Viking Lander camera images of the Sun were used to compute total normal atmospheric optical depth at the two landing sites over a period of about 900 Mars days. The data set contains 1044 measurements of optical depth and associated error estimates. The optical depths were derived from Sun diode images, which were obtained by the Lander cameras at a wavelength of 0.67 micrometers. Note that no images of Phobos were used in deriving this data set. Studies of diurnal, seasonal, and annual variations in optical depth, surface-based observations of great dust storms, and analyses of the contributions of ice haze and dust particles to the observed optical depth, have been carried out using these data. See COLBURN_ETAL_1989, POLLACK_ETAL_1977, and POLLACK_ETAL_1977 for further information on the derivation and interpretation of this data set.