Concept art for theTAU spacecraft, a 1980s era study which would have used an interstellar precursor probe to expand the baseline for calculating stellar parallax in support of Astrometry.
The history of astrometry is linked to the history ofstar catalogues, which gave astronomers reference points for objects in the sky so they could track their movements. This can be dated back to theancient Greek astronomerHipparchus, who around 190 BC used the catalogue of his predecessorsTimocharis andAristillus to discover Earth'sprecession. In doing so, he also developed the brightness scale still in use today.[1] Hipparchus compiled a catalogue with at least 850 stars and their positions.[2] Hipparchus's successor,Ptolemy, included a catalogue of 1,022 stars in his work theAlmagest, giving their location, coordinates, and brightness.[3]
In the 10th century, the Iranian astronomerAbd al-Rahman al-Sufi carried out observations on the stars and described their positions,magnitudes andstar color; furthermore, he provided drawings for each constellation, which are depicted in hisBook of Fixed Stars. Egyptian mathematicianIbn Yunus observed more than 10,000 entries for the Sun's position for many years using a largeastrolabe with a diameter of nearly 1.4 metres. His observations oneclipses were still used centuries later in Canadian–American astronomerSimon Newcomb's investigations on the motion of the Moon, while his other observations of the motions of the planets Jupiter and Saturn inspired French scholarLaplace'sObliquity of the Ecliptic andInequalities of Jupiter and Saturn.[4] In the 15th century, theTimurid astronomerUlugh Beg compiled theZij-i-Sultani, in which he catalogued 1,019 stars. Like the earlier catalogs of Hipparchus and Ptolemy, Ulugh Beg's catalogue is estimated to have been precise to within approximately 20minutes of arc.[5]
Being very difficult to measure, only about 60 stellar parallaxes had been obtained by the end of the 19th century, mostly by use of thefilar micrometer.Astrographs using astronomicalphotographic plates sped the process in the early 20th century. Automated plate-measuring machines[9] and more sophisticated computer technology of the 1960s allowed more efficient compilation ofstar catalogues. Started in the late 19th century, the projectCarte du Ciel to improve star mapping could not be finished but made photography a common technique for astrometry.[10] In the 1980s,charge-coupled devices (CCDs) replaced photographic plates and reduced optical uncertainties to one milliarcsecond. This technology made astrometry less expensive, opening the field to an amateur audience.[citation needed]
In 1989, theEuropean Space Agency'sHipparcos 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, andproper 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 stars to within 20-30mas (milliarcseconds). Additional catalogues were compiled for the 23,882 double and multiple stars and 11,597variable stars also analyzed during the Hipparcos mission.[11]In 2013, theGaia satellite was launched and improved the accuracy ofHipparcos.[12]The precision was improved by a factor of 100 and enabled the mapping of a billion stars.[13]Today, the catalogue most often used isUSNO-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, 7,435Schmidt camera plates were used to complete several sky surveys that make the data in USNO-B1.0 accurate to within 0.2 arcsec.[14]
Diagram showing how a smaller object (such as anextrasolar planet) orbiting a larger object (such as astar) could produce changes in position and velocity of the latter as they orbit their commoncenter of mass (red cross).Motion ofbarycenter of solar system relative to the Sun
Astrometry has also been used to support claims ofextrasolar planet detection by measuring the displacement the proposed planets cause in their parent star's apparent position on the sky, due to their mutual orbit around the center of mass of the system. Astrometry is more accurate in space missions that are not affected by the distorting effects of the Earth's atmosphere.[15] NASA's plannedSpace Interferometry Mission (SIM PlanetQuest) (now cancelled) was to utilize astrometric techniques to detectterrestrial planets orbiting 200 or so of the nearestsolar-type stars. The European Space Agency'sGaia Mission, launched in 2013, applies astrometric techniques in its stellar census. In addition to the detection of exoplanets,[16] it can also be used to determine their mass.[17]
Astrometric measurements are used byastrophysicists to constrain certain models incelestial mechanics. By measuring the velocities ofpulsars, it is possible to put a limit on theasymmetry ofsupernova explosions. Also, astrometric results are used to determine the distribution ofdark matter in the galaxy.
Astronomers use astrometric techniques for the tracking ofnear-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 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 itsorbital elements, can be obtained.[18]Asteroid impact avoidance is among the purposes.
Quaoar andSedna are two trans-Neptuniandwarf planets discovered in this way byMichael E. Brown and others at Caltech using thePalomar Observatory'sSamuel 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 the Solar System and its interrelated past, present, and future with others in the Universe.[19][20]
A fundamental aspect of astrometry is error correction. Various factors introduce errors into the measurement of stellar positions, including atmospheric conditions, imperfections in the instruments and errors by the observer or the measuring instruments. Many of these errors can be reduced by various techniques, such as through instrument improvements and compensations to the data. The results are thenanalyzed usingstatistical methods to compute data estimates and error ranges.[21]
^Walter, Hans G. (2000).Astrometry of fundamental catalogues: the evolution from optical to radio reference frames. New York: Springer.ISBN3-540-67436-5.
^Kanas, Nick (2007).Star maps: history, artistry, and cartography. Springer. p. 109.ISBN978-0-387-71668-8.
