Uranus has twenty-seven named moons. Five of them are massive enough to have achieved hydrostatic equilibrium and so would be considered dwarf planets if they were in orbit about the Sun. There are also thirteen inner moons that orbit within Uranus' ring system, and another nine outer irregular moons.

The first two moons to be discovered, Titania and Oberon, were spotted by William Herschel on March 13, 1787. Two more, Ariel and Umbriel, were discovered by William Lassell in 1851. In 1852, Herschel's son John Herschel gave the four then-known moons their names. In 1948 Gerard Kuiper discovered the smallest moon out of the five, Miranda.

The flyby of the Voyager 2 space probe in January 1986 led to the discovery of a further 10 inner moons, and another satellite Perdita was later found after studying old Voyager photographs. Two more small inner moons were discovered by astronomers using the Hubble Space Telescope. Until 1997, Uranus was the only giant planet with no known irregular satellites. Since then, nine distant irregular moons have been identified using ground-based telescopes.

The region between the main rings and Miranda appears to be very crowded. The small moons there are constantly perturbed by each other. The system is chaotic and apparently unstable, and simulations show that the moons may perturb each other into crossing orbits which may result in collisions between the moons. Desdemona may collide with either Cressida or Juliet within the next 100 million years.

Unlike most planetary moons, which are named from antiquity, all the moons of Uranus are named after characters from the works of Shakespeare and Alexander Pope's work The Rape of the Lock.

The moons

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. Irregular (captured) moons with prograde orbits are shown in light grey, those with retrograde orbits in dark grey.

Notes: * Negative orbital periods indicate a retrograde orbit around Uranus (opposite to the planet's rotation).

Order	Name (spheroidal moons in bold) ()			Image	Mean diameter (km)	Mass (kg)	Semi-major axis (km)	Orbital period (day)	Inclination (°) (to Uranus' equator)	Discovery date
1	Uranus VI	Cordelia	kɔrˈdiːliə		42 ± 6	5.0?	49 751	0.335 034	0.084 79°	1986
2	Uranus VII	Ophelia	oʊˈfiːliə		46 ± 8	5.1?	53 764	0.376 400	0.103 6°	1986
3	Uranus VIII	Bianca	biːˈɑːŋkə		54 ± 4	9.2?	59 165	0.434 579	0.193°	1986
4	Uranus IX	Cressida	ˈkrɛsɨdə		82 ± 4	34?	61 766	0.463 570	0.006°	1986
5	Uranus X	Desdemona	ˌdɛzdɨˈmoʊnə		68 ± 8	23?	62 658	0.473 650	0.111 25°	1986
6	Uranus XI	Juliet	ˈdʒuːliət		106 ± 8	82?	64 360	0.493 065	0.065°	1986
7	Uranus XII	Portia	ˈpɔrʃiə		140 ± 8	170?	66 097	0.513 196	0.059°	1986
8	Uranus XIII	Rosalind	ˈrɒzəlɨnd		72 ± 12	25?	69 927	0.558 460	0.279°	1986
9	Uranus XXVII	Cupid	ˈkjuːpɨd		~ 18	0.38?	74 800	0.618	0.1°	2003
10	Uranus XIV	Belinda	bɨˈlɪndə		90 ± 16	49?	75 255	0.623 527	0.031°	1986
11	Uranus XXV	Perdita	ˈpɝdɨtə		30 ± 6	1.8?	76 420	0.638	0.0°	1986
12	Uranus XV	Puck	ˈpʌk		162 ± 4	290?	86 004	0.761 833	0.319 2°	1985
13	Uranus XXVI	Mab	ˈmæb		~ 25	1.0?	97 734	0.923	0.1335°	2003
14	Uranus V	Miranda	mɨˈrændə		471.6 ± 1.4	6 600 ± 700	129 390	1.413 479	4.232°	1948
15	Uranus I	Ariel	ˈɛəriəl		1157.8 ± 1.2	135 000 ± 12 000	191 020	2.520 379	0.260°	1851
16	Uranus II	Umbriel	ˈʌmbriəl		1169.4 ± 5.6	117 000 ± 13 000	266 300	4.144177	0.205°	1851
17	Uranus III	Titania	tɨˈtɑːnjə		1577.8 ± 3.6	353 000 ± 9 000	435 910	8.705 872	0.340°	1787
18	Uranus IV	Oberon	ˈoʊbərɒn		1522.8 ± 5.2	301 000 ± 7 000	583 520	13.463 239	0.058°	1787
19	Uranus XXII	Francisco	frænˈsɪskoʊ		~ 22	0.72?	4 276 000	-266.56*	147.459°	2001
20	Uranus XVI	Caliban	ˈkælɨbæn		~ 72	25?	7 231 000	-579.73*	139.885°	1997
21	Uranus XX	Stephano	ˈstɛfənoʊ		~ 32	2.2?	8 004 000	-677.37*	141.873°	1999
22	Uranus XXI	Trinculo	ˈtrɪŋkjʊloʊ		~ 18	0.39?	8 504 000	-749.24*	166.252°	2001
23	Uranus XVII	Sycorax	ˈsɪkəræks		~ 150	230?	12 179 000	-1288.28*	152.456°	1997
24	Uranus XXIII	Margaret	ˈmɑrgərɨt		~ 20	0.54?	14 345 000	+1687.01	51.455°	2003
25	Uranus XVIII	Prospero	ˈprɒspəroʊ		~ 50	8.5?	16 256 000	-1978.29*	146.017°	1999
26	Uranus XIX	Setebos	ˈsɛtɨbʌs		~ 48	7.5?	17 418 000	-2225.21*	145.883°	1999
27	Uranus XXIV	Ferdinand	ˈfɝdɨnænd		~ 20	0.54?	20 901 000	-2887.21*	167.346°	2001

