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Published online by Cambridge University Press: 14 August 2015
A critical analysis of selected high-quality photometric observations of Mars indicates that: (1) The phase function is concave upward out to at least 40° phase. No sudden brightening occurs at opposition, but the curvature increases at small phase. (2) Large systematic differences (0.1–0.2 mag.) exist between different observers' data. However, the small random scatter attributable to Mars (0.01–0.02 mag.) in the better series suggests that these differences represent systematic errors in data reduction, not variations in the planet's brightness. (3) The disentangling of seasonal, diurnal, and phase effects leaves considerable ambiguity; more observations are needed, over a long time, with a stable instrumental system. However, even the present data are sufficient to expose substantial errors in published phase curves of Mars (and consequently, in interpretations based on them).