Federal Observatory, Observatory in District 6, Zurich, Switzerland

The Federal Observatory is a building on Schmelzbergstrasse 25 near the ETH Zurich main building, designed by Gottfried Semper and completed in 1864. Its observation terrace carries a small observatory added in 1910–1911 and a solar observation tower built in 1950–1951. It is the only Semper building largely undamaged over time and is a Swiss cultural property of national significance.

Gottfried Semper, the new polytechnic's first architecture professor, saw the foundation stone laid on 27 March 1862; the observatory was completed in 1864. Its first director, Rudolf Wolf, began long-term sunspot observation there, and the site later coordinated international sunspot counts and published the Zurich Sunspot Number. A small observatory was added in 1910–11 and a solar observation tower in 1950–51; operations ceased on 1 April 1980.

The building now houses the Collegium Helveticum, an interdisciplinary research institute of ETH Zurich. Since 2 December 2022, an exhibition of astronomical instruments from Rudolf Wolf's collection has been open to the public in the foyer. The Zurich Sunspot Number, developed here, remains the standard measure of solar activity.

The observatory stands at Schmelzbergstrasse 25, near the ETH Zurich main building in District 6. Astronomical observation ended here on 1 April 1980 because the neighbouring cantonal hospital blocked the view, so the site is visited for its architecture and instruments rather than for stargazing. The foyer holds a public exhibition of Rudolf Wolf's instrument collection, open since 2 December 2022.

The observatory's Zurich Sunspot Number, devised under director Rudolf Wolf, is still the standard scale for solar activity. Wolf led the observatory from 1864 to 1893, and since 2 December 2022 an exhibition of his astronomical instruments has been open to the public in the foyer.