Uranus, the seventh planet of the Solar System, has 27 known moons, most of which are named after characters that appear in, or are mentioned in, the works of William Shakespeare and Alexander Pope. Uranus’s moons are divided into three groups: thirteen inner moons, five major moons, and nine irregular moons. The inner moons are small dark bodies that share common properties and origins with Uranus’s rings.
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. The Roman numbering scheme of Uranus’s moons was in a state of flux for a considerable time, and publications hesitated between Herschel’s designations (where Titania and Oberon are Uranus II and IV) and William Lassell’s (where they are sometimes I and II). With the confirmation of Ariel and Umbriel, Lassell numbered the moons I through IV from Uranus outward, and this finally stuck. In 1852, Herschel’s son John Herschel gave the four then-known moons their names.
In 1948, Gerard Kuiper at the McDonald Observatory discovered the smallest and the last of the five large, spherical moons, Miranda. Decades later, the flyby of the Voyager 2 space probe in January 1986 led to the discovery of ten further inner moons. Another satellite, Perdita, was discovered in 1999 after studying old Voyager photographs.
Uranus was the last giant planet without any known irregular moons, but since 1997 nine distant irregular moons have been identified using ground-based telescopes. Two more small inner moons, Cupid and Mab, were discovered using the Hubble Space Telescope in 2003. As of 2016, the moon Margaret was the last Uranian moon discovered.
- The Rape of the Lock (a poem by Alexander Pope):
- Ariel, Umbriel, Belinda
- Plays by William Shakespeare:
- A Midsummer Night’s Dream: Titania, Oberon, Puck
- The Tempest: (Ariel), Miranda, Caliban, Sycorax, Prospero, Setebos, Stephano, Trinculo, Francisco, Ferdinand
- King Lear: Cordelia
- Hamlet: Ophelia
- The Taming of the Shrew: Bianca
- Troilus and Cressida: Cressida
- Othello: Desdemona
- Romeo and Juliet: Juliet, Mab
- The Merchant of Venice: Portia
- As You Like It: Rosalind
- Much Ado About Nothing: Margaret
- The Winter’s Tale: Perdita
- Timon of Athens: Cupid
Characteristics and groups
As of 2016, Uranus is known to have 13 inner moons. Their orbits lie inside that of Miranda. All inner moons are intimately connected with the rings of Uranus, which probably resulted from the fragmentation of one or several small inner moons. The two innermost moons (Cordelia and Ophelia) are shepherds of Uranus’s ring, whereas the small moon Mab is a source of Uranus’s outermost ring. There may be two additional small (2–7 km in radius) undiscovered shepherd moons located about 100 km exterior to Uranus’ α and β rings.
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%. They are composed of water ice contaminated with a dark material, probably radiation-processed organics.
The small inner moons constantly perturb each other. The system is chaotic and apparently unstable. Simulations show that the moons may perturb each other into crossing orbits, which may eventually result in collisions between the moons. Desdemona may collide with either Cressida or Juliet within the next 100 million years.
Uranus has five major moons: Miranda, Ariel, Umbriel, Titania, and Oberon. They range in diameter from 472 km for Miranda to 1578 km for Titania. All these moons are relatively dark objects: their geometrical albedo varies between 30 and 50%, whereas their Bond albedo is between 10 and 23%. Umbriel is the darkest moon and Ariel the brightest. The masses of the moons range from 6.7 × 1019 kg (Miranda) to 3.5 × 1021 kg (Titania). For comparison, the Moon has a mass of 7.5 × 1022 kg. The major moons of Uranus are thought to have formed in the accretion disc, which existed around Uranus for some time after its formation or resulted from a large impact suffered by Uranus early in its history.
All major moons comprise approximately equal amounts rock and ice, except Miranda, which is made primarily of ice. The ice component may include ammonia and carbon dioxide. Their surfaces are heavily cratered, though all of them (except Umbriel) show signs of endogenic resurfacing in the form of lineaments (canyons) and, in the case of Miranda, ovoid race-track like structures called coronae. Extensional processes associated with upwelling diapirs are likely responsible for the origin of the coronae. Ariel appears to have the youngest surface with the fewest impact craters, while Umbriel’s appears oldest. A past 3:1 orbital resonance between Miranda and Umbriel and a past 4:1 resonance between Ariel and Titania are thought to be responsible for the heating that caused substantial endogenic activity on Miranda and Ariel. One piece of evidence for such a past resonance is Miranda’s unusually high orbital inclination (4.34°) for a body so close to the planet.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. 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.
As of 2005 Uranus is known to have nine irregular moons, which orbit it at a distance much greater than that of Oberon, the furthest of the large moons. All the irregular moons are probably captured objects that were trapped by Uranus soon after its formation. The diagram illustrates the orbits of those irregular moons discovered so far. The moons above the X axis are prograde, those beneath are retrograde. The radius of the Uranian Hill sphere is approximately 73 million km.
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. 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.
|1||VI||¡Cordelia||40 ± 6
(50 × 36)
|2||VII||¡Ophelia||43 ± 8
(54 × 38)
|3||VIII||¡Bianca||51 ± 4
(64 × 46)
|4||IX||¡Cressida||80 ± 4
(92 × 74)
|5||X||¡Desdemona||64 ± 8
(90 × 54)
|6||XI||¡Juliet||94 ± 8
(150 × 74)
|7||XII||¡Portia||135 ± 8
(156 × 126)
|8||XIII||¡Rosalind||72 ± 12||0.25||69940||0.558460||0.279°||0.00011||1986||Synnott
|9||XXVII||¡Cupid||≈ 18||0.0038||74800||0.618||0.1°||0.0013||2003||Showalter and
||90 ± 16
(128 × 64)
|11||XXV||¡Perdita||30 ± 6||0.018||76400||0.638||0.0°||0.0012||1999||Karkoschka
||162 ± 4||2.90||86010||0.761833||0.3192°||0.00012||1985||Synnott
||≈ 25||0.01||97700||0.923||0.1335°||0.0025||2003||Showalter and
||471.6 ± 1.4
(481 × 468 × 466)
(1162 × 1156 × 1155)
|19||XXII||‡Francisco||≈ 22||0.0072||4276000||−266.56||147.459°||0.1459||2003[j]||Holman et al.|
|0.25||7230000||−579.50||139.885°||0.1587||1997||Gladman et al.|
|21||XX||‡Stephano||≈ 32||0.022||8002000||−676.50||141.873°||0.2292||1999||Gladman et al.|
|22||XXI||‡Trinculo||≈ 18||0.0039||8571000||−758.10||166.252°||0.2200||2001||Holman et al.|
|2.30||12179000||−1283.4||152.456°||0.5224||1997||Nicholson et al.|
|24||XXIII||±Margaret||≈ 20||0.0054||14345000||1694.8||51.455°||0.6608||2003||Sheppard and
|25||XVIII||‡Prospero||≈ 50||0.085||16418000||−1992.8||146.017°||0.4448||1999||Holman et al.|
|26||XIX||‡Setebos||≈ 48||0.075||17459000||−2202.3||145.883°||0.5914||1999||Kavelaars et al.|
|27||XXIV||‡Ferdinand||≈ 20||0.0054||20900000||−2823.4||167.346°||0.3682||2003[j]||Holman et al.|