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Logo of the IAU, initialism both in French and English (UAI for Union astronomique internationale and IAU for International Astronomical Union)

The International Astronomical Union (IAU) unites national astronomical societies from around the world. It also acts as the internationally recognized authority for assigning designations to celestial bodies (stars, planets, asteroids, etc.) and any surface features on them, and is a member of the International Council for Science (ICSU). The main aim of the IAU is to promote and safeguard the science of astronomy in all its aspects through international cooperation. Headquartered in Paris, France, its individual members are professional astronomers from all over the world, at the Ph.D. level or beyond, and active in professional research and education in astronomy. The IAU maintains friendly relations with organizations that include amateur astronomers in their membership. National Members are usually those with a significant level of professional astronomy.

Working groups include the Working Group for Planetary System Nomenclature (WGPSN), which maintains the astronomical naming conventions and planetary nomenclature for planetary bodies. The IAU is also responsible for the system of astronomical telegrams which are produced and distributed on its behalf by the Central Bureau for Astronomical Telegrams. The Minor Planet Center (MPC), a clearinghouse for all non-planetary or non-moon bodies in the solar system, also operates under the IAU.

The IAU was founded in 1919, as a merger of various international projects including the Carte du Ciel, the Solar Union and the International Time Bureau (Bureau International de l'Heure). The first appointed President was Benjamin Baillaud. Pieter Johannes van Rhijn served as president from 1932 to 1958.

The IAU has 9,785 individual members, all of whom are professional astronomers and most of whom hold PhDs. There are also 63 national members who represent countries affiliated with the IAU. 87% of individual members are male; 13% are female. The union's current president is astronomer Catherine J. Cesarsky.

The sovereign body of the IAU is its General Assembly, which comprises all members. The Assembly determines IAU policy, approves the Statutes and By-Laws of the Union (and amendments proposed thereto) and elects various committees.

The right to vote on matters brought before the Assembly varies according to the type of business under discussion. The Statutes consider such business to be divided into two categories:

• issues of a "primarily scientific nature" (as determined by the Executive Committee), upon which voting is restricted to individual members, and
• all other matters (such as Statute revision and procedural questions), upon which voting is restricted to the representatives of national members.

On budget matters (which fall into the second category), votes are weighted according to the relative subscription levels of the national members. A second category vote requires a turnout of at least two thirds of national members in order to be valid. An absolute majority is sufficient for approval in any vote, except for Statute revision which requires a two-thirds majority. An equality of votes is resolved by the vote of the President of the Union.

The XXVIth General Assembly of the International Astronomical Union was held from August 14 to August 25, 2006 in Prague, Czech Republic. On 15 August the Assembly decided to restore to individual members the right to vote on scientific matters, which had been removed from them at the XXVth Assembly in 2003. Among the business before the Assembly was a proposal to adopt a formal definition of planet. The proposed definition would have created 12 known planets in our solar system, adding initially the asteroid Ceres, Pluto's present moon Charon and Eris and would retain Pluto as a planet. However this proposed definition was rejected and on August 24 the Assembly passed a resolution that redefined the definition of a planet, which classified Ceres, Eris and Pluto as dwarf planets, and reduced the number of planets in the solar system to 8. The number having registered their attendance at the Assembly at the time of the vote stood at 2411,^[1] but only 424 attended the plenary session, not all of whom voted. Note that the authority of the IAU to name celestial bodies was discussed publicly after the August 24th, 2006. Parts of the scientific community did not agree with this ruling, and in the following public debate many laypersons expressed disagreement with the vote.

The IAU General Assembly meets every three years, with the exception of WWII. Past meetings include:

• XXVIth IAU General Assembly (2006): Prague, Czech Republic
• XXVth IAU General Assembly (2003): Sydney, Australia
• XXIVth IAU General Assembly (2000): Manchester, United Kingdom
• XXIIIrd IAU General Assembly (1997): Kyoto, Japan
• XXIInd IAU General Assembly (1994): The Hague, Netherlands
• XXIst IAU General Assembly (1991): Buenos Aires, Argentina
• XXth IAU General Assembly (1988): Baltimore, Maryland, United States
• XIXth IAU General Assembly (1985): New Delhi, India
• XVIIIth IAU General Assembly (1982): Patras, Greece
• XVIIth IAU General Assembly (1979): Montreal, Quebec, Canada
• XVIth IAU General Assembly (1976): Grenoble, France
• Extraordinary IAU General Assembly¹ (1973): Warsaw, Poland
• XVth IAU General Assembly (1973): Sydney, Australia
• XIVth IAU General Assembly (1970): Brighton, United Kingdom
• XIIIth IAU General Assembly (1967): Prague, Czechoslovakia
• XIIth IAU General Assembly (1964): Hamburg, West Germany
• XIth IAU General Assembly (1961): Berkeley, California, United States
• Xth IAU General Assembly (1958): Moscow, Soviet Union
• IXth IAU General Assembly (1955): Dublin, Ireland
• VIIIth IAU General Assembly (1952): Rome, Italy
• VIIth IAU General Assembly (1948): Zürich, Switzerland
• VIth IAU General Assembly (1938): Stockholm, Sweden
• Vth IAU General Assembly (1935): Paris, France
• IVth IAU General Assembly (1932): Cambridge, Massachusetts, United States
• IIIrd IAU General Assembly (1928): Leiden, Netherlands
• IInd IAU General Assembly (1925): Cambridge, United Kingdom
• Ist IAU General Assembly (1922): Rome, Italy

¹ Held to commemorate the 500th anniversary of the birth of Nicholas Copernicus [1]

• The XXVIIth General Assembly is planned for 2009 in Rio de Janeiro, Brazil.
• The XXVIIIth General Assembly is planned for 2012 in Beijing, China.

• 2006 redefinition of planet
• Astronomical naming conventions
• Planetary nomenclature

• Website of the International Astronomical Union
• XXVIth General Assembly 2006

• Statutes of the IAU, VII: General Assembly, ss. 13-15

1. ^ IAU General Assembly Welcome page 24 August 2006.