Standing just to the north of the historic Morrow Plots, the University of Illinois Astronomical Observatory was designed by Charles Gunn. In 1895, the Illinois state legislature allotted funds to replace the university's previous observatory. It was built in 1896 using brick and limestone and features an octagonal observation tower that rises 35 feet above the ground. Since its construction, the observatory has contributed greatly to advances in astronomy. For example, during Joel Stebbins' time at the university, he developed the selenium photoelectric cell, which allowed astronomers to measure stars' magnitudes with greater accuracy.
This site was added to the National Register of Historic places in 1986 and became a National Historic Landmark in 1989.
Joel Stebbins (1878-1966), Director of the University of Illinois ObservatoryStebbins earned his doctorate from the University of California, Berkeley in 1903. He then came to the University of Illinois, where he worked at the Observatory and taught classes in Math and Astronomy. At the time, the university did not have a separate Astronomy Department, nor did it supply an adequate budget for operating the Observatory. Stebbins successfully requested the creation of a Department of Astronomy, which opened for the 1905-06 school year with a budget of $750. His research at the Observatory changed the way celestial magnitude's were measured. Before 1907, methods for ascertaining stars' magnitudes were limited. For example, astronomers could estimate a star's magnitudes visually, which often led to a substantial margin of error. In the 1850s, Harvard's William Bond introduced the method of photographic photometry, which involved producing stellar images (with a photometer such as the one developed by Edward Pickering in the 1870s) and comparing their sizes and densities. Joel Stebbins sought to make improvements on these methods. In a 1957 speech before the American Astronomical Society in 1957, he explained the process of developing a new method of photometry, with the help of his wife, May Stebbins. In his words:Not enjoying home alone, she [May Stebbins] found that if she came to the observatory and acted as recorder, she could get me home earlier. She wrote down the numbers as the observer called them, but after some nights of recording a hundred readings to get just one magnitude, she said it was pretty slow business. I responded that someday we would do this by electricity. That was a fatal remark. Thereafter she would often prod me with the question: When are you going to change to electricity? It happened that within a two or three months the department of physics gave an open house, and one of the exhibits was in charge of a young instructor F.C. Brown. He showed how when he turned on a lamp to illuminate a selenium cell, a bell would ring; when the lamp was off, the bell would stop. Here was the idea; why not turn a star on to a cell on a telescope and measure the current? (Stebbins, Early Photometry, 507; quoted in History of the Observatory).For several years, Stebbins and his colleagues continually worked to develop and improve selenium cell photometry, and then more advanced photoelectric cells. Stebbins left Illinois in 1922. He was elected to the National Academy of Science the same year. His career contributions were vital to modern photoelectric astronomy.