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The Alexandrian School and the Ptolemaic System
The apex of Greek astronomy was reached in the Hellenistic period by the Alexandrian school. Aristarchus (c.310–c.230 B.C.) determined the sizes and distances of the moon and sun relative to the earth and advocated a heliocentric (sun-centered) cosmology. Although there were errors in his assumptions, his approach was truly scientific; his work was the first serious attempt to make a scale model of the universe. The first accurate measurement of the actual (as opposed to relative) size of the earth was made by Eratosthenes (284–192 B.C.). His method was based on the angular difference in the sun's position at the high noon of the summer solstice in two cities whose distance apart was known.
The greatest astronomer of antiquity was Hipparchus (190–120 B.C.). He developed trigonometry and used it to determine astronomical distances from the observed angular positions of celestial bodies. He recognized that astronomy requires accurate and systematic observations extended over long time periods. He therefore made great use of old observations, comparing them to his own. Many of his observations, particularly of the planets, were intended for future astronomers. He devised a geocentric system of cycles and epicycles (a compounding of circular motions) to account for the movements of the sun and moon.
Ptolemy (A.D. 85–165) applied the scheme of epicycles to the planets as well. The resulting Ptolemaic system was a geometrical representation of the solar system that predicted the motions of the planets with considerable accuracy. Among his other achievements was an accurate measurement of the distance to the moon by a parallax technique. His 13-volume treatise, the Almagest, summarized much of ancient astronomical knowledge and, in many translations, was the definitive authority for the next 14 centuries.
Development of Modern Astronomy
The Copernican Revolution
Brahe and Kepler
Isaac Newton (1642–1727), possibly the greatest scientific genius of all time, succeeded in uniting the sciences of astronomy and physics. His laws of motion and theory of universal gravitation provided a physical, dynamic basis for the merely descriptive laws of Kepler. Until well into the 19th cent., all progress in astronomy was essentially an extension of Newton's work. Edmond Halley's prediction that the comet of 1682 would return in 1758 was refined by A. C. Clairault, who included the perturbing effects of Jupiter and Saturn on the orbit to calculate the nearly exact date of the return of the comet. In 1781, William Herschel accidentally discovered a new planet, eventually named Uranus. Discrepancies between the observed and theoretical orbits of Uranus indicated the existence of a still more distant planet that was affecting Uranus's motion. J. C. Adams and U. J. J. Le Verrier independently calculated the position where the new planet, Neptune, was actually discovered (1846). Similar calculations for a large “Planet X” led in 1930 to the discovery of Pluto, now classed as a dwarf planet.
By the early 19th cent., the science of celestial mechanics had reached a highly developed state at the hands of Leonhard Euler, J. L. Lagrange, P. S. Laplace, and others. Powerful new mathematical techniques allowed solution of most of the remaining problems in classical gravitational theory as applied to the solar system. In 1801, Giuseppe Piazzi discovered Ceres, the first of many asteroids. When Ceres was lost to view, C. F. Gauss applied the advanced gravitational techniques to compute the position where the asteroid was subsequently rediscovered. In 1838, F. W. Bessel made the first measurement of the distance to a star; using the method of parallax with the earth's orbit as a baseline, he determined the distance of the star 61 Cygni to be 60 trillion mi (about 10 light-years), a figure later shown to be 40% too large.
Modern Techniques, Discoveries, and Theories
Astronomy was revolutionized in the second half of the 19th cent. by the introduction of techniques based on photography and spectroscopy. Interest shifted from determining the positions and distances of stars to studying their physical composition (see stellar structure and stellar evolution). The dark lines in the solar spectrum that had been observed by W. H. Wollaston and Joseph von Fraunhofer were interpreted in an elementary fashion by G. R. Kirchhoff on the basis of classical physics, although a complete explanation came only with the quantum theory. Between 1911 and 1913, Ejnar Hertzsprung and H. N. Russell studied the relation between the colors and luminosities of typical stars (see Hertzsprung-Russell diagram). With the construction of ever more powerful telescopes (see observatory), the boundaries of the known universe constantly increased. E. P. Hubble's study of the distant galaxies led him to conclude that the universe is expanding (see Hubble's law). Using Cepheid variables as distance indicators, Harlow Shapley determined the size and shape of our galaxy, the Milky Way. During World War II Walter Baade defined two “populations” of stars, and suggested that an examination of these different types might trace the spiral shape of our own galaxy (see stellar populations). In 1951 a Yerkes Observatory group led by William W. Morgan detected evidence of two spiral arms in the Milky Way galaxy.
