5 Gaia web site: http://sci.esa.int/Gaia site: web Gaia 6 June 2009 June are emerging about the nature of our Galaxy. Galaxy. our of nature the about emerging are More detailed information can be found on the the on found be can information detailed More technologies developed by creative engineers. creative by developed technologies scientists all over the world, and important conclusions conclusions important and world, the over all scientists of the Universe combined with the most cutting-edge cutting-edge most the with combined Universe the of The results from Hipparcos are being analysed by by analysed being are Hipparcos from results The expression of a widespread curiosity about the nature nature the about curiosity widespread a of expression 118218 stars to a precision of around 1 milliarcsecond. milliarcsecond. 1 around of precision a to stars 118218 trying to answer for many centuries. It is the the is It centuries. many for answer to trying created with the positions, distances and motions of of motions and distances positions, the with created will bring light to questions that astronomers have been been have astronomers that questions to light bring will accuracies obtained from the ground. A catalogue was was catalogue A ground. the from obtained accuracies Gaia represents the dream of many generations as it it as generations many of dream the represents Gaia achieving an improvement of about 100 compared to to compared 100 about of improvement an achieving orbit, the Hipparcos satellite observed the whole sky, sky, whole the observed satellite Hipparcos the orbit, ear Y of them in the solar neighbourhood. solar the in them of revolutionised our knowledge of star positions. From its its From positions. star of knowledge our revolutionised search for extrasolar planets by detecting thousands thousands detecting by planets extrasolar for search 150 BC 1600 1800 2000 1800 1600 BC 150 , which has has , which Hipparcos satellite, astrometric . It will also revolutionise the the revolutionise also will It . Way Milky the Galaxy, own (ESA) launched the first first the launched (ESA) Agency Space European understanding the origin and evolution of our our of evolution and origin the understanding science: But things changed for astrometry in 1989, as the the as 1989, in astrometry for changed things But Gaia most challenging yet fundamental questions of modern modern of questions fundamental yet challenging most 0.00001 ambitious and its ultimate aim is to solve one of the the of one solve to is aim ultimate its and ambitious Invention of telescope of Invention arcsecond, limited mostly by atmospheric effects. atmospheric by mostly limited arcsecond, The science case for Gaia is extremely broad and and broad extremely is Gaia for case science The 0.0001 precision obtainable from Earth, of approximately 0.1 0.1 approximately of Earth, from obtainable precision From Hipparchus to Gaia to Hipparchus From very difficult, because it had reached the best best the reached had it because difficult, very ARCOS HIPP 0.001 to occur. Progress in astrometry meanwhile became became meanwhile astrometry in Progress occur. to in astronomy enabled this change change this enabled astronomy in plates photographic 0.01 ASTROMETRY composition, temperature and nature) and the use of of use the and nature) and temperature composition, Gaia arcseconds chemical their determine to objects by emitted Late 20th century 20th Late 0.1 (which studies the light light the studies (which spectroscopy like techniques instead of only measuring their position. New New position. their measuring only of instead HISTORY OF OF HISTORY 1 19th century 19th on learning more about the nature of celestial objects objects celestial of nature the about more learning on stars. stars. , astronomy focused its research research its focused astronomy , century 20th the n I 18th century 18th 10 distance of 1000 km!) and even better for brighter brighter for better even and km!) 1000 of distance to measuring the diameter of a human hair at a a at hair human a of diameter the measuring to 17th century 17th stars and of our place in the Universe. the in place our of and stars 100 accuracy will be about 20 microarcseconds (equivalent (equivalent microarcseconds 20 about be will accuracy distances was a turning point in our understanding of of understanding our in point turning a was distances ycho Brahe ycho T distances and also velocities about 1 billion stars. Its Its stars. billion 1 about velocities also and distances confirmation that stars lay at very large but still finite finite still but large very at lay stars that confirmation 1000 with positions, positions, with Galaxy our of map three-dimensional the first stellar parallaxes in the 1830s. The The 1830s. the in parallaxes stellar first the dynamic precise an extremely to create Hipparchus increase in precision was fundamental for measuring measuring for fundamental was precision in increase Gaia of Books