Uranus and the shape of elliptical rings

Abstract

AN interesting question raised by the discovery^1–3 of rings around Uranus concerns the ellipticity of the ε ring, the outermost of the five rings detected. When the star SAO158687 passed behind the Uranus system (with a closest approach to the centre of the planet of about 25,000 km) its light was occulted twice by the ε ring. The first part of the ring to occult was about 100 km wide and 51,700 km from the planet's centre; the second part was about 40 km wide and 51,050 km from the centre¹ (J. L. Elliot, personal communication). These two points were separated in angle by about 120°. The average transmission of starlight during these events, 35% and 10%, respectively, lead to optical depths that are, within experimental error, inversly proportional to the two widths (J. L. Elliot, personal communication), strongly suggesting that there is the same amount of material in the rings at the two points sampled. Here we account for the variable width of the ring by differences in the orbital eccentricities of the individual particles composing the ring.