Various rival theories of the origin and overall structure of the universe, e.g., the big bang and steady state theories, have been formulated (see cosmology). Albert Einstein's theory of relativity plays a central role in all modern cosmological theories. In 1963, the moon passed in front of the radio source 3C-273, allowing Cyril Hazard to calculate the exact position of the source. With this information, Maarten Schmidt photographed the object's spectrum using the 200-in. (5-m) reflector on Palomar Mt., then the world's largest telescope. He interpreted the result as coming from an object, now known as a quasar, at an extreme distance and receding from us at a substantial fraction of the speed of light. In 1967 Antony Hewish and Jocelyn Bell Burnell discovered a radio source a few hundred light years away featuring regular pulses at intervals of about 1 second with an accuracy of repetition of one-millionth of a second. This was the first discovered pulsar, a rapidly spinning neutron star emitting lighthouse-type beams of energy, the end result of the death of a star in a supernova explosion.
The discovery by Karl Jansky in 1931 that radio signals were emitted by celestial bodies initiated the science of radio astronomy. Most recently, the frontiers of astronomy have been expanded by space exploration. Perturbations and interference from the earth's atmosphere make space-based observations necessary for infrared, ultraviolet, gamma-ray, and X-ray astronomy. The Surveyor and Apollo spacecraft of the late 1960s and early 1970s helped launch the new field of astrogeology. A series of interplanetary probes, such as Mariner 2 (1962) and 5 (1967) to Venus, Mariner 4 (1965) and 6 (1969) to Mars, and Voyager 1 (1979) and 2 (1979), provided a wealth of data about Jupiter, Saturn, Uranus, and Neptune; more recently, the Magellan probe to Venus (1990) and the Galileo probe to Jupiter (1995) have continued this line of research (see satellite, artificial; space probe). The Hubble Space Telescope, launched in 1990, has made possible visual observations of a quality far exceeding those of earthbound instruments.
See A. Berry, Short History of Astronomy (1961); J. L. Dreyer, History of Astronomy from Thales to Kepler (2d ed. 1953); A. Koyré, The Astronomical Revolution (1973); P. Maffei, Beyond the Moon (1978); P. Moore, ed. The International Encyclopedia of Astronomy (1987); S. Maran, ed., The Astronomy and Astrophysics Encyclopedia (1991); C. Peterson and J. C. Brandt, Astronomy with the Hubble Space Telescope (1995).
astrometry(ă-strom -ĕ-tree) (positional astronomy) The branch of astronomy concerned with the measurement of the positions of celestial bodies in the sky, with the factors, such as precession, nutation, and proper motion, that cause the positions to change with time, and with the concepts and observational methods involved in making the measurements. In photographic astrometry the positions of stars, planets, etc., are determined in relation to reference stars on photographic plates. In meridian astrometry the positions of celestial bodies are determined relative to the motions of the Earth. See also long-focus photographic astrometry; radio astrometry.
a branch of astronomy; its task is to construct a basic inertial coordinate system for astronomical measurements in conjunction with other branches of astronomy—celestial mechanics and stellar astronomy—and to determine the exact locations and motions of various celestial objects from observations. One of the tasks of astrometry is to study the rotation of the earth, including polar motion (latitude service) and irregularities of rotation, including the problem of calculating time (time service). The methods of astrometry are used to measure the parallaxes and angular diameters of celestial objects and the dimensions and locations of their surface details. Of great importance in astrometry are instrument-methodology questions concerning the development of improved methods of observation and new instrument designs and the detailed study of instruments and various factors influencing the accuracy of measurements, such as thermal gradients and atmospheric refraction. Astrometry also includes spherical astronomy, which investigates mathematical methods of studying the observable distribution and motion of celestial objects, and practical astronomy, which studies the methods and instruments needed for determining time, geographical coordinates, and azimuthal directions on the earth. In the 1950’s and 1960’s new tasks arose as a result of progress in space research: determination of the coordinates of objects that move quickly across the sky (artificial satellites), astrometric measurements from spacecraft and the surface of the moon, the location of artificial satellites and space probes, the location of objects on the moon, the planets, and so forth. The results of astrometric work are widely used in other branches of astronomy—celestial mechanics, astrophysics, and stellar astronomy—as well as in geodesy and geophysics.
