| Abstract: | Photographic measures of surface features on Mercury have led to a rotation period of 58.663 +/- 0.021 days, which is in good agreement with the 58.646-day period required by a predicted 2:3 resonance between the axial and orbital periods. The incorrect interpretation of earlier visual and photographic observations which supported an 88-day rotation period appears to be partially explained by peculiar characteristics associated with the observability of various hermo-graphic longitudes. The apparent contrast of most of the recorded surface features is marginal for visual observation when viewed through the terrestrial daytime sky. The intrinsic contrast of a relatively conspicuous feature was measured as 0.20, a value lower than that of typical markings observed on the moon and Mars. |
