Astronomers as diplomats : when the IAU builds bridges between nations This book illuminates a few highly significant events in history in which astronomers have helped keep contacts between astronomers of different states in moments of international political tensions or even crises. The chapters written by 20 international authors focus on four periods where astronomers were particularly active in international relations: 1. The WWI period the epoch of the creation of the IAU in the context of the simultaneous creation of other scientific unions. The book also singles out the important role of A.S. Eddington and his network “across forbidden borders”. 2. The Cold war period and its consequences when several countries were divided between opposite blocs. “The China crisis” is told here from different viewpoints by Chinese astronomers both from the mainland and from Taiwan in parallel with the evolution of astronomy in South and North Korea. Germany’s twisted path in its membership of the IAU from its admission in 1951 to its reunification in 1991 is shown as another example. 3. The book then highlights a third period when radio astronomers in particular were very active in “building bridges” between East and West. It also tells the history of how the apparently innocuous issue of the “lunar nomenclature” became extremely sensitive. The part ends on two chapters on Russian robotic missions and lunar surface features as well on the Russian participation in the “International Virtual Observatory” project. 4. The fourth part reports for the first time on the “hidden story” of the relations between the IAU and the United Nations after the “Moon race” when the United Nations decided to challenge the IAU’s authority on “extraterrestrial names”. The final chapter reviews how twenty years later UNESCO and the IAU had become strong partners in the difficult but highly successful organization of the International Year of Astronomy (2002 2009) and of the “Astronomy and World Heritage” intitiative (2008).