Measurements of Secchi depth in Garner Lake, TN, were made with and without a viewing tube at approximately 20-minute intervals from sunrise to sunset on 12 days throughout a year. The goal was to determine which factors affect the apparent Secchi depth as a function of time of day as well as date. The data show that on any given day the apparent (measured) Secchi depth depends on solar altitude. This relationship has been formally modeled. A key point in the model is that the magnitude of the observed Secchi depth is determined by the trajectory of abeamof sunlight through the water. The model calculations allow the Secchi depth observed at any date, time, or geographical location to be normalized to a standard, the value that would be found with the sun in the zenith. Use of such a normalization offers the opportunity to accurately quantify and study Secchi depth variations over time, or differences between lakes in different geographical locations. Based on the experience gained in this experiment, several suggestions are offered on how to obtain the most accurate measurements on any given day.