Yerkes Observatory

Yerkes Observatory (/ˈjɜːrkiːz/ ^ⓘ YUR-keez) is an astronomical observatory located in Williams Bay, Wisconsin, United States. The observatory was operated by the University of Chicago Department of Astronomy and Astrophysics from its founding in 1897 until 2018. Ownership was transferred to the non-profit Yerkes Future Foundation (YFF) in May 2020, which began millions of dollars of restoration and renovation of the historic building and grounds. Yerkes re-opened for public tours and programming in May 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 aperture (102 cm) doublet lens refracting telescope, the largest refractor ever successfully used for astronomical observation. The Swedish 1-m Solar Telescope in the Canary Islands has a slightly larger 43-in diameter (109 cm) lens but only 39-in (99 cm) clear aperture and is primarily dedicated to solar observations. There are two smaller domes at Yerkes in which are currently mounted 41-inch (104 cm) and 24-inch (60 cm) reflecting telescopes. The observatory also holds a collection of over 170,000 photographic plates.

The Yerkes 40-inch was the largest refracting-type telescope in the world when it was dedicated in 1897. Another large telescope of this period was the Great Melbourne Telescope, which was a reflector. In the United States, the Lick refractor had just a few years earlier come online in 1888 in California with a 36-inch (91 cm)lens.

Prior to its installation, the telescope on its enormous German equatorial mount was shown at the World's Columbian Exhibition in Chicago, Illinois during the time the observatory was under construction.

The observatory was a center for serious astronomical research for more than 100 years. By the 21st century, however, the historic telescope had reached the end of its research life. The University of Chicago closed the observatory in October 2018. In November 2019, it was announced that the university would transfer Yerkes Observatory to the non-profit Yerkes Future Foundation (YFF). The transfer of ownership took place on May 1, 2020.