Margaret, also known as Uranus XXIII and previously as S/2003 U 3, is an irregular moon of Uranus. It is the only known irregular moon of Uranus that follows a prograde orbit, meaning it orbits in the same direction as the planet's orbit around the Sun. It was discovered by Scott S. Sheppard and David C. Jewitt at Mauna Kea Observatory on 29 August 2003, although it had been briefly observed earlier by Matthew J. Holman and JJ Kavelaars on 13 August 2001. The moon is formally named after Margaret, the servant of Hero in William Shakespeare's play Much Ado About Nothing, while also alluding to the discoverers' family members. Margaret follows a distant, highly inclined, and eccentric orbit around Uranus, revolving at an average distance of 14.4 million km (8.9 million mi) with an orbital period of 4.53 years (1,655 days). It has one of the most eccentric orbits of any known moon in the Solar System. The Sun's gravitational influence places the moon in the Kozai resonance, which causes large, synchronized variations in its orbital eccentricity and inclination over a 500-year period. Margaret is estimated to have a diameter of 20 km (12 mi), though most of its physical properties are unknown.
Discovery Margaret was initially spotted on 13 August 2001 by astronomers Matthew J. Holman and JJ Kavelaars, during their search for distant moons of Uranus at Cerro Tololo Inter-American Observatory. The group had planned to reobserve Margaret by September, but poor observational circumstances prevented them from doing so. With only one night of observations, the moon's orbit could not be determined and it became lost. Margaret was not spotted again until 29 August 2003, when Scott S. Sheppard and David C. Jewitt began searching for Uranian moons using the 8.3-meter (27 ft) Subaru Telescope at Mauna Kea Observatory. After they reobserved Margaret with Mauna Kea's Gemini North telescope on 20 September, the head of the Minor Planet Center, Brian G. Marsden, recognized that it was the same moon observed by Holman and Kavelaars in 2001. Kavelaars confirmed Marsden's suspicion after he identified a single Canada–France–Hawaii Telescope image of Margaret from 25 August 2001. Once follow-up observations ended on 30 September 2003, the Minor Planet Center announced the discovery of Margaret on 7 October. NASA and the International Astronomical Union consider Sheppard and Jewitt as the official discoverers of Margaret. For several years after its discovery, Margaret remained poorly observed and still had an uncertain orbit, which put it at risk of being lost again. To improve the understanding of its orbit, a team led by Robert A. Jacobson and Marina Brozović conducted a campaign to reobserve Margaret and several other poorly observed moons from 2009 to 2011. Despite these observations, Margaret's orbital uncertainty persisted and remained unobserved until 8 September 2021, when Sheppard began another search for Uranian moons with the Subaru Telescope. As of 2026, Margaret's observation arc spans 20.32 years (7,421 days) with its last recorded observation on 7 December 2021.
Name When the discovery of Margaret was announced, it was given the temporary provisional designation "S/2003 U 3" by the Minor Planet Center. It was later named and given the Roman numeral designation Uranus XXIII by the International Astronomical Union's Working Group for Planetary System Nomenclature on 29 December 2005. The moon was named after the servant of Hero from William Shakespeare's play, Much Ado About Nothing. In a 2014 interview with the Folger Shakespeare Library, Sheppard disclosed that he chose the name "Margaret" because it matched his mother's name. He got the idea of naming a moon after his mother when he searched up her name on the Web in 2002. Kavelaars, who initially spotted Margaret in 2001, supported Sheppard's name choice because it matched the middle name of his elder daughter, Ruth Ann Margaret Kavelaars.
Orbit Margaret is one of Uranus's irregular moons, which follow wide, eccentric, and inclined orbits in contrast to the regular moons of Uranus. It is the only known prograde irregular moon of Uranus, orbiting in the same direction as the planet's orbit around the Sun. Irregular moons like Margaret are thought to have been captured by their planet during the Solar System's formation, although the exact process for this is uncertain. Simulations by Matija Ćuk and Brett Gladman in 2006 showed that prograde irregular moons like Margaret could have been directly captured by Uranus if their orbits became stabilized by a 1:2 orbital resonance between Saturn and Uranus. The Uranian irregular moons are loosely bound by Uranus's gravity due to their great distance from the planet, which makes their orbits susceptible to gravitational perturbations by the Sun and other planets. This results in significant changes to their orbits over short periods of time, so a simple Keplerian elliptical orbit cannot accurately describe their long-term orbital motions. Instead, proper or mean orbital elements are used to describe the long-term orbits of irregular moons more accurately, since these are calculated by averaging out the perturbed orbit over an extended period of time. Over a 10,000-year time span, Margaret's semi-major axis from Uranus averages at about 14.4 million km (8.9 million mi; 0.096 AU), with an average orbital period of 4.53 years (1,655 days). Its orbit is markedly more eccentric and inclined compared to other Uranian irregular moons, having an average eccentricity of 0.64 and an average inclination of 61° with respect to the ecliptic. Compared to other known moons of the Solar System as of 2026, Margaret has the fifth-highest average orbital eccentricity.
Kozai resonance
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