Astrometry is the science which deals with the positions and motions of celestial objects. Astrometry is now one of many fields of research within astronomy
Measurements of distances to celestial objects by triangulation for example is at the core of astrometry and it forms the basis of all astrophysics; without knowing the distances to planets, satellites, stars, and galaxies, no correct understanding of the cosmos in which we live can be achieved.
It 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 Way.
History
The history of astrometry is linked to the history of star catalogues, which gave astronomers reference points for objects in the sky so they could track their movements. This can be dated back to Hipparchus, who around 190 BC used the catalogue of his predecessors Timocharis and Aristillus to discover the earths precession. In doing so, he also developed the brightness scale still in use today.
Hipparchus compiled a catalogue with at least 850 stars and their positions.
Hipparchuss successor, Ptolemy, included a catalogue of 1,022 stars in his work the Almagest, giving their location, coordinates, and brightness.
In 1989, the European Space Agencys Hipparcos satellite took astrometry into orbit, where it could be less affected by mechanical forces of the Earth and optical distortions from its atmosphere. Operated from 1989 to 1993, Hipparcos measured large and small angles on the sky with much greater precision than any previous optical telescopes. During its 4-year run, the positions, parallaxes, and proper motions of 118,218 stars were determined with an unprecedented degree of accuracy.
A new "Tycho catalog" drew together a database of 1,058,332 to within 20-30 mas (milliarcseconds). Additional catalogues were compiled for the 23,882 double/multiple stars and 11,597 variable stars also analyzed during the Hipparcos mission.
As of 2014, the catalogue most often used is USNO-B1.0, an all-sky catalogue that tracks proper motions, positions, magnitudes and other characteristics for over one billion stellar objects. During the past 50 years, 7,435 Schmidt camera plates were used to complete several sky surveys that make the data in USNO-B1.0 accurate to within 0.2 arcsec
Gaia - An astrometry mission
Astrometry is about measuring angles, dealing with errors in angular measures and changes of angles with time (angular velocity), and derivation of astrophysical quantities from those measurements. A full circle can be divided into 360 degrees. A degree is subdivided into 60 arcminutes (arcmin) and 1 arcmin equals 60 arcseconds (arcsec). The full moon in the sky substends an angle of about 1/2 degree or 30 arcmin as seen from earth. The smallest angular separations or resolutions seen through an ordinary telescope on the ground is about 1 arcsec, limited by the turbulence of earths atmosphere. Progress in astrometry in the recent decade called for smaller angular units. A milliarcsecond (mas) is 1/1000 arcsec and a microarcsecond is 1/1000 mas. The diameter of a large coin as seen from a distance of about 6000 km (New York to London) corresponds to an angle of 1 mas.
Applications
Apart from the fundamental function of providing astronomers with a reference frame to report their observations in, astrometry is also fundamental for fields like celestial mechanics, stellar dynamics and galactic astronomy. In observational astronomy, astrometric techniques help identify stellar objects by their unique motions. It is instrumental for keeping time, in that UTC is basically the atomic time synchronized to Earths rotation by means of exact observations. Astrometry is an important step in the cosmic distance ladder because it establishes parallax distance estimates for stars in the Milky Way.
Astrometry has also been used to support claims of extrasolar planet detection by measuring the displacement the proposed planets cause in their parent stars apparent position on the sky, due to their mutual orbit around the center of mass of the system. Although, as of 2009, none of the extrasolar planets detected by ground-based astrometry has been verified in subsequent studies, astrometry is expected to be more accurate in space missions that are not affected by the distorting effects of the Earths atmosphere.
NASAs planned Space Interferometry Mission (SIM PlanetQuest) (now cancelled) was to utilize astrometric techniques to detect terrestrial planets orbiting 200 or so of the nearest solar-type stars, and the European Space Agencys Gaia Mission, launched in 2013, which will be applying astrometric techniques in its stellar census.
