Spurious moons Īfter Herschel discovered Titania and Oberon on January 11, 1787, he subsequently believed that he had observed four other moons: two on January 18 and February 9, 1790, and two more on February 28 and March 26, 1794. As of 2020, the moon Margaret was the last Uranian moon discovered, and its characteristics were published in October 2003. Two more small inner moons, Cupid and Mab, were discovered using the Hubble Space Telescope in 2003. 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. Another satellite, Perdita, was discovered in 1999 after studying old Voyager photographs. Decades later, the flyby of the Voyager 2 space probe in January 1986 led to the discovery of ten further inner moons. In 1948, Gerard Kuiper at the McDonald Observatory discovered the smallest and the last of the five large, spherical moons, Miranda. No other discoveries were made for almost another century. In 1852, Herschel's son John Herschel gave the four then-known moons their names. With the confirmation of Ariel and Umbriel, Lassell numbered the moons I through IV from Uranus outward, and this finally stuck. 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). Eventually, the next two moons, Ariel and Umbriel, were discovered by William Lassell in 1851. 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. For nearly 50 years, Herschel's instrument was the only one with which the moons had been seen. Later, Herschel thought he had discovered up to six moons (see below) and perhaps even a ring. 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. The remaining moons were discovered after 1985, either during the Voyager 2 flyby mission or with the aid of advanced Earth-based telescopes. These five may be in hydrostatic equilibrium, and so would be considered dwarf planets if they were in direct orbit about the Sun. The other three ellipsoidal moons were discovered in 1851 by William Lassell ( Ariel and Umbriel) and in 1948 by Gerard Kuiper ( Miranda). William Herschel discovered the first two moons, Titania and Oberon, in 1787. Uranus's irregular moons have elliptical and strongly inclined (mostly retrograde) orbits at large distances from the planet. The orbits of the regular moons are nearly coplanar with Uranus's equator, which is tilted 97.77° to its orbit. The largest of these five, Titania, is 1,578 km in diameter and the eighth-largest moon in the Solar System, about one-twentieth the mass of the Earth's Moon. The five major moons are ellipsoidal, indicating that they reached hydrostatic equilibrium at some point in their past (and may still be in equilibrium), and four of them show signs of internally driven processes such as canyon formation and volcanism on their surfaces. The inner moons are small dark bodies that share common properties and origins with Uranus's rings. The inner and major moons all have prograde orbits, while orbits of the irregulars are mostly retrograde. Uranus's moons are divided into three groups: thirteen inner moons, five major moons, and nine irregular moons. 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. Six largest moons of Uranus as captured by the James Webb Space Telescope: Ariel, Puck, Miranda, Umbriel, Titania and Oberon.
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