January 11 


11 January 1787

William Herschel, An account of the discovery of two satellites revolving round the Georgian planet. Philosophical Transactions of the Royal Society of London, Vol. 77, Tab. VII, p. 128

Titania (Uranus III) and Oberon (Uranus IV), the two largest moons of Uranus, were discovered by William Herschel (1738–1822) on January 11, 1787. 

   Titania is the largest moon of Uranus and the eighth-largest moon in the Solar System, with a diameter of 1,577 km. The moon orbits Uranus at the distance of about 436,000 kilometres, being the second farthest from the planet among its five major moons after Oberon. Its orbital period is around 8.7 days, coincident with its rotational period. The orbit lies inside Uranus's magnetosphere. 

   Titania consists of approximately equal amounts of ice and rock, and is probably differentiated into a rocky core and an icy mantle. A layer of liquid water may be present at the core–mantle boundary. Its surface, which is relatively dark and slightly red in color, appears to have been shaped by both impacts and endogenic processes, and is cut by a system of enormous canyons and scarps. Although Titania is covered with numerous impact craters reaching up to 326 kilometres in diameter, it is less heavily cratered than Oberon. The moon is named after the queen of the fairies in Shakespeare's A Midsummer Night's Dream.

Titania imaged by Voyager 2 spacecraft in 1986. Credit: NASA/JPL

   Oberon is the outermost and second-largest major moon of Uranus and the tenth-largest moon in the Solar System, with a diameter of 1,523 km. The moon orbits Uranus at the distance of about 583,500 kilometres. Its orbital period is around 13.5 days, coincident with its rotational period. The orbit lies partially outside Uranus's magnetosphere.

Oberon consists of approximately equal amounts of ice and rock, and is probably differentiated into a rocky core and an icy mantle. A layer of liquid water may be present at the boundary between the mantle and the core. The surface of Oberon, which is dark and slightly red in color, appears to have been primarily shaped by asteroid and comet impacts. It is covered by numerous impact craters reaching 210 km in diameter. The moon is named after the king of the fairies in Shakespeare's A Midsummer Night's Dream.

Oberon imaged by Voyager 2 spacecraft on January 24 1986. Credit: NASA/JPL-Caltech/Kevin M. Gill

William Herschel, portrait by Lemuel Francis Abbott, 1785


© 2026, Andrew Mirecki



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