The fundamental task of astrometry includes the compilation of catalogs of thepositions and proper motions of stars and the determination of the values of astronomical constants. The classical method of determining the coordinates of celestial bodies involves observing their transit across the meridian with the aid of the transit instrument, vertical circle, or meridian circle. The right ascensions of celestial bodies are determined from the time of their transit, and their declinations from their zenith distances. The reference coordinates (vernal equinox) are determined from observations of the sun and planets. During analysis, results of observations are freed from the influence of the bending of the light rays as they pass through the atmosphere (refraction), from the motion of the earth’s axis in space caused by the gravitational attraction of the sun and the moon (precession and nutation), from the effect resulting from the relative motion of the celestial object and the observer (aberration of light), from changes in latitude resulting from the motion of the earth’s poles, and from various instrumental errors, personal errors of the observer, and so forth. There are absolute, or independent, determinations of coordinates, for which all necessary data—azimuth of the instrument, zero-point of its circle, latitude, refraction constant, and others—are obtained from observations; there are also relative, or differential, determinations of coordinates, which consist in measuring the coordinates of celestial bodies with respect to reference stars, whose exact locations are taken from a catalog. Measurements of coordinates made by refractors with position micrometers and photographic determinations are of the differential type.
The results of determining the coordinates of stars are published in the form of star catalogs. In view of the impossibility of fully taking into account all factors that influence results of observations, star catalogs are burdened with systematic errors which are detected through comparison with other star catalogs. Each absolute catalog (obtained from absolute observations) specifies an independent coordinate system. The accuracy of determining star coordinates is characterized by the probable error of one observation, which in the mid-20th century is close to ± 0.3” arc of the great circle. The chief task of fundamental astrometry is to construct a basic system of celestial coordinates which takes the form of a fundamental star catalog with the most accurate positions and proper motions of selected, so-called fundamental, stars. This problem is being solved by a collective revision of many catalogs, for the most part absolute catalogs, which were compiled by various observatories. Contemporary fundamental catalogs contain star coordinates with probable errors no greater than ± 0.1”. The apparent and average star positions from fundamental catalogs, calculated for each day of every year, are published in astronomical annuals.
The determination of the proper motions of stars is one of the most intricate problems of astrometry because of the slow transit of the stars across the sky; for most stars, it is less than 0.01” per year. The proper motions are usually determined by comparing the coordinates of stars in new and old catalogs, reduced to one system; however, the result is greatly affected by errors in the catalogs. More precise values for the proper motions are obtained through photographic methods by comparing photographs of a certain part of the sky taken with a given instrument at intervals of several decades. The calculation of absolute proper motions also takes into account the motions of reference stars. In the 1940’s in the USSR work began on determining the absolute motion of stars by way of their astrometric link to remote galaxies which are located millions of parsecs from us and are practically motionless in the sky.
Study of the rotation and motion of the earth’s poles in astrometry is based on the precise data of geographical latitude and time. As early as the end of the 18th century, L. Euler concluded that if the earth’s axis of rotation did not coincide with one of the axes of its inertial ellipsoid, then the axis must move within the earth in a conical path, causing periodic changes in the geographical coordinates of points on the earth’s surface. Later, this phenomenon was verified by astronomical observations, during which it was also discovered that there existed a small annual wave in the motion of the earth’s axis of rotation caused by changes in the earth’s moments of inertia owing to seasonal displacements of masses (mainly, air) on its surface. The International Latitude Service was established at the end of the 19th century for the detailed study of this phenomenon, which is governed by the earth’s internal structure. This service, which was later reorganized into the International Polar Motion Service, established a number of stations, including one in Russia (presently, in Kitab). Study of the changes in latitude and polar motion is also regularly conducted at observatories in Pul-kovo and Poltava (USSR), Greenwich (England), Paris (France), Washington (USA), and other sites.
