In 2019, the International Astronomical Union (IAU) celebrated its 100th anniversary. To commemorate this milestone, the IAU organised a year-long celebration to increase awareness of a century of astronomical discoveries as well as to support and improve the use of astronomy as a tool for education, development and diplomacy under the central theme "Under One Sky".

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The IAU100 initiative exceeded initial expectations with over 5000 registered activities in 143 countries, and facilitated the direct involvement of an estimated 5 to 10 million people along with an  estimated further reach of 100 million people through communication activities. The IAU 100th Anniversary Celebrations Final Report showcases the implemented activities, event highlights, finances, communications, impact numbers and IAU100 legacy.