeridian circleThe meridian circle, transit circle, or transit telescope is an instrument for observing the time of stars passing the meridian, at the same time measuring its angular distance from the zenith...
s (visual, photographic or CCDCharge-coupled deviceA charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
) - Azimuth measurements
- for the exact orientation of geodetic networkGeodetic networkA geodetic network is a network of triangles which are measured exactly by techniques of terrestrial surveying or by satellite geodesy.In "classical geodesy" this is done by triangulation, based on measurements of angles and of some spare distances; the precise orientation to the geographic north...
s - for mutual transformationTransformation (mathematics)In mathematics, a transformation could be any function mapping a set X on to another set or on to itself. However, often the set X has some additional algebraic or geometric structure and the term "transformation" refers to a function from X to itself that preserves this structure.Examples include...
s between terrestrial and space methods - for improved accuracy by means of "Laplace points" at special fixed points
- for the exact orientation of geodetic network
- Vertical deflectionVertical deflectionThe vertical deflection at a point on the earth is a measure of how far the direction of the local gravity field has been shifted by local anomalies such as nearby mountains....
measurements and their use- in geoid determination
- in mathematical reductionReduction (mathematics)In mathematics, reduction refers to the rewriting of an expression into a simpler form. For example, the process of rewriting a fraction into one with the smallest whole-number denominator possible is called "reducing a fraction"...
of very precise networks - for geophysical and geological purposes (see above)
- Modern spatial methods
- VLBI with radio sources (quasarQuasarA quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s) - AstrometryAstrometryAstrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...
of stars by scanning satellites like HipparcosHipparcosHipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
or the future GaiaGaia probeGaia is a European Space Agency astrometry space mission, and a successor to the ESA Hipparcos mission. It was included within the context of the ESA Horizon 2000 Plus long-term scientific programme in 2000. Arianespace expects to launch Gaia for the ESA in March 2013, using a Soyuz rocket from...
.
- VLBI with radio sources (quasar
The accuracy of these methods depends on the instrument
Measuring instrument
In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item...
and its spectral wavelength, the measuring or scanning method, the time amount (versus economy), the atmospheric
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
situation, the stability of the surface resp. the satellite, on mechanical and temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
effects to the instrument, on the experience and skill of the observer
Observation
Observation is either an activity of a living being, such as a human, consisting of receiving knowledge of the outside world through the senses, or the recording of data using scientific instruments. The term may also refer to any data collected during this activity...
, and on the accuracy of the physical-mathematical models
Mathematical model
A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used not only in the natural sciences and engineering disciplines A mathematical model is a...
.
Therefore the accuracy reaches from 60" (navigation, ~1 mile) to 0,001" and better (a few cm; satellites, VLBI), e.g.:
- angleAngleIn geometry, an angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle.Angles are usually presumed to be in a Euclidean plane with the circle taken for standard with regard to direction. In fact, an angle is frequently viewed as a measure of an circular arc...
s (verticalVertical directionIn astronomy, geography, geometry and related sciences and contexts, a direction passing by a given point is said to be vertical if it is locally aligned with the gradient of the gravity field, i.e., with the direction of the gravitational force at that point...
deflections and azimuths) ±1" up to 0,1" - geoid determination & height systems ca. 5 cm up to 0,2 cm
- astronomicalAstrometryAstrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...
lat/long and star positions ±1" up to 0,01" - HIPPARCOSHipparcosHipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
star positions ±0,001" - VLBI quasar positions and Earth's rotation poles 0,001 to 0,0001" (cm...mm)
See also
- AstronomyAstronomyAstronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, stellar triangulationStellar triangulationStellar triangulation is a method of geodesy which uses cosmic instead of terrestrial targets. It was first done by the Finnish geodesist Väisälä in 1959, who made astrometric photographs of the sky at two stations together with a balloon probe between them....
, spherical trigonometrySpherical trigonometrySpherical trigonometry is a branch of spherical geometry which deals with polygons on the sphere and the relationships between the sides and the angles... - SatelliteSatelliteIn the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
, space techniquesSpace techniquesIn geodesy, the term Space techniques includes modern measuring methods which make use of artificial satellites, interplanetary space probes and of quasars....
, electro-opticsElectro-opticsElectro-optics is a branch of technology involving components, devices and systems which operate by modification of the optical properties of a material by an electric field...
, CCDCharge-coupled deviceA charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time... - TriangulationTriangulationIn trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...
, tacheometer, passage instrument - Astro navigation, Karl RamsayerKarl RamsayerKarl Ramsayer was a German geodesist and is well known as one of the most important scientists in geodetic astronomy and in electronic navigation....