Around the middle of the 20th century, it was finally established that the period of rotation of the earth around its axis is not strictly constant. There are three types of irregularities: (1) a slow, age-old retardation of rotation, arising mainly from tidal friction in the oceans (the length of the day increases by about 0.001 sec per century); (2) anomalous, sometimes erratic, fluctuations that change the length of the day by periods of up to 0.005 sec, the source of which has not been established yet; and (3) periodic seasonal variations in the length of the day of up to 0.001 sec, resulting mainly from atmospheric circulation. The first two phenomena were discovered through the study of the motion of the moon over an extended period, specifically through the analysis of deviations from the theoretical times of solar and lunar eclipses observed in antiquity. Seasonal variations in the earth’s rotation were established by comparing the astronomical determinations of time with those given by quartz and later by atomic clocks. Thus, it was discovered that global time, which is based on the period of rotation of the earth, is not uniform. Since a uniform system of time computation is essential in various scientific problems, including the study of the motion of celestial bodies and the prediction of their positions (ephemeris), in 1950 the concepts of ephemeris time and atomic time were introduced. Ephemeris time is based on the revolution of the earth around the sun and is determined from lunar observations; atomic time is based on molecular and atomic standards of frequency. As a result of these concepts, regular observation of the moon and the most accurate determination of astronomical times from the stars became particularly important in astrometry. Photographic methods were introduced and used with classical meridian observations to determine the positions of the moon. The most accurate determinations of time from the stars (with errors of less than ± 0.01 sec) are made with the aid of photoelectrical transit instruments and also with photographic zenith tubes and prismatic astrolabes. Work on the accurate determination of time, which is being carried out in various countries, is coordinated by the International Time Bureau located in Paris. In the USSR there is a Soviet Time Service, headed by the Committee of Standards, Measures, and Measuring Devices under the Council of Ministries of the USSR.
The results of astrometric observations serve as material for determining the system of astronomical constants. The refinement of the precession constant, the determination of the direction and velocity of the sun’s motion among the stars, and the determination of the parameters of the galaxy’srotation are made through the statistical processing of the proper motions of stars and also their radial velocity. The nutation constant is determined by analyzing long-term latitude observations. The sun’s parallax and the related astronomical unit and aberration constant were determined by astrometric methods until the middle of the 20th century. However, since 1960 they have been calculated with much greater accuracy by radar observations of planets.
Astrometry is the most ancient branch of astronomy. Star catalogs were prepared in China as early as the fourth century B.C. by Shih Shen. The ancient Greek astronomer Hipparchus discovered the phenomenon of precession and prepared a catalog of 1,022 stars which became part of Ptolemy’s astronomical treatise the Almagest. In the 15th century these stars were once again observed by Ulug Beg at the observatory near Samarkand. The most accurate observations by the naked eye were made in the 16th century by Tycho Brahe of Denmark at the Uraniborg Observatory and in the 17th century by J. Hevelius in Gdansk (Poland). Tycho’s observations served as the material on the basis of which the German astronomer J. Kepler derived the laws of planetary motion. Modern astrometry is considered to have originated with the work of the Greenwich Astronomical Observatory, where in the first half of the 18th century J. Bradley (England) discovered aberration of light and nutation of the earth’s axis and conducted observations of 3,268 stars with the transit instrument and mural quadrant. The catalog compiled later from Bradley’s observations played a major role in determining the precession constant and in studying the proper motions of stars. Significant contributions to astrometry were made by the German astronomer F. Bessel, who proposed rational methods for processing observations and for studying instruments. A new period in astrometry was begun by the work at the Pulkovo Observatory (presently, the Central Astronomical Observatory of the Academy of Sciences of the USSR), established in 1839. Owing to the concern of its founder, V. Ia. Struve, the observatory was equipped from the very beginning with first-class instruments and subsequently received wide renown for the high accuracy of its star catalogs. Major contributions in astrometry in the 19th and 20th centuries were also made by observatories in Germany, France, the USA (Washington), South Africa (Capetown), and other countries. InGermany and the USA, work on the compilation of fundamental catalogs has been conducted since the 1870’s. The fundamental catalogs of the German Astronomical Society (Astronomische Gesellschaft) are considered to be the most accurate. On the recommendation of the International Astronomical Union, beginning in 1940, the third fundamental catalog of the German Astronomical Society (FK3) and, beginning in 1962, the fourth (FK4), were accepted for all astronomical annuals. The American Boss catalog (GC) which contains 33,342 stars, is widely used, especially in stellar astronomy.
