THE BRIGHTNESS, ALBEDO, AND TEMPORAL VARIABILITY OF NEPTUNE

Since 1972, the brightness of Neptune at 4720 and 5510 angstrom has slowly varied with an amplitude of 4%, apparently anticorrelated with cyclic solar activity. In addition, there is a secular trend towards redder color. The night-to-night variation of brightness tends to be greater in seasons when the planet is relatively bright. From annual spectrophotometric observations at 8 angstrom resolution, 3295-8880 angstrom, we have computed the geometric albedo spectrum for 1982, when the planet was relatively faint, and for 1987, when it was relatively bright. The two spectra do not differ substantially from each other, but yield a significantly higher albedo in the ultraviolet compared with the values published by Neff et al. based on 1981 observations and a different solar irradiance spectrum.