Birth of Photoelectric Photometry

     The University of Illinois Observatory is historically significant for the development of selenium and photoelectric cell photometry that revolutionized the measurement of celestial magnitudes. The determination of stellar magnitudes is one of the most fundamental measurements in astronomy. During the years from 1907 to 1922, Observatory director Joel Stebbins worked with physicists F.C. Brown, W.F. Schulz, and Jakob Kunz to develop and refine electric photometry.

     In 1910, Stebbins published a study of the eclipsing binary star Algol ( Persei) using a photometer based on a selenium cell.  This work was the most accurate set of photometric observations up to that time and was the first demonstration of the secondary eclipse, proof of the photometer’s superiority to other methods. Stebbins continued to study eclipsing variable stars and refining the instrument. By 1914, he replaced the selenium cell with a photoelectric cell improving sensitivity. The use of electricity for empirically gathering astronomical data revolutionized astronomy.

As a result of Stebbins' work beginning at Illinois and continuing at the University of Wisconsin, photoelectric photometry became the standard technique for determining stellar magnitudes in the 20th Century. It transformed the measurement of astronomical radiation from imprecise visual and photographic methods, to a quantifiable science.

     In 1989, the United States Department of the Interior designated the Observatory and National Historic Landmark. 

     This exhibit highlights some of the collection that was used in the pioneering research.