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B. The Historical Development of Astrometry

According to a strict definition, astrometry is the branch of astronomy devoted to the detemination of the positions of celestial bodies, and is therefore also known as positional astronomy. Position measurements, however, naturally lead to the direct determination of quantities such as the motions, distances and dimensions of the observed objects, which are needed for many astrophysical investigations. In particular, distance measurements are useful in calibrating the extragalactic distance scale and can be used in conjunction with photometric measurements to estimate luminosities. For this reason, astrometry has in time also come to indicate the measurement of these quantities. In very general terms, anything in the universe which is somehow distributed, moves or has a dimension or shape accessible to measurement is within the domain of astrometry. Thanks to its fundamental nature, throughout human history astrometry has repeatedly led to significant changes in our perception of the world ([Hoskin 1999]), and thanks to recent technological advances is likely to continue to do so is the future ([Kovalevsky 1995]). Without any claim to completeness, and considering that GAIA is chiefly an astrometric mission, it is therefore interesting and useful to briefly review the most significant phases of the development of this discipline up until the birth of contemporary astronomy.

In its original form astrometry certainly was the first science practised by man. Long before the invention of writing, archeological records show how the recognition of a pattern in the Sun's, the other stars' and the Moon's apparent motions deeply impressed our ancestors, who used to keep track of time by systematically observing the sky. Even greater must have been the sensation and the interest caused by the seemingly irregular apparent motions of the planets, to which some religious meaning was generally ascribed.

Although sophisticated astronomies also developed elsewhere, e.g. in China and in the Americas, historically astronomy as we know it today emerged in the Near East and in Europe, at the time of the Babylonians and the Greeks, respectively. The approach to astronomy by these two peoples was remarkably different, in that while the Babylonians attached great importance to the accurate determination and prediction of the motions of the planets, the Greek tradition, a result of an intense mathematical and philosophical activity, was particularly committed to develop a geometrical model of the universe describing these motions. The achievements of Greek astronomy include e.g. the proof of Earth's sphericity by the Pythagorean philosophers and the accurate determination of Earth's radius by Eratosthenes.

Later, the conquests of Alexander the Great and the beginning of the Hellenistic era caused these two complementary approaches to merge, giving rise to an astronomy in which the models' predictions were routinely compared with observations, much like in the modern scientific method. Around the middle of the the second century BC Hipparchus, arguably the greatest of ancient astronomers, determined the Earth-Moon distance by measuring the Moon's parallax, discovered the precession of the equinoxes and compiled the first star catalogue of which we have an historical record, containing about 1000 stars divided in six classes of brightness, or magnitudes. However, since the original written materials by these civilizations are very limited, most of what we know about Hellenistic astronomy, including Hipparchus' results, has come down to us thanks to Ptolemy's Almagest, a remarkable work of synthesis written in the second century AD and profoundly influenced by the Aristotelean view of the world.

Notwithstanding these accomplishments, the complicated models of Hellenistic astronomers could not predict the long-term motions of planets with an acceptable accuracy, and as a matter of fact the goal of a model capable to do so was not attained until the seventeenth century. Several factors led to this long standstill. From an observational vantage point, the poor accuracy of the available instruments did not allow observers to measure what was then considered, provided that the stars were at a finite distance, the only possible direct proof of the Earth's motion around the Sun, namely the stars' parallaxes. Crucial was then the abiding influence on western thought throughout the Middle Ages of the Aristotelian-Ptolemaic Weltanschauung, which was summarized in the Almagest and supported by the Catholic Church. According to this view, the Earth was a spherical body at rest at the center of the universe, the ``fixed stars'' described uniform circular motions around it, and the seven ``wandering stars'' (the Sun, the Moon and the five then-known planets) followed combinations of uniform circular motions. Since the true non-uniform elliptical motions taking place in the Solar System cannot be described in these terms, in order to improve the accuracy of their predictions, the geometric models started to get unbelievably complicated. Furthermore, these models also predicted some phenomena, such as a strong variation in the Moon's apparent size, which were not observed.

Despite these inconsistencies, the first heliocentric mathematical model of the Solar System was only developed in the first half of the sixteenth century by Nicolaus Copernicus. Although it did not significantly improve the accuracy of predictions, nor abandoned the assumption of uniform circular motions, his model was fundamental in casting doubt on the geocentric prejudice.

