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).

Thierry Montmerle has been General Secretary of the IAU from 2012 to 2015. In the framework of the centenary celebrations of the creation of the IAU and four other International Unions in 1919, he and co-Editor Danielle Fauque have organized an international conference in Paris, where the IAU Secretariat in currently located, honoring in particular the first President of the IAU, Benjamin Baillaud, who was Director of the Paris Observatory at the time. While this book is based on the conference, it has been vastly expanded to include more contributions. He has written two papers on the history of the IAU in the "IAU Centenary Symposium" held during the IAU General Assembly in Vienna (2018). During his long career in astrophysics (PhD Paris, 1975), he has edited many proceedings of international conferences (including the IAU General Assembly in Honolulu, 2015, edited by Cambridge University Press), and also contributed to books edited by Springer (Encyclopedia of Astrobiology, 2015+; Young Sun, Early Earth, and the Origin of Life, 2012).

Danielle Fauque graduated in physics, chemistry, and history of science. She is a member of the International Academy of History of Science. She is specialized in history of astronomical instruments (thesis on the history of the heliometer), astronomical navigation, optics and chemistry. She and co-editor Thierry Montmerle have organized a conference on history of IAU in Paris in October 2019. She has been a member of various committees (organization and scientific programme) of the International Union of Pure and Applied Chemistry (IUPAC) Congress in Paris in July 2019. She co-edited a special issue of Chemistry International, the IUPAC magazine on the history of IUPAC. She published papers on the history of the scientific unions, the last one with R. Fox (Oxford), in Acta Historica Leopoldina (2021), on the International Research Council (1919-1931). She has been also a member of the Editorial Council of the RHS for several years. Since 1983, she has published several papers on history of science in collective books (physics, optics, astronomy, science teaching history, scientific travels, chemistry) and continues to publish and organize national and international meetings on the history of science.

Part I: The creation of the IAU

- Precursors to the IAU: Paris Observatory and the Carte du Ciel project

- A hundred years ago: the birth of the IAU as a model for the other scientific unions

- The IAU first President: Benjamin Baillaud and the international cooperation in astronomy

- The foundation of the Bureau International de l'Heure

- The "Eddington network": crossing the borders between enemy countries *

 Part II: International tensions and crises after WWII

- "Exchange of astronomers": resurrecting international cooperation in the aftermath of WWII *

- Germany joining the IAU (1951): a complicated story

- When China left the IAU (1960-1980)

- Chinese astronomy at the time of the Cultural Revolution *

- USA vs. USSR: the Moon race and conflicts about lunar nomenclature

- The Moon race: Russian robotic missions and landing sites surface features

- "Extraterrestrial names": when the UN challenged the IAU *

 Part III: The IAU and the world today

- Astronomy and science diplomacy *

- Two Koreas as IAU members *

- The IAU and the public worldwide: the 2009 International Year of Astronomy

- The IAU and world education, development, and outreach *

 Index (Names & Institutions)