In 1918 the only Mexican institution dedicated professionally to astronomical work was the National Astronomical Observatory, which had begun operations in 1878. In that year of the 20th century, Mexico was immersed in the armed movement known as the Mexican Revolution, which strongly affected the social and economic structure of the country. The Mexican astronomers saw their group drastically reduced and their infrastructure limited; however, they made great efforts to continue their participation in the international project of the Carte du Ciel, since they were part of the 18 observatories that originally participated in it. Despite the limitations they suffered, they soon joined the group of seven nations that in 1919 formed the IAU, and two members of that observatory actively participated in the first general assembly held in Rome in May 1922.

At the Sydney IAU General Assembly (GA) the statutes were modified to remove the votes by individual members. Few noticed this at the time but the subsequent reaction by disenfranchised members led us to revise this position in Prague. The need to have a members’ vote on the status of Pluto was complicated by these changes and the drama behind the scene at the Prague GA where the planet definition was resolved is not well known despite the huge public impact of this GA. I will describe some of the activities of the executive and its working groups during this very exciting GA. The IAU structures served us well during this process but of course there were also many lessons learned.

Nicolae Donitch gave a special role to Romania’s membership of the IAU, even though it had basically adhered to it in 1922, but only on the 12th of July 1928 was Romania’s membership in the IAU recognized. He was a passionate observer of eclipses, of the zodiacal light, and a good instrument builder. His tortuous destiny was somehow similar to the destiny of many Romanians from Eastern Moldova: he was born in Chişinău, the capital of Bessarabia at that time; he studied in Odessa; after the first revolution in February 1917 he left Russia and remained in Odessa until the arrival of the Bolsheviks in 1920, who would completely destroy his laboratory. He then moved to his private Astrophysics Observatory in Dubăsarii Vechi, which he had set up in 1908, and stayed until 1940, when Bessarabia was once again torn away from Romania. He relocated to Bucharest, but soon after, when communists came to power, he had to leave the country on a permanent basis. As he severed all the ties with people from the country that had fallen into the red zone of communism, we completely lost track of him. It was only recently that, after thorough research, we could discover his traces, and learn of his last years of life.

The so-called China crisis, well documented in History of the IAU by Adriaan Blaauw and in Under the Same Starry Sky: History of the IAU by Chengqi Fu and Shuhua Ye, refers to the withdrawal in 1960 of the People’s Republic of China (PRC) from the Union. The crisis stemmed from the admission by the IAU, amidst strong protest from PRC and some other member countries, of the Republic of China (ROC) to the Union, creating the so-called “Two Chinas” – or “One China, one Taiwan” problem. The crisis directly led to the absence of mainland Chinese astronomers from the stage of international collaborations and exchanges, and was only solved two decades later. The solution, accepted by all the parties involved, is that China is to have two adhering organizations, with mainland China astronomers represented by the Chinese Astronomical Society located in Nanjing (China Nanjing) and China Taiwan astronomers represented by the Academia Sinica located in Taipei (China Taipei). The denominations “China Nanjing” and “China Taipei” represent the IAU official resolution and should be used in all IAU events.

The China crisis, probably the most serious one in IAU history, was a painful lesson in the 100-year development of the Union. Yet, with its eventual solution, the Union has emerged stronger, upholding its spirit of promoting astronomical development through international collaboration of astronomers from all regions and countries, regardless of the political systems, religion, ethnicity, gender or level of astronomical development.

The history of the International Astronomical Union (IAU) meetings goes back to 1922 when the first IAU General Assembly (GA) was held in Rome, Italy, following the IAU creation in 1919. However, until 1953, no individual symposia were organized and the GAs were the only official gatherings for astronomers. All together, eight IAU GAs were held during 1922–1952. The IAU Symposium 1 was held in 1953 in Groningen, Netherlands. Starting with 1955, several IAU symposia were regularly held in different places, and since 1959, the IAU also began to organize colloquia to discuss relatively smaller topics. Twenty IAU colloquia numbered as I–XX were held in the period 1959–1971, and another series of IAU colloquia was organized in 1968–2005, numbered as Nos. 1–200. At present IAU symposia are the only official scientific meetings, nine of them being organized every year. IAU S349 “Under One Sky: the IAU Centenary Symposium”, held in Vienna during the IAU GA XXX, was the last one by number in 2018. Thus, the IAU has a 65-year history of symposia and all together 348 such meetings have been held, on average 5–6 annually. At present most of the IAU symposia during the years of GA are being organized in the framework of the GA, there being typically six symposia during each GA. All together, 31 IAU GA have been organized during the years 1922–2018, including 30 regular ones and one Extraordinary GA (1973 in Warsaw, Poland), typically once every three years. Since 1974, the IAU has also organized regional meetings in Europe, Asia and Pacific (APRIM), Latin America (LARIM), and the Middle East and Africa (MEARIM). The European ones were discontinued in 1990 after the creation of the European Astronomical Society (EAS) and the organization of the yearly JENAM/EWASS. The 348 IAU symposia have been organized in 43 countries. We give the statistics of all IAU symposia by year of organization, by various topics of astronomy and astrophysics, and by host countries and cities.