Little The . Gaia will use the most advanced technology technology advanced most the use will Gaia . Gaia called accuracies of fractions of a second of arc. This This arc. of second a of fractions of accuracies launch a much more powerful astrometric satellite satellite astrometric powerful more much a launch and measurements were possible with with possible were measurements and further Positional accuracy through History through accuracy Positional Following the success of Hipparcos, ESA is planning to to planning is ESA Hipparcos, of success the Following engraving techniques advanced advanced techniques engraving , century 19th the In - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Astrometry is the oldest branch of astronomy. It The radial velocity is easily found by observing the In 129 B.C. and only with the help of the naked eye, the Tycho's observations of the planets throughout their studies the geometrical relationships between spectrum of a star, but finding the proper motion is Greek astronomer Hipparchus was the first to orbit with unprecedented accuracy allowed Kepler to objects in the sky and their apparent and true tougher and requires careful observation of the complete a catalogue of a thousand stars, specifying discover that planets move in elliptical orbits. motions. movement of the star with respect to others over a their relative brightness and position with an accuracy number of years. of about one degree, i.e. the angle equivalent to the In 1609, the telescope was invented, opening new To find the distance to a star we use a concept called apparent height of a person at a distance of 100 worlds to human scrutiny. But the telescope alone the parallax. If we observe a star from the Earth and Measuring distances and motions of stars is metres. This is considered to represent the birth of wasn't of much use for measuring angles. It took some record its position with respect to the background fundamental to our understanding of the nature of the the science of astrometry. time to devise an instrument which would make use of stars and then repeat this measurement 6 months Universe. Knowing the distance to a star, we can the improved sight available with the telescope, but later, when the Earth is on the opposite side of its deduce its true luminosity and size and so we can After Hipparchus, progress in the accuracy of angular which would also permit a high angular accuracy. orbit around the Sun, we see that the position of the derive essential information about its nature and age. measurements was slight until the 16th century. A star appears to shift with respect to the background. On the other hand, knowing the motion of stars we can revolution came with Tycho Brahe (1546-1601), a In the 17th century the filar micrometer was invented, This apparent angular displacement of a star is what deduce not only where they were millions of years ago Danish astronomer, who fixed star positions to about a consisting of two wires mounted in the field of view of is called the stellar parallax. By measuring the but also what their positions will be in the future. minute of arc, i.e. one sixtieth of a degree. He a telescope which moved towards and away from each parallax, we can deduce the distance to a nearby star designed, built and calibrated a wide variety of viewing other with a screw. The number of turns of the screw using simple geometry. But stellar parallax is a Ancient civilizations already realised that objects in instruments like the sextant or the mural quadrant and indicated the angle subtended by the object in the sky. difficult quantity to measure as it is extremely small the sky appear to move in a regular manner which can changed observational practice profoundly. This allowed breaking the barrier of accuracy imposed for all but the nearest few hundred stars. be useful for determining directions and time on the by the limited resolution of the human eye, which Earth. The need to solve problems originating from cannot distinguish angles below 1 minute of arc. early communities (e.g. establishing accurately the optimum moment for planting and harvesting) In the 18th century, knowledge of materials and Sun constituted a starting point for precision astrometry. workshop techniques improved significantly allowing instrument makers to engrave angular scales like the Parallax Making accurate angular measurements and cataloguing astronomical circle with high precision. Accuracies celestial positions has been the fundamental task of improved to the order of arcseconds, which allowed the Earth astronomy until the 19th century and still constitutes a detection of stellar aberration in 1725, the first direct basic element of astronomical research. The angles proof that the Earth was moving through space. This Astrometry also determines how celestial objects are involved are extremely small and improving the finally confirmed the controversial Copernican theory moving in space relative to each other.