Yerkes Observatory is an astronomical observatory located in Williams Bay, Wisconsin, U.S.A. It was operated by the University of Chicago Department of Astronomy and Astrophysics from its founding in 1897 to 2020. Ownership was transferred to the non-profit Yerkes Future Foundation in May 2020, which began restoration and renovation of the historic building and grounds. Re-opening for public visits and programming is expected in 2022.
The observatory, often called "the birthplace of modern astrophysics", was founded in 1892 by astronomer George Ellery Hale and financed by businessman Charles T. Yerkes.
It represented a shift in the thinking about observatories, from their being mere housing for telescopes and observers, to the early-20th-century concept of observation equipment integrated with laboratory space for physics and chemistry analysis.
The observatory's main dome houses a 40 in-diameter doublet lens refracting telescope, the largest refractor ever successfully used for astronomy. Two smaller domes house 40-inch and 24-inch reflecting telescopes. There are several smaller telescopes – some permanently mounted – that are primarily used for educational purposes.