on axis/orbital eccentricity. The inner group includes those satellites closer to Uranus (a < 0.15 rH) and moderately eccentric (~0.2), namely Francisco, Caliban, Stephano, and Trinculo. The outer group (a > 0.15 rH) includes satellites with high eccentricity (~0.5): Sycorax, Prospero, Setebos, and Ferdinand. The intermediate inclinations 60° < i < 140° are devoid of known moons due to the Kozai instability. In this instability region, solar perturbations at apoapse cause the moons to acquire large eccentricities that lead to collisions with inner satellites or ejection. The lifetime of moons in the instability region is from 10 million to a billion years. Margaret is the only known irregular prograde moon of Uranus, and it currently has the most eccentric orbit of any moon in the Solar System, though Neptune's moon Nereid has a higher mean eccentricity. As of 2008, Margaret's eccentricity is 0.7979. List The Uranian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in light blue and bolded. The inner and major moons all have prograde orbits. Irregular moons with retrograde orbits are shown in dark grey. Margaret, the only known irregular moon of Uranus with a prograde orbit, is shown in light grey. The orbits and mean distances of the irregular moons are variable over short timescales due to frequent planetary and solar perturbations, therefore the listed orbital elements of all irregular moons are averaged over a 8,000-year numerical integration by Brozović and Jacobson (2009). Their orbital elements are all based on the epoch of 1 January 2000 Terrestrial Time. Sources: NASA/NSSDC, Sheppard, et al. 2005. For the recently discovered outer irregular moons (Francisco through Ferdinand) the most accurate orbital data can be generated with the Minor Planet Center's Natural Satellites Ephemeris Service. The irregulars are significantly perturbed by the Sun. See also List of natural satellites Notes References External links Scott S. Sheppard: Uranus Moons Simulation Showing the location of Uranus's Moons Gazetteer of Planetary Nomenclature—Uranus (USGS) Lists of moons