International Astronomical Union

The International Astronomical Union (IAU or UAI) is an association of professional astronomers, which takes initiatives to promote cooperation in astronomical science, among them, standardizing the designations of bodies and standardizing recommended (i.e., nominal) values of measurable quantities, e.g., the radius of the Sun. This assists communication and collaboration among researchers regarding observation data, modeling, etc. Thus, for example, two researchers each modeling the Sun can avoid results incompatible solely because they selected different measures of its radius. The IAU famously redefined the definition of planet such that Pluto was relegated to the status of "dwarf planet".

Also among IAU's activities has been dissemination of alerts (by mail) on recent astronomical phenomena, IAU Circulars (IAUC), first mailed in 1920 and still being mailed through 2016.