Hale decided, at least initially, to focus his efforts in solar research, his own area of study. He sent invitations to academies and scientific societies in Europe. Of the 17 invited academies, only the Prussian Academy of Sciences declined, and the rest assisted the meeting organized by Hale at the International Congress of Science at St Louis in 1904 (which coincided with the St Louis Exposition). In the meeting, they appointed a committee composed of a representative from each participating country. The committee created a constitution for the now named International Union for Cooperation in Solar Research. In their second conference (Oxford, September 1905), they focused on scientific problems over bureaucratic minutia and produced many important results: Alfred Fowler presented results about the spectrum of sunspots and Hale and Deslandres suggested several applications for the spectroheliograph (which later helped Hale discover the magnetic fields in sunspots), among other things. They were published in the Transactions of the Union, which became a recurring publication after each meeting (a tradition the IAU also followed).
The Union was accepted by the International Association of Academies in 1907 (it had not been before due to Prussian opposition) and a third conference was held in Paris, also in 1907. In the fourth conference, held in Mount Wilson, California, in 1910, stellar research and astrophysics were also included in the programme, in an effort to expand the Union’s research to other areas of astronomy that might benefit from international collaboration. The conferences were meant to take place every three years, but, after the one held at Bonn in 1913, the First World War (1914-18) broke out, and international collaboration was stopped to a halt. The long war, and the bitterness that followed it, lead to the disappearance of the Union, as well as the International Association of Academies, which also held its last meeting in 1913.
The desire for collaboration to bring science forward, however, resurfaced once the War ended. The National Research Council of the United States, where George Hale now worked, joined forces with the Royal Society of London and the Paris Academy of Sciences to create the International Research Council in Brussels, in 1919. The Council, formed then by 12 countries, had the objective of promoting international cooperation in the sciences. To this end, it created a number of internal unions that dealt with different areas of study. The International Astronomical Union was one of these, officially created on 28 July 1919.
The IAU took upon the duties previously held by the International Union for Cooperation on Solar Research, but also by other international astronomical groups. One of these was the Carte du Ciel (Map of the Sky), one of the oldest international astronomical projects. It had been launched in Paris, in 1887, and it had the objective of carrying out a photographic map of the entire celestial sphere. Eighteen observatories collaborated on this colossal task, which would require of at least 20600 photographic frames. The IAU continued the project (put in charge of Commission 23) until 1970, when it was left unfinished. The first President of the IAU, Edouard Benjamin Baillaud, then Director of the Paris Observatory, had been one of the people in charge of the Carte du Ciel before the Great War. Baillaud was also the founder of the Bureau International de l’Heure (International Time Bureau), which researched time keeping. It was assimilated by the IAU, becoming Commission 31.
The IAU soon became the leading astronomical organization. The 7 countries that had joined in 1920 increased to 31 by 1949, and even scientists from countries that had not joined yet (like Germany or Austria) regularly attended the meetings. The IAU held their General Assemblies approximately every 3 years, with the first one taking place in Rome, in 1922. After the 1938 meeting in Stockholm, the General Assemblies stopped due to the Second World War, but they soon restarted with the August 1948 meeting in Zurich.
In the Assembly, the members chose an Executive Committee, formed by a President, several Vice-Presidents and a General Secretary (which also acted as a treasurer). Their duty was to manage the IAU’s finances and prepare the next General Assembly. The IAU established different commissions (there were 41 by 1949) which covered different branches of astronomy. In 1958, under General Secretary Donald Sadler, the IAU started publishing the biannual Information Bulletin, which served to keep all members up to date with the goings-on of the Union. The IAU Circulars were a series of postcards with information on recent discoveries that required prompt dissemination.
In 1973, the General Assembly was to be celebrated in Sidney, Australia. This was the first time an important meeting of this kind was to be held in the southern hemisphere. However, this decision put the IAU in danger of splitting up: 1973 was the year of the quincentenary of Nicolaus Copernicus’ birth, so Polish astronomer Wilhelmina Iwanowska proposed that the General Assembly be held in Poland, in celebration of such an occasion. Many astronomers from Eastern Europe supported this proposal, discouraged by the idea of going on a long and expensive trip to Australia. Others, however, pointed out that changing the site of the Assembly at such a late date would be an offense to all of the members of the IAU that lived in the southern hemisphere, Australians in particular, since they had taken that same trip in all previous meetings. The Executive Committee, led by President Otto Heckmann, reached a middle ground. They celebrated the 1973 General Assembly in Sidney, as planned, but they also held an Extraordinary General Assembly in Warsaw one month later (circumventing the clause in the IAU Constitution that impeded them from holding two General Assemblies in the same year).
The Chinese (through the National Committee of Astronomy at Nanjing) joined the IAU in 1935, but they left in 1961, just prior to the General Assembly in Berkeley, California. This was a reaction to the acceptance of Taiwan (through the Astronomical Society of Taipei) to the IAU. Taiwan, or officially the Republic of China, claimed to be the legitimate government of all of China. It enjoyed the political protection of the United States, but it was not recognised by the People’s Republic of China. Many astronomers considered the exit of China a big loss, since astronomy had been present in China for longer than in Europe, but the Executive Committee was not willing to go back on its acceptance of the Taiwanese (a measure other international scientific unions had taken when faced with a similar dilemma). The Cultural Revolution that took place in China in the following years only made it harder for the Chinese to return to the IAU. Negotiations only started in 1979, years after the United Nations had stopped recognising the sovereignty of Taiwan. The President of the IAU, Adriaan Blaauw, held several meetings with a Chinese delegation, led by astronomer Chang Yu-Che, before and after the Montreal General Assembly, in which the Taiwanese were also involved. The negotiations were successful and the Chinese re-joined the IAU. In 2012, the XVIII General Assembly was held in Beijing.
In 1988, under the proposal of President Jorge Sahade, the IAU created the position of President-Elect, which future presidents would occupy for 3 years before taking charge of the presidency. This way, they would familiarise themselves with the inner workings of the IAU, resulting in them being more prepared for their job. Under the presidency of Lodewijk Woltjer, in an effort to stop younger members from disconnecting from the General Assemblies, the Hague General Assembly of 1994 included more scientific content, incorporating 6 symposia and colloquia (instead of holding them independently). The IAU also inaugurated its official webpage in December 1994 and, in the last decade of the 20th Century, started publishing their Informational Bulletins in digital form, starting with No. 74 (January 1995). In 2006, the IAU started issuing the electronic IAU Newsletters, containing short announcements of urgent nature.
The IAU has always been concerned with the dissemination of astronomy and with the public understanding of the importance of the science. In 2009, under President Catherine Cesarsky, it celebrated the International Year of Astronomy, coinciding with the 90th anniversary of the institution and the 400th of Galileo's first telescope observations and the publication of Kepler’s Astronomia Nova.