A major international undertaking, sponsored around 1870 by the German Astronomical Society, was the compilation of meridian zone catalogs, which included the positions of all stars to the ninth magnitude. About 40 catalogs containing more than 400,000 stars were published. Around 1930 and again around 1960, the stars of the northern sky detailed in these catalogs were observed in Germany through photographic methods using wide-angle astrographs; the proper motions of 270,000 stars were derived. Huge photographic catalogs of the stars were also compiled at Pulkovo (the zones from +70° to the north pole), at the Yale Observatory in the USA (the zones from +30° to -30° and others), and at Capetown (from -30° to the south pole). The Carte du Ciel, the largest international effort to photograph the entire sky using normal astrographs, was organized by French astronomers in 1887. The goal of this undertaking was to prepare a catalog of coordinates for about 3.5 million stars to the 11th stellar magnitude and a star map for those to the 14th magnitude. Numerous catalogs and maps were published for the northern and southern skies. In 1906, the Dutch astronomer J. Kapteyn proposed a “select area” plan, which provided for a detailed study of the various characteristics of many thousands of stars in 206 small areas equally distributed across the entire sky. Based on this plan, the Soviet astronomer A. N. Deich in 1941 completed the study of the motions of 18,000 stars in areas of the sky’s northern hemisphere begun by S. K. Kostinskii, one of the founders of photographic astrometry. Similar works were done in the USA and Great Britain.
In the 1930’s, based on the observations of five Soviet and several foreign observatories, the Catalog of Geodetic Stars was compiled, containing about 3,000 stars of the northern sky to the sixth magnitude. This catalog is widely used by time services and in geodesy. In 1939, Soviet astrometry began a major effort to compile the Fundamental Catalog of Faint Stars using meridian observations of tens of thousands of stars and photographic observations of asteroids and distant galaxies. In the 1950’s this problem was combined with the international efforts of compiling a catalog of about 40,000 reference faint stars located throughout the sky. Major contributions in observing the southern hemisphere were made by the Chilean expedition of the Pulkovo Observatory.
The methods of photographic astrometry are also used to determine the proper motions of stars and the parallaxes of stars, to measure binary stars, to observe planets and asteroids, and to study artificial earth satellites. Parallaxes are determined with the help of the more long-focusing astro-graphs (focal lengths of from 7 to 19 m); this work is systematically being conducted by observatories in the USA, South Africa, and other countries. Observations of artificial satellites are made using special wide-angle satellite cameras with automatic shutters, which guarantees time exposures with accuracies of 0.001 sec. Synchronous (simultaneous from different points) astrometric observations of high-altitude artificial earth satellites have been conducted since 1961, making it possible to study some geodesy problems in new ways (satellite geodesy).
Today, visual observations on refractors with position micrometers are confined to the measurement of close binary stars in order to study their orbital motion. In this field a major effort was made in the 19th century by the Pulkovo astronomers V. Ia. Struve and O. V. Struve. Micrometric measurements of asteroids and comets relative to reference stars, widely used in the 19th century, and measurements of the lunar disk using the heliometer have been replaced almost everywhere by photographic measurements. Precise measurements of binary stars and of stellar diameters are accomplished with interferometers; this method is also successfully used in radio astronomy for determining the angular dimensions of radio-wave sources. Much work is being done in studying the configuration and libration of the moon and on the measurements of the photographs of the moon’s surface by the Central Astronomical Observatory of the Academy of Sciences of the Ukrainian SSR at Kiev and by the V. P. Engel’gardt Astronomical Observatory near Kazan.
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