Discussion

Abstract

Kopal: I should like to express our sincere thanks to professor Minnaert for so lucid a survey of such fascinating field as represented by the photometric methods in the service of lunar topography; and to make two brief remarks. First, the shadow method does not permit us to measure altitude differences between any two points of the lunar surface, but only between those which are situated on a “parallel of illumination” which passes through the shadow-casting obstacle; and only within such fanwise spread of it as is afforded by the libration in latitude. The second point which I should like to make concerns the need of separating local albedo and inclination in lunar terrain studies. I agree with professor Minnaert that this is impossible as long as observations have been carried out in only one colour. However, colorimetric studies of the Moon indicate that a change in local albedo is usually (if not always) accompanied by a change in colour index which, small as it is, can nowadays be measured well within the limits of observational precision. Therefore, the best way of ensuring that what we measure as an albedo change should be interpreted as a change in slope, and not in reflectivity, would be to verify that change in albedo is not accompanied by a change in colour index; for only when this is the case can we surmise to our satisfaction that it is the inclination, and not reflectivity, which has undergone a local change.