Astrometry is a branch of astronomy that studies stellar motions (i.e., studies the geometric relationships between celestial objects, their motion, and their position). 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.

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 earth’s 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.

We use the concept of parallax to calculate the distance of a star. If we observe a star from Earth and note its position relative to more distant reference stars, and then repeat this measurement 6 months later, when Earth is on the opposite side in its orbit around the Sun, we see that the star appears to have shifted relative to the reference stars. This apparent shift is called parallax, and from it we can derive the distance to the star by making use of elementary geometry. Parallax is a very difficult quantity to measure, because it is smaller the farther away the stars are, and is therefore only measurable for the nearest few hundred stars.

Astrometry is also concerned with measuring the relative velocity of celestial objects. To do this, it is necessary to measure two components of the motion: the radial velocity, which is the speed at which the star is moving away from or approaching us, and the proper motion, which is the speed at which it is moving across the sky.

The radial velocity is easily determined by observing the spectrum of a star. But self-motion is more difficult and requires high-precision measurements of the star’s motion relative to others over many years. Knowing the distance and motion of stars is critical to understanding the Universe. From the distance of a star, it is possible to trace its intrinsic luminosity and size, and thus derive information about its age and composition. Furthermore, if the velocity and direction of motion are known, it becomes possible to know where the stars were in the past and where they will be in the future.

History of astrometry

Ancient civilizations had already understood that celestial objects move in a regular way, and that this can help us to determine positions on the surface of the Earth, and to measure the passage of time. The need to orient themselves in their movements and to determine the most propitious moment to sow or reap constitutes the initial motive for the development of astrometry.

Accurate measurement of the positions of celestial bodies was the fundamental task of astronomers until the nineteenth century, and is still a basic aspect of modern astronomical research. The angles that are sought to be measured are extremely small, and improving the accuracy of measurements is a constant goal of astronomers. The increased precision of measurements was a consequence of the development of new and more advanced observational instruments, and led to fundamental changes in scientific knowledge.

In 129 BC and with the naked eye, the Greek astronomer Hipparchus completed the first catalog with 1000 stars, of which he indicated the apparent brightness and position, the latter with an accuracy of about one degree, that is the angle equivalent to the height of a person at a distance of 100 meters. This event represents the birth of astrometry as a science. After Hipparchus, and until the 16th century, measurement techniques did not progress much. A revolution in this field occurred with Tycho Brahe (1546-1601), a Danish astronomer, who was able to measure the position of the stars with an accuracy of one minute of arc, or one-sixtieth of a degree. Tycho designed, built and calibrated a wide variety of instruments, such as the sextant or the wall quadrant, and profoundly changed the way of observing the stars. It was Tycho’s observations of the orbits of the planets with unprecedented precision that enabled Kepler to discover that the planets move in elliptical orbits.

In 1609 the telescope was invented, and this opened new worlds to human observation. But the telescope by itself did not allow for improved angular measurements. It still took a number of years before a new instrument was invented that could take advantage of the improved capabilities of the telescope but also allowed greater accuracy in measuring angles.

In the 17th century, the invention of the wire micrometer made it possible to overcome the barrier imposed by the limited angular resolution of the eye (about one minute of arc). The wire micrometer consists of two metal wires mounted in the field of view of the telescope. The relative position of the two wires is controlled by turning a screw, and can be read on a graduated scale. From this is traced the angular distance of the two stars on which the wires are positioned.

In the eighteenth century, new knowledge about materials and their processing techniques made it possible to build high-precision angular measurers (astronomical circles). In 1725, thanks to the angular precision of a few seconds of arc allowed by the new technologies, it became possible to measure for the first time the stellar aberration, the first direct measurement of the motion of the Earth in space. This definitively confirmed the controversial Copernican theory that the Earth moved around the Sun and not vice versa. Another important discovery of the eighteenth century was the observation of the motion of stars in space made by Edmund Halley.

In the nineteenth century, engraving techniques improved further and it became possible to measure angles with an accuracy of fractions of a second of arc. This allowed to measure the first stellar parallaxes in about 1830. The confirmation that stars are at very large but finite distances was a fundamental turning point in the understanding of stars and our place in the Universe.

In the 20th century, astronomy is dedicated to understanding the nature of celestial bodies, rather than just measuring their positions. This change is made possible by new techniques such as spectroscopy (which studies starlight to determine its chemical composition, temperature, and nature), and the use of photographic plates. Progress in astrometry in the meantime became very difficult, because the maximum precision obtainable from Earth, about 0.1 arc seconds, had been reached, limited mainly by atmospheric effects.

Things changed in 1989, when the European Space Agency (ESA) put into orbit the first astrometric satellite Hipparcos, which revolutionized our knowledge of stellar distances. Freed from atmospheric limitations, Hipparcos was able to measure the position, distance and motion of nearly 120,000 stars across the sky, with an accuracy of about 1 thousandth of an arc second, 100 times greater than that obtainable from Earth.

Following the success of Hipparcos, ESA has designed Gaia, a much more powerful and advanced astrometric satellite, able to build a three-dimensional dynamic map of our galaxy by measuring the position, distance, and velocities of a billion stars, with an astrometric accuracy 10 to 100 times higher than Hipparcos. For the brightest stars, its accuracy is about 10 microseconds of arc.

Astrometry research areas

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.

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 Einstein’s 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).


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