{"nrhp_refnum":"86003155","name":"University of Illinois Astronomical Observatory","other_names":[],"category":"building","location":{"address":"901 S. Mathews Ave.","address_restricted":false,"city":"Urbana","county":"Champaign County","state":"IL","lat":40.10501801800007,"lon":-88.22622842799996,"geom_source":"nps_gis"},"listed_date":"1986-11-06","is_nhl":false,"nhl_designated_date":null,"areas_of_significance":["Science"],"significance_levels":["national"],"summary":{"what_it_is":"A one-story T-plan buff Roman brick astronomical observatory at 901 South Mathews Avenue on the University of Illinois campus in Urbana, built in 1896 to designs by architect Charles A. Gunn, with a 35-foot observation tower and dome.","why_significant":"The observatory is where Joel Stebbins, working with F.C. Brown and Jacob Kunz, developed the selenium cell and photoelectric cell for astronomical photometry between 1907 and 1922. This work made photoelectric measurement of stellar magnitudes the standard technique. The building keeps its original tower, equatorial room, dome, pier and twelve-inch Warner and Swasey refractor.","narrative":"Built in 1896 on the University of Illinois campus in Urbana, this small brick observatory was designed by architect Charles A. Gunn and built by Bevis and Company. It has a T-shaped plan, buff Roman brick walls, limestone sills and lintels, and a round tower holding a dome and a twelve-inch Warner and Swasey refractor telescope. Between 1907 and 1922, director Joel Stebbins, physics instructor F.C. Brown and physics professor Jacob Kunz used that telescope to develop first the selenium cell and then the alkali-metal photoelectric cell for measuring starlight. Their work replaced visual and photographic estimates of stellar brightness with precise electrical measurement, which became the standard method in astronomy. Additions from 1956 and 1966 enlarged the building, but the tower, equatorial room, dome, pier and telescope remain original. The observatory is still used for teaching, though it needs repairs to its deck, cornice and dome.","period_of_significance":"1900-1922 (specific dates 1907-1922)","architect_builder":"C.A. Gunn, architect; Bevis & Co., contractors","architectural_style":null,"criteria":[],"status_note":"Occupied and still used as a teaching observatory. Additions were built in 1956 and 1966. The building is structurally sound but has suffered from deferred maintenance, including a deteriorating observation deck, cornice and dome surface, and water damage in the equatorial room.","vacant_flag":false,"machine_generated":true,"model":"claude-sonnet-5-5","prompt_version":"v1.0-2026-10-29","generated_at":"2026-10-29 02:55:35.793525-05","source_pdf":"https://s3.amazonaws.com/NARAprodstorage/opastorage/live/44/8918/28891844/content/electronic-records/rg-079/NPS_IL/86003155.pdf"},"photo":{"url_800":"https://nationalhistoricalregister.com/media/86003155/b24df7403e-800.webp","url_1600":"https://nationalhistoricalregister.com/media/86003155/b24df7403e-1600.webp","licence":"CC BY 2.0","author":null,"credit_html":"<a href=\"https://commons.wikimedia.org/wiki/File:Champaign-Urbana_area_IMG_1138.jpg\" rel=\"noopener\">Unknown author</a>, <a href=\"https://creativecommons.org/licenses/by/2.0\" rel=\"license noopener\">CC BY 2.0</a>, via Wikimedia Commons","source_page":"https://commons.wikimedia.org/wiki/File:Champaign-Urbana_area_IMG_1138.jpg"},"district":null,"sources":{"nps_snapshot_date":"2026-05-22","nara_catalog":"https://catalog.archives.gov/id/28891844","wikipedia":"https://en.wikipedia.org/wiki/Astronomical_Observatory_(University_of_Illinois_Urbana-Champaign)","wikidata":"https://www.wikidata.org/wiki/Q3348346"},"indexable":true,"url":"https://nationalhistoricalregister.com/illinois/champaign-county/university-of-illinois-astronomical-observatory-86003155/"}