Note on a Photometric Determination of the Moon’s Form from Eclipse Observations

Abstract

In reference to the foregoing communication presented by Dr. Eckhardt, I should like to propose another method for photometric measurement of the shape of the Moon’s limb during annular solar eclipses. The method consists in rotating a system of two prisms A and B around the optical axis xx of the objective (see Figure 1). The prism A collects light from a sector of the annulus A, the focal plane being located at the exit face of prism B. A slit S is located close to this face, rotating with the prism and oriented normally to the limbs of the Sun and the Moon. A field lens, following the slit, forms an image of the objective on the photocathode of a photomultiplier, and the photocurrent is recorded between the second and third contacts of the eclipse.