Sources: NASA/NSSDC, University of Hawaii and Natural Satellites Ephemeris Service (for the outer satellites). These sources give no information on the masses for the small satellites.

Irregular moons

The diagram illustrates the orbits of the irregular moons of Uranus discovered so far. The eccentricity of the orbits is represented by the yellow segments (extending from the pericentre to the apocentre) with the inclination represented on Y axis. The satellites above the axis are prograde, the satellites beneath are retrograde. The X axis is labelled in Gm (million km) and the fraction of the Hill sphere's (gravitational influence) radius (approximately 70 million km for Uranus).

Unlike for Jupiter's irregulars, no correlation axis versus inclination can be found among the known population. Instead, the retrograde moons can be divided into two groups based on axis/eccentricity. The inner group includes satellites closer to Uranus (a < 0.15 r_H) and moderately eccentric (~0.2), namely: Francisco, Caliban, Stephano and Trinculo. The outer group (a > 0.15 r_H) includes satellites with high eccentricity (~0.5): Sycorax, Prospero, Setebos and Ferdinand.

Naming notes

The sources of the names are the following works:

• The Rape of the Lock, 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

Some asteroids share the same names as moons of Uranus: 171 Ophelia, 218 Bianca, 593 Titania, 666 Desdemona, 763 Cupido and 2758 Cordelia. See also Name conflicts of solar system objects.

Spurious moons

Titania and Oberon were discovered by Herschel on January 11, 1787. Subsequently, Herschel observed, or believed that he observed, four other moons, two in 1790 (January 18 and February 9) and two more in 1794 (February 28 and March 26). 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 beside Titania and Oberon, did not correspond to any of Herschel's four additional satellites in orbital characteristics. It was therefore concluded that Herschel's four satellites were spurious, probably arising from the misidentification of small stars in the vicinity of Uranus as satellites, and the credit for the discovery of Ariel and Umbriel was given to Lassell. 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 and 107.69 days (exterior to Oberon).

References

External links

• Uranus' Moons by NASA's Solar System Exploration
• NASA's Hubble Discovers New Rings and Moons Around Uranus (STScI-2005-33, December 22, 2005)
• Ring diagram (Courtesy of Astronomy Magazine 2005)