Introduction to: Contributions to the history of astrometry Dear reader, Copenhagen 10 June 2008 On my website you may find the file www.astro.ku.dk/~erik/History.pdf containing links to a collection of “Contributions to the history of astrometry”, presently with seven items: No. 1: Bengt Strömgren and modern astrometry: Development of photoelectric astrometry including the Hipparcos mission No. 1A: Bengt Strömgren and modern ... (Short version) No. No. No. No. No. 2: 3: 4: 5: 6: Lennart Lindegren’s first years with Hipparcos From the Roemer mission to Gaia Development of astrometric accuracy (only partly ready) Three lectures about astrometry of all times Miraculous approval of Hipparcos in 1980 Item No. 1 is a report about Hipparcos history: Bengt Strömgren and modern astrometry Development of photoelectric astrometry including the Hipparcos mission No.1A is a short version of this report posted at the IAU Symposium No. 254, June 9-13, 2008, dedicated to Professor Bengt Strömgren (1908-1987). It contains a view of this history which is partly new to most readers. It is therefore important for me to be historically correct, and I am grateful to Catherine Turon, Jean Kovalevsky, and Lennart Lindegren for comments to earlier versions and for their agreement to the present text. I also acknowledge comments from Andrew Murray, Edward van den Heuvel, and colleagues in Copenhagen. It is suggested first to read the overview given in No.1A, the poster. A comprehensive exposition of the historical issues is provided in Nos. 1, 2, and 6 which were widely distributed in May 2008. Best regards PS on 18 June: I followed and enjoyed the symposium all last week. I noticed that all speakers mentioning BS's impact on science forgot about astrometry, but Gerry Gilmore has agreed with my conclusion: without Bengt Strömgren there would have been no Hipparcos space astrometry mission. When asked further he said: “Without BS the study of the Milky Way might have been delayed by an entire generation because everyone else moved to the exciting extragalactic astronomy in those years. But BS emphasized the importance of understanding our own Milky Way, as is obvious for everyone today. He was a great visionary.” Erik Høg First page of No.1A, the poster: Bengt Strömgren and modern astrometry Development of photoelectric astrometry including the Hipparcos mission Erik Høg, Niels Bohr Institute, Copenhagen ABSTRACT: Bengt Strömgren is known as the famous astrophysicist and as a leading figure in many astronomical enterprises. Less wellknown, perhaps, is his role in modern astrometry although this is equally significant. There is an unbroken chain of actions from his ideas and experiments with photoelectric astrometry since 1925 over the new meridian circle in Denmark in the 1950s up to the Hipparcos and Tycho Catalogues published in 1997. 1. Introduction This account follows a chain of actions beginning with an experiment on photoelectric astrometry in 1925 and culminating with the Hipparcos mission at the end of the 20th century, thus, in 1925 began a new era of positional astronomy comparable in significance to that of Tycho Brahe four centuries earlier. This brief account is far from being a complete history of Hipparcos, nor, of course, of the many other developments of photoelectric astrometry in the same period. Note: This is a shorter text than to the proceedings of the IAU Symposium 254 where references are given; see www.astro.ku.dk/~erik/History.pdf. 5. Conclusion Bengt Strömgren appears clearly at the root of my contributions to astrometry, including Hipparcos, and he was directly active before the mission approval in 1980 in order to ensure Danish and Swedish support. He would have seen the Gaia mission (to be launched in 2011) with astrometry, photometry and radial velocities as the ultimate fulfilment of his quest since the 1930s for comprehensive studies of our Milky Way. It seems from the unbroken chain of actions defined here that there would have been no Hipparcos, no space astrometry with a scanning satellite, if any of the five persons Bengt Strömgren, myself, Pierre Lacroute, Jean Kovalevsky or Lennart Lindegren had been absent from the scene before 1980.