A new satellite of Uranus

Abstract

While observing the occultation of the star SAO 158687 by Uranus on 10 March 1977 we came across an unexpected event suggesting the existence of a hitherto unknown satellite of the planet. The observations were carried out at the Kavalur Observatory (lat. 12° 34' 32″ N: long 78° 49' 54″ E) using a near infrared photometer consisting of a refrigerated EMI 9558B phototube and Kodak Wratten 89B filter attached to the Cassegrain focus of the 102-cm telescope. The individual deflections due to the star and the planet were measured during the previous night, as well as a few hours before the predicted instant of occultation. A 16-arc s diaphragm was used during occultation, whereby the planet and the star could be conveniently contained for several hours around the epoch of occultation. Guiding and monitoring of the objects were done through the 20-cm guide telescope, with periodic checking of the centering in the main telescope. As indicated by the prediction, we expected only a grazing occultation to be visible at Kavalur, resulting in a slight diminution in the intensity of starlight owing to the extinction effect by differential refraction in the outer layers of the planet's atmosphere. A very shallow depression of 0.046 magnitudes in the light of SAO 158687 was indeed observed.