Moons of Uranus

The first two moons to be discovered were Titania and Oberon, which were spotted by Sir William Herschel on January 11, 1787, six years after he had discovered the planet itself. Later, Herschel thought he had discovered up to six moons (see below) and perhaps even a ring. For nearly 50 years, Herschel's instrument was the only one with which the moons had been seen. In the 1840s, better instruments and a more favorable position of Uranus in the sky led to sporadic indications of satellites additional to Titania and Oberon. Eventually, the next two moons, Ariel and Umbriel, were discovered by William Lassell in 1851. With the confirmation of Ariel and Umbriel, Lassell numbered the moons I through IV from Uranus outward, and this finally stuck. Decades later, the flyby of the Voyager 2 space probe in January 1986 led to the discovery of ten further inner moons. Spurious moons After Herschel discovered Titania and Oberon on 11 January 1787, he subsequently believed that he had observed four other moons: two on 18 January and 9 February 1790, and two more on 28 February and 26 March 1794. It was thus believed for many decades thereafter that Uranus had a system of six satellites, though the four latter moons were never confirmed by any other astronomer. Lassell's observations of 1851, in which he discovered Ariel and Umbriel, however, failed to support Herschel's observations; Ariel and Umbriel, which Herschel certainly ought to have seen if he had seen any satellites besides Titania and Oberon, did not correspond to any of Herschel's four additional satellites in orbital characteristics. Herschel's four spurious satellites were thought to have sidereal periods of 5.89 days (interior to Titania), 10.96 days (between Titania and Oberon), 38.08 days, and 107.69 days (exterior to Oberon). It was therefore concluded that Herschel's four satellites were spurious, probably arising from the misidentification of faint stars in the vicinity of Uranus as satellites, and the credit for the discovery of Ariel and Umbriel was given to Lassell. == Names ==

Although the first two Uranian moons were discovered in 1787, they were not named until 1852, a year after two more moons had been discovered. The responsibility for naming was taken by John Herschel, son of the discoverer of Uranus. Herschel, instead of assigning names from Greek mythology, named the moons after magical spirits in English literature: the fairies Oberon and Titania from William Shakespeare's ''A Midsummer Night's Dream'', and the sylph Ariel and gnome Umbriel from Alexander Pope's The Rape of the Lock (Ariel is also a sprite in Shakespeare's The Tempest). It is uncertain if John Herschel was the originator of the names, or if it was instead William Lassell (who discovered Ariel and Umbriel) who chose the names and asked Herschel for permission. Subsequent names, rather than continuing the airy spirits theme (only Puck and Mab continued the trend), have focused on Herschel's source material. In 1949, the fifth moon, Miranda, was named by its discoverer Gerard Kuiper after a thoroughly mortal character in Shakespeare's The Tempest. The current IAU practice is to name moons after characters from Shakespeare's plays and The Rape of the Lock (although at present only Ariel, Umbriel, and Belinda have names drawn from the latter; all the rest are from Shakespeare). The outer retrograde moons are all named after characters from one play, The Tempest; the sole known outer prograde moon, Margaret, is named from Much Ado About Nothing. Some asteroids, also named after the same Shakespearean characters, share names with moons of Uranus: 171 Ophelia, 218 Bianca, 593 Titania, 666 Desdemona, 763 Cupido, and 2758 Cordelia. == Characteristics and groups ==

The Uranian satellite system is the least massive among those of the giant planets. Indeed, the combined mass of the five major satellites is less than half that of Triton (the seventh-largest moon in the Solar System) alone. The largest of the satellites, Titania, has a radius of 788.9 km, At 162 km, Puck is the largest of the inner moons of Uranus and the only one imaged by Voyager 2 in any detail. Puck and Mab are the two outermost inner satellites of Uranus. All inner moons are dark objects; their geometrical albedo is less than 10%. and Cupid will likely collide with Belinda in the next 10 million years; Perdita and Juliet may be involved in later collisions. Because of this, the rings and inner moons may be under constant flux, with moons colliding and re-accreting on short timescales. This view is supported by their large thermal inertia, a surface property they share with dwarf planets like Pluto and Haumea. It differs strongly from the thermal behaviour of the Uranian irregular moons that is comparable to classical trans-Neptunian objects. The largest Uranian moons may be internally differentiated, with rocky cores at their centers surrounded by ice mantles. Titania and Oberon may harbor liquid water oceans at the core/mantle boundary. The major moons of Uranus are airless bodies. For instance, Titania was shown to possess no atmosphere at a pressure larger than 10–20 nanobar. The path of the Sun in the local sky over the course of a local day during Uranus's and its major moons' summer solstice is quite different from that seen on most other Solar System worlds. The major moons have almost exactly the same rotational axial tilt as Uranus (their axes are parallel to that of Uranus). The Sun would appear to follow a circular path around Uranus's celestial pole in the sky, at the closest about 7 degrees from it, during the hemispheric summer. Near the equator, it would be seen nearly due north or due south (depending on the season). At latitudes higher than 7°, the Sun would trace a circular path about 15 degrees in diameter in the sky, and never set during the hemispheric summer, moving to a position over the celestial equator during the Uranian equinox, and then invisible below the horizon during the hemispheric winter. Irregular moons ) in the horizontal axis and orbital inclination in the vertical axis. The semi-major axis values are expressed as a fraction of the planet's Hill sphere's radius, while the inclination is expressed in degrees from the ecliptic. The radius of the Uranian Hill sphere is approximately 73 million km. The relative sizes of moons are indicated by the size of their symbols, and the Caliban group of Uranian moons is labeled. Data as of February 2024. Uranus's irregular moons range in size from 120 to 200 km (Sycorax) to under 10 km (S/2023 U 1). Due to the small number of known Uranian irregular moons, it is not yet clear which of them belong to groups with similar orbital characteristics. The only known group among Uranus's irregular moons is the Caliban group, which is clustered at orbital distances between and inclinations between 141°–144°. The Caliban group includes three retrograde moons, which are Caliban, S/2023 U 1, and Stephano. 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 has one of the most eccentric orbits of any moon in the Solar System. == List ==

and orbital distances for Uranus's rings and moon system at various scales. Open the image for full resolution. 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 10,000-year numerical integration by Sheppard et al. (2024). These may differ from osculating orbital elements provided by other sources. The orbital elements of the inner moons listed here are based on the epoch of 1 January 2025; those for the major moons are based on the epoch of 1 January 2000; while orbital elements of irregular satellites are based on the epoch of 1 January 2020. } || S/2023 U 1♦ || || bgcolor="black" | || 13.7 || || || || || 143.9 || 0.250 || 2023 || 2024 || Sheppard et al. || Caliban == See also ==