apoapsis estimated by different sources at from 0.008 to 0.06. The mean size of the ring particles is 0.2–20.0 m, and the mean separation is around 4.5 times their radius. The ring is almost devoid of dust, possibly due to the aerodynamic drag from Uranus's extended atmospheric corona. Due to its razor-thin nature the ε ring is invisible when viewed edge-on. This happened in 2007 when a ring plane-crossing was observed. The Voyager 2 spacecraft observed a strange signal from the ε ring during the radio occultation experiment. The signal looked like a strong enhancement of the forward-scattering at the wavelength 3.6 cm near ring's apoapsis. Such strong scattering requires the existence of a coherent structure. That the ε ring does have such a fine structure has been confirmed by many occultation observations. The ε ring seems to consist of a number of narrow and optically dense ringlets, some of which may have incomplete arcs. The ε ring is known to have interior and exterior shepherd moons—Cordelia and Ophelia, respectively. The inner edge of the ring is in 24:25 resonance with Cordelia, and the outer edge is in 14:13 resonance with Ophelia. The masses of the moons need to be at least three times the mass of the ring to confine it effectively. The mass of the ε ring is estimated to be about 1016 kg. δ (delta) ring The δ ring is circular and slightly inclined. It shows significant unexplained azimuthal variations in normal optical depth and width. One possible explanation is that the ring has an azimuthal wave-like structure, excited by a small moonlet just inside it. The sharp outer edge of the δ ring is in 23:22 resonance with Cordelia. The δ ring consists of two components: a narrow optically dense component and a broad inward shoulder with low optical depth. The width of the narrow component is 4.1–6.1 km and the equivalent depth is about 2.2 km, which corresponds to a normal optical depth of about 0.3–0.6. The ring's broad component is about 10–12 km wide and its equivalent depth is close to 0.3 km, indicating a low normal optical depth of 3 × 10−2. This is known only from occultation data because Voyager 2's imaging experiment failed to resolve the δ ring. When observed in forward-scattering geometry by Voyager 2, the δ ring appeared relatively bright, which is compatible with the presence of dust in