What can we say about the Shape of the Moon?

Abstract

After a brief history, we propose to discuss the seven principal series of observations and their characteristics. From the subsequent comparison there will emerge, on one hand, systematic amounts by which to reduce the altitude data to a common level; and, on the other, values of the absolute accuracy which amounts to approximately ±1.1 km for three series, and for the others to approximately ±1.8 km. The causes of the various differences will be investigated.

Plans are disclosed to compile (similarly as was done for trigonometric parallaxes and radial velocities) a “Preliminary General Catalogue of Absolute Heights on the Moon”. At the present stage of preliminary studies we can expect to be able to list approximately 410 certain and 600 less reliable heights. It is also planned to draw a contour map. Preliminary studies of the map of the Mare Cognitum and the LAC charts Nos. 59 and 78 of ACIC showed that local characteristics (as, for example, degrees of steepness at the shores of the maria) must be taken into account.

We shall discuss the various attempts to represent the shape of the Moon by a tri-axial ellipsoid or by a series of spherical harmonics. It seems to be more appropriate to regard the Moon as a sphere covered with extended “bulges” (mostly the continents) and “craters” (largely the maria). The departures of the heights of these formations from the mean Moon-level exceed but rarely ± 2 km.