In the second half of the sixteenth century, in order to provide the first observational test between the two cosmologies, Tycho Brahe constructed a whole new range of instruments and carried out a long and intense programme of observations of stellar and planetary positions, achieving an accuracy of about 1 arcmin, close to the resolution limit of the naked eye. Even with the aid of this superb instrumentation, however, he could not detect the stars' parallaxes, the apparent annual movements that were to be expected if they were being observed from a moving Earth, and thus concluded that the Earth was actually at rest.

On the contrary, through a careful analysis of Tycho's very data, Johannes Kepler was soon able to derive the three laws that now bear his name, which described the planetary orbits in terms of non-uniform elliptical motions in the framework of an heliocentric cosmology. Still, such sophisticated mathematical relationships were difficult to verify on the basis of the available observations, nor they were strictly valid owing to planetary perturbations, so that at first their strength was more in their formal elegance than in the accuracy of their predictions.

At the same time, the introduction of the spyglass for use in astronomical observations by Galileo Galilei greatly expanded the range of celestial phenomena that could be object of quantitative study. Thanks to a substantial increase in both resolution and sensitivity with respect to the naked eye, Galileo was able to observe for the first time the Moon's maria, Venus' phases, the sunspots, Jupiter's Medicean satellites and Saturn's rings and to resolve the Milky Way into a swarm of faint stars.

As further evidence of the universality of orbital motions throughout the Solar System and beyond was being gathered, a shift from a kynematical to a dynamical interpretation of the observations gradually took place. Towards the end on the seventeenth century, Isaac Newton finally managed to put observational evidence and Kepler's laws together in a coherent fully-general picture, in so doing laying the foundations of differential calculus as well as modern physics. The development of the mathematical techniques that were necessary to calculate in detail the predictions of his fundamental principles of dymanics and law of universal gravitation gave rise to celestial mechanics, which through the eighteenth and nineteenth century would have been the central problem in theoretical astronomy.

As for the observations, now that the Earth was firmly believed to orbit the Sun, the main problem was the determination of the stars' parallaxes, which was particularly difficult owing to the poorly understood atmospheric refraction as well as to other as yet unknown phenomena. In the course of this long-standing struggle, astronomers discovered most optical effects affecting the observations and inaugurated several research fields which now are the astrometrists' main objects of study. In the 1670s, by observing the eclipses of Jupiter's satellites, Ole Rømer showed that the speed of light was finite. In 1718, Edmond Halley measured for the first time the proper motions of three of the brightest stars in the sky, by comparing modern positions with those measured by Hipparchus. Around 1730, while he was trying to determine a star's parallax, James Bradley discovered the aberration of starlight and the nutation of Earth's rotation axis. The first effect, in particular, given the finite speed of light, provided an independent proof of the Earth's motion around the Sun. In 1783 William Herschel found indications that the Solar System as a whole is travelling in the direction of the Hercules constellation. In 1802 William Herschel discovered that several of the couples of stars that are observed to lie near to each other in the sky (double stars) are in fact orbiting a common center, and thus are pairs of physically connected companions (binary stars), bound together by gravitational forceB.1. Finally, in 1838 Friedrich Wilhelm Bessel, followed a few weeks later by Thomas Anderson, managed to measure the parallax of a star.

In summary, until the end of the nineteenth century, all astronomical observations were directed towards obtaining positions and brightnesses of as many celestial bodies as accurately as possible, and thus were astrometric in nature. In the last century, however, the development of physical astronomy, or astrophysics, which has its characteristic tool in spectroscopic measurements, attracted more interest and generated more excitement, so that sometimes one has the mistaken impression that astrometry is old-fashioned or even unimportant. Such an opinion is remarkably wrong not only because positions, parallaxes, proper motions, masses and radii of stars, which can only be obtained through astrometric techniques, are fundamental quantities in many domains of astrophysics, but also because the whole subject, thanks to the development of new techniques, has recently undergone a renaissance. In particular, the advent of astronomical satellites has significantly extended the observable wavelength region and increased the resolution of several orders of magnitude, bringing to the clarification of long-standing problems as well as to the discovery of numerous phenomena that now characterize contemporary astronomy. Even so, due to budgetary reasons most astrometric observations will still be carried out from the ground, and great care must therefore be given to the improvement of existing techniques for ground-based observations as well as to the development of new ones.

next up previous contents
Next: C. ESA and Space Up: thesis Previous: A.2 Photometric Quantities   Contents
Mattia Vaccari 2000-12-05