Union Observatory was an astronomical observatory in the suburb Observatory, Johannesburg, South Africa. It bears IAU code 078.
Known as the Transvaal Observatory until 1912, it became the Republic Observatory in 1961. Well remembered for the quality of its Directors, work done on minor planets and the discovery of Proxima Centauri, growing light pollution problems in Johannesburg led to its closure in 1971-1972.
At that time the South African government decided to amalgamate all astronomical research into one body, which later became known as the South African Astronomical Observatory (SAAO); it has its headquarters in Cape Town and has Sutherland as its outstation. The main Cape telescopes were moved to Sutherland, and the Radcliffe Observatory at Pretoria was also dismantled.
Union Observatory went through a number of name changes:
Transvaal Meteorological Department 1903 – 1909
Transvaal Observatory 1909 – 1912
Union Observatory 1912 – 1961
Republic Observatory 1961 – 1971