Diameter, flattening and optical properties of the upper atmosphere of Neptune as derived from the occultation of BD -17 4388.

The occulation of BD-17 4388 by Neptune that occurred on April 7, 1968 was observed photometrically in Japan and Australia. Precise photometric curves obtained at Dodaira and Okayama observatories were carefully studied and this paper presents the results of this new reduction (Kovalevsky et Link, 1968). The principle of the astrometric reduction of the occulation is described. The phenomenon is studied in projection on a plane perpendicular to the Earth-Star direction with an angiilar scale corresponding to the actual distance Earth-Neptune. The apparent geocentric motion of Neptune is deduced from a series of positional observations by Miller on April 5th to April 8th. It is in good agreement with the ephemerides and gives the scaling with a maximum error of 0.001. The formulae relating the relative position of Neptune and the star at a given time, the equatorial radius of Neptune and eventually its apparent flattening are given (8) and (10). It is assumed that the apparent direction of the axis of flattening is known from other sources (Gffl and Gault, 1968). What is actually obtained is the apparent dimensions and shape of a certain atmospheric layer at which the ratio of the light flux from the star arriving at the Earth is reduced to one half, and which we call the hall-intensity layer. As the results of the reduction of the actual observations, we obtained the following results: Position of Neptune versus the star at U.T.