exhibit regular variations in brightness and width. They are brightest and widest 30° from the apoapsis and dimmest and narrowest 30° from the periapsis. The α and β rings have sizable orbital eccentricity and non-negligible inclination. The widths of these rings are 4.8–10 km and 6.1–11.4 km, respectively. The equivalent optical depths are 3.29 km and 2.14 km, resulting in normal optical depths of 0.3–0.7 and 0.2–0.35, respectively. During a ring plane-crossing event in 2007 the rings disappeared, which means they are geometrically thin like the ε ring and devoid of dust. The same event revealed a thick and optically thin dust band just outside the β ring, which was also observed earlier by Voyager 2. The masses of the α and β rings are estimated to be about 5 kg (each)—half the mass of the ε ring. Rings 6, 5 and 4 Rings 6, 5 and 4 are the innermost and dimmest of Uranus's narrow rings. They are the most inclined rings, and their orbital eccentricities are the largest excluding the ε ring. In fact, their inclinations (0.06°, 0.05° and 0.03°) were large enough for Voyager 2 to observe their elevations above the Uranian equatorial plane, which were 24–46 km. Rings 6, 5 and 4 are also the narrowest rings of Uranus, measuring 1.6–2.2 km, 1.9–4.9 km and 2.4–4.4 km wide, respectively. Their equivalent depths are 0.41 km, 0.91 and 0.71 km resulting in normal optical depth 0.18–0.25, 0.18–0.48 and 0.16–0.3. They were not visible during a ring plane-crossing event in 2007 due to their narrowness and lack of dust. Dusty rings λ (lambda) ring The λ ring was one of two rings discovered by Voyager 2 in 1986. It is a narrow, faint ring located just inside the ε ring, between it and the shepherd moon Cordelia. This moon clears a dark lane just inside the λ ring. When viewed in back-scattered light, the λ ring is extremely narrow—about 1–2 km—and has the equivalent optical depth 0.1–0.2 km at the wavelength 2.2 μm. The normal optical depth is 0.1–0.2. The optical depth of the λ ring shows strong wavelength dependence, which is atypical for the Uranian ring system. The equivalent depth is as high as 0.36 km in the ultraviolet part of the spectrum, which explains why λ ring was initially detected only in UV stellar occultations by Voyager 2. The detection during a stellar occultation at the