Secular brightness increases of Titan, Uranus, and Neptune, 1972–1976

Abstract

Abstract The brightnesses of Titan, Uranus, and Neptune in b (4718 AA) and y (5508 AA) have increased linearly since 1972 at rates ranging from 0.005 to 0.025 mag yr −1 . The observations were made differentially on a number of nights each season with respect to a network of comparison stars whose relative magnitudes were determined by independent measurements. Solar phase coefficients were derived for each object, and all observations have been normalized to zero solar phase angle and mean heliocentric distances. No explanation for the changes has been found, but a possible influence of solar activity upon planetary albedo is suggested by the fact that all of the objects observed have brightened during the declining half of the solar cycle.