Astrometry
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 Earth's precession. 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 the Almagest, giving their location, coordinates, and brightness.[3]
astrometry
James Bradley first tried to measure stellar parallaxes in 1729. The stellar movement proved too insignificant for his telescope, but he instead discovered the aberration of light and the nutation of the Earth's axis. His cataloguing of 3222 stars was refined in 1807 by Friedrich Bessel, the father of modern astrometry. He made the first measurement of stellar parallax: 0.3 arcsec for the binary star 61 Cygni.
Being very difficult to measure, only about 60 stellar parallaxes had been obtained by the end of the 19th century, mostly by use of the filar micrometer. Astrographs using astronomical photographic plates sped the process in the early 20th century. Automated plate-measuring machines[8] and more sophisticated computer technology of the 1960s allowed more efficient compilation of star catalogues. Started in the late 19th century, the project Carte du Ciel to improve star mapping couldn't be finished but made photography a common technique for astrometry.[9] 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, the European Space Agency's 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 stars to within 20-30 mas (milliarcseconds). Additional catalogues were compiled for the 23,882 double and multiple stars and 11,597 variable stars also analyzed during the Hipparcos mission.[10]In 2013, the Gaia satellite was launched and improved the accuracy of Hipparcos.[11]The precision was improved by a factor of 100 and enabled the mapping of a billion stars.[12]Today, 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.[13]
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 essentially the atomic time synchronized to Earth's rotation by means of exact astronomical observations. Astrometry is an important step in the cosmic distance ladder because it establishes parallax distance estimates for stars in the Milky Way.
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 then analyzed using statistical methods to compute data estimates and error ranges.[20]
Astrometry is the science (and art!) of precision measurement of stars' locations in the sky. When planet hunters use astrometry, they look for a minute but regular wobble in a star's position compared to the positions of other stars. If such a periodic shift is detected, it is almost certain that the star is being orbited by an unseen companion planet.
Astrometry is the oldest method used to search for extrasolar planets. As early as 1943 astronomer Kaj Strand, working at the Sproul Observatory at Swarthmore College announced that his astrometric measurements revealed the presence of a planet orbiting the star 61 Cygni. Although the announcement was greeted with enthusiasm at the time, the claim has remained unproven and astronomers today are highly skeptical of Strand's results. The tradition of planet hunting through astrometry nevertheless remained strong at Sproul, where Strand's announcement was followed decades later by two other contentious claims. In 1960 Sproul astronomer Sarah Lippincott published a paper claiming that the star Lalande 21185 was orbited by a planet of roughly ten Jupiter masses, and in 1963 the observatory's director, Peter Van de Kamp, announced the discovery of a planet orbiting Barnard's Star.
Astrometry is one of the most sensitive methods for detection of extrasolar planets. Unlike transit photometry, astrometry does not depend on the distant planet being in near-perfect alignment with the line of sight from the Earth, and it can therefore be a applied to a far greater number of stars. Furthermore, unlike the radial velocity method, astrometry provides an accurate estimate of a planet's mass, and not just a minimum figure.
In several of its key characteristics, astrometry is an excellent complement to the spectroscopic method. Whereas spectroscopy works best when a planet's orbital plane is edge-on when observed from Earth, astrometry is most effective when the orbital plane is face-on, or perpendicular to an observer's line of sight. This is because astrometric observations cannot detect a star's displacement towards or away from Earth, as this does not produce any change in the star's position in the sky. Astrometry can only detect that component of a star's wobble that moves it to a different location in the sky - i.e. perpendicular to the line of sight of the Earth-bound observer. The closer a planet's orbital plane is to a face-on position when seen from Earth, the larger the component of its movement that can be astrometrically measured.
Furthermore, whereas spectroscopy is at its best in detecting planets with short periods, orbiting very close to their stars, astrometry will excel in detecting stars of long periods, orbiting further away. This is because a planet with a long orbit causes a greater displacement of its star's location during the course of its orbit than a planet that remains in close proximity to its star. In other words, in contrast to spectroscopy, the sensitivity of astrometric detections actually grows with the increasing distance of a planet from its star. This means that astrometry can, in theory, detect relatively small planets orbiting far from their stars -- a crucial advantage for scientists looking for Earthlike planets rather than the hot Jupiters favored by spectroscopy.
Discovering extrasolar planets through astrometry is extremely hard to do -- so hard that it hasn't yet succeeded. It requires a degree of precision that has seldom been achieved even with the largest and most advanced telescopes.
Even improved accuracy cannot change some fundamental limitations of the astrometric approach. Astrometry, by its very nature, is highly sensitive to the distance of a celestial object from Earth. This is because the same actual displacement in an object's position would appear as a greater change in position in the night sky if that object was close by than if it were further away. Therefore, while astronomers believe that astrometry will be very useful for detecting planets in the solar neighborhood, the method will be far less effective when applied to more distant objects.
Finally, there is an inherent difficulty in observing planets with long periods, the very planets that astrometry should excel in. In order to detect a planet, it is necessary to observe the repeated periodic displacements of its parent star. This means that the star needs to be observed for longer than a single orbital period. When dealing with planets of long periods, comparable to those of our own Solar System, this can obviously be a problem. A star must be observed continuously for years or even decades before the presence or absence of a planet can be established.
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. Historically, astrometry was all that astronomy was about until about the 19th century. Toward the end of the 19th century not only the directions, i.e. angles between celestial objects as seen on the celestial sphere were measured but also the "quality of light", specifically the light intensity (photometry) and color (spectroscopy, light intensity as function of color or wavelength). This was the birth of astrophysics. The term astrophysics, often used to distinguish most of current astronomical research from the classical astronomy (i.e. astrometry) is misleading, because astrometry also is certainly part of physics or astrophysics. 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. 041b061a72