PRELIMINARY RESULTS OF SPECTROGRAPHIC OBSERVATIONS FOR ROTATION OF NEPTUNE

Abstract

Information concerning the rotation of the planet Neptune may be derived by three independent methods of investigation: (a) by photometric observations of the planet; (b) from perturbations of the satellite's orbit due to oblateness of the planet; (c) by spectrographic observations of the inclination of the spectral lines when the slit is placed parallel to the planet's equator. Neptune presents a disk about 2. 5 in diameter, apparently perfectly circular and without surface markings. Photometric observations of the planet made by Maxwell Hall2 at Jamaica in 1883 and 1915 indicated a variation in light with a period of 7.835 hours. A more recent discussion in 1924 of all photometric data, by Opik and Livlander3 yields two superposed periods of 7.707 and 7.836 hours. They suggest that the two periods arise from the relative drifts of darkened areas at different latitudes. It is easily shown that the ellipticity of the disk for such a period of rotation would be of the order of l/12th, i.e., the difference between polar and equatorial diameters would be 0.2, a quantity which could hardly have escaped detection. This lends considerable weight to the suggestion by Jackson4 that the actual period may be some multiple of 7.8 hours such as would be caused by the presence of spots differing in longitude. It is well known that the inclination of the plane of the satellite's orbit shows a steady variation, having changed about 10° in the interval 1850 to 1900. On the assumption that this