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In the 19th century, engraving techniques advanced further and measurements were possible with accuracies of fractions of a second of arc. This increase in precision was fundamental for measuring the first stellar parallaxes in the 1830s. The confirmation that stars lay at very large but still finite distances was a turning point in our understanding of stars and of our place in the Universe.
In the 20th century, astronomy focused its research on learning more about the nature of celestial objects instead of only measuring their position. New techniques like spectroscopy (which studies the light emitted by objects to determine their chemical composition, temperature and nature) and the use of photographic plates in astronomy enabled this change to occur. Progress in astrometry meanwhile became very difficult, because it had reached the best precision obtainable from Earth, of approximately 0.1 arcsecond, limited mostly by atmospheric effects.
But things changed for astrometry in 1989, as the European Space Agency (ESA) launched the first astrometric satellite, Hipparcos, which has revolutionised our knowledge of star positions. From its orbit, the Hipparcos satellite observed the whole sky, achieving an improvement of about 100 compared to accuracies obtained from the ground. A catalogue was created with the positions, distances and motions of 120 000 stars to a precision of around 1 milliarcsecond. The results from Hipparcos are being analysed by scientists all over the world, and important conclusions are emerging about the nature of our Galaxy.
Click on the thumbnail below to explore spectroscopy and how it helps us to better understand the nature of celestial objects:
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