Uranus, the seventh planet of the Solar System, has 27 known moons, all of which are named after characters from the works of William Shakespeare and Alexander Pope.

William Herschel discovered the first two moons, Titania and Oberon, in 1787, and the other spherical moons were discovered in 1851 by William Lassell (Ariel and Umbriel) and in 1948 by Gerard Kuiper (Miranda).

The remaining moons were discovered after 1985, either during the Voyager 2 flyby mission or with the aid of advanced Earth-based telescopes.

Uranian 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 the planet’s rings.

The five major moons are massive enough to have achieved hydrostatic equilibrium, and four of them show signs of internally driven processes such as canyon formation and volcanism on their surfaces.

The largest of these five, Titania, is 1,578 km in diameter and the eighth-largest moon in the Solar System, and about 20 times less massive than Earth’s Moon. Uranus’s irregular moons have elliptical and are strongly inclined and orbit at great distances from the planet.

The first two moons to be discovered, Titania and Oberon, 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 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.

No other discoveries were made for almost another century. 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 retroactively discovered in 1999 after studying old Voyager photographs.

Uranus was the last giant planet without any known irregular satellites, 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. The moon Margaret was the last Uranian moon discovered as of 2008.

Spurious moons

After Herschel discovered Titania and Oberon on January 11, 1787, he subsequently believed that he observed four other moons; two on January 18 and February 9, 1790, and two more on February 28 and March 26, 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 beside 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 and 107.69 days (exterior to Oberon).[14] 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.

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