Astrometric measurements are used by astrophysicists to constrain certain models in celestial mechanics. By measuring the velocities of pulsars, it is possible to put a limit on the asymmetry of supernova explosions. Also, astrometric results are used to determine the distribution of dark matter in the galaxy.
Astronomers use astrometric techniques for the tracking of near-Earth objects. Astrometry is responsible for the detection of many record-breaking solar system objects. To find such objects astrometrically, astronomers use telescopes to survey the sky and large-area cameras to take pictures at various determined intervals.
By studying these images, they can detect solar system objects by their movements relative to the background stars, which remain fixed. Once a movement per unit time is observed, astronomers compensate for the parallax caused by the earth’s motion during this time and the heliocentric distance to this object is calculated. Using this distance and other photographs, more information about the object, including its orbital elements, can be obtained.
50000 Quaoar and 90377 Sedna are two solar system objects discovered in this way by Michael E. Brown and others at Caltech using the Palomar Observatorys Samuel Oschin telescope of 48 inches (1.2 m) and the Palomar-Quest large-area CCD camera. The ability of astronomers to track the positions and movements of such celestial bodies is crucial to the understanding of our Solar System and its interrelated past, present, and future with others in our Universe
The area of research in astrometry can be divided by technique or objects of study. Looking at the electromagnetic spectrum there are 2 distinct "windows" for ground-based observations. Radio astrometry uses large radio telescopes and interferometric techniques, while various optical and near-infrared astrometry techniques use more or less traditional telescopes. Another way to categorize astrometry is the differentiation between wide-angle, absolute and narrow-angle, differential observations. Both categories can be found in either radio or optical astrometry. Wide-angle observations often contribute to defining a reference frame or obtaining absolute proper motions and parallaxes. Narrow-field observations are typically even more precise than wide-angle measurements, however they obtain only relative positions and motions in narrow fields of view.
Some of the important techniques used in astrometry today are a) interferometry at radio and optical wavelengths, including VLBI (see above), and the fine guidance sensors aboard the Hubble Space Telescope, b) speckle interferometry for double star observations, c) direct imaging onto 2-dimensional detectors like the charge-coupled device (CCD) and measuring photographic plates for early epoch data, and d) drift scanning by ground-based and space-based instruments.
Microlensing, Astrometry and Other Methods
Objects of research in astrometry range from monitoring the rotation of the earth and continental drifts, motions of solar system objects (planets, natural and artificial satellites, space navigation, asteroids), and the prediction of their future positions (ephemerides), to the kinematical and dynamical studies of our Milky Way galaxy and beyond. For practical applications by the general astronomical community the Hipparcos stars are too bright and too few and far apart. The densification of the optical reference frame toward more and fainter stars is a subject of star cataloging astrometry. Trigonometric distance measures (parallaxes) are the basis for the entire cosmic distance scale and our knowledge about types of stars (giants and dwarfs) and their absolute luminosities. Observations of the motions of star clusters and satellite galaxies around our Galaxy give insight into the distribution of matter, in particular, dark matter is a hot topic today.
Finally, astrometry overlaps with theoretical astrophysics and cosmology beyond local investigations of the distribution of dark matter. Historically astrometry provided the critical empirical evidence in support of Einsteins general theory of relativity by directly measuring the bending of light near a massive body (total solar eclipse observations) and the observation of perihelion motion of the planet Mercury. Astrometry will soon engage in further testing of the theory of gravitation by experiments of ever higher accuracy, e.g. with the Gaia and SIM space missions (see below).
Internationally, astrometric research is represented by Commission 8 (Astrometry) of the IAU. Astrometry ties also into celestial mechanics (IAU Commission 7), the system of astronomical constants and time (Com. 31), ephemerides (Com. 4), galactic structure (Com. 33), double stars (Com. 26), and others. Most of these commissions are grouped together under the IAU Division I (Fundamental Astronomy).