Analysis of plasma data from the geosynchronous satellites ATS 5 and 6 shows that the satellites vary rapidly in potential as they pass into and out of the earth's shadow. These variations result from the loss of photoelectrons as the sun is eclipsed. A model of this phenomenon which includes the effects of atmospheric attenuation of the solar flux between ~1000 and ~3000 ¿ and spacecraft charge buildup is used to predict the potential variations observed by ATS 5 and 6. The model predicts potentials within ¿700 V of the observed potentials in the range O to --10,000 V.