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The meridian transit circle


The meridian instrument is a tool for astrometry. What is astrometry? It is the measure of sky and celestial bodies. It is astrometry that allows the definition of a fixed reference to make measurements. What is fixed, what is moving? And compared to what? The path will be long from Galilean relativity to general relativity and astrometry will be indispensable. But what will these measures be used for? Only the measure of the sky can make us understanding what is around us. The simple observation of the sky does not make possible the knowledge of the distances which separate us from the visible stars. Hypotheses have been made, and they have been overturned or confirmed by the measurement of positions and, above all, the movements of celestial bodies. It was only during the eighteenth century that the distance from the Earth to the Sun was determined, and then, step by step, the knowledge of the size of the visible universe! Astrometry has always remained the fundamental tool for advancing our knowledge of the universe.
Image of the sky: field of 11x11 minutes of degree;
the Abbadia transit circle may only measure bright stars which will be used as references
Credit : DSS 

To know more on astrometry and meridian transit, see here the lesson (given in french at Abbadia) on astrometry.