INTERPLANETARY NAVIGATION STUDIES: LIGHT CENTERS OF TERRESTRIAL PLANETS
GEOPHYSICS CORP OF AMERICA BOSTON MASS
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PHASE SHIFT FOR A LAMBERT SPHERE A theoretical estimate is made of the differences in direction between an observer and a light center of a planet when the phase angle is not zero. The Lambert sphere is a sphere whose surface is a perfect diffusor it is the simplest model of reflection by a planet. For actual planets, the phase functions decrease much faster away from full phase than for a Lambert sphere. When the phase angle approaches 180 degrees, the actual planets will depart fron the Lambert sphere in different directions according to the nature of the reflecting surface. OBSERVED PHASE DEFECTS OF MERCURY AND VENUS The fractions of the disk illuminated are compared as between computed and observed values. Curves for Venus tend to show the effect of limb darkening at small phase angles and of atmospheric scattering at large phase angles. The curve for Mercury probably reflects the systematic errors of measurements. LIGHT CENTER OF MERCURY The observational evidence on the departures between the center of light and the center of mass of mercury is analyzed.