We have detected a component of sky brightness varying with a frequency of 120 Hertz during high speed photometric observations of 40 Eridani with the 30-cm telescope of the University of British Columbia. The observations were made in 1974, but only recently has their significance to the field of urban illumination been realized. The 120 Hertz component was detected by Fourier analysis of data sets of the star and of the sky alone. We confirmed that the 120 Hertz component was due to urban lighting by direct observation of a “standard source,” a lamp on the campus.