1787

Titania and Oberon

William Herschel (1738–1822), Caroline Herschel (1750–1848), John Herschel (1792–1871), William Lassell (1799–1880)

After the discovery of Uranus in 1781, English astronomer William Herschel continued to observe the planet with successively larger telescopes. The larger mirrors and focal lengths of his instruments enabled both the increased light-gathering potential to detect fainter objects, and the increased resolution to distinguish these objects from others nearby. Better technology and careful observations, as well as assistance from his sister and fellow astronomer Caroline Herschel, led to the discovery of new moons around both Uranus and Saturn.

Herschel discovered two moons orbiting Uranus on the same night in early 1787. By monitoring their motions and assuming that the moons orbit in or close to the planet’s equatorial plane, Herschel and other astronomers were able to quickly deduce that the tilt of the Uranian spin axis—what astronomers call the obliquity—was dramatic: Uranus is tilted on its side, with an obliquity of about 98 degrees. A planet’s tilt is the “reason for the seasons.” Earth’s tilt of 23.5 degrees causes our familiar winters, springs, summers, and falls, as well as the six-month-long polar days and nights in the Arctic and Antarctic. On Uranus and its moons, the entire northern and southern hemispheres experience “polar days” and “polar nights” that are forty-two Earth years long! Extreme tilts lead to extreme seasons.

Herschel’s son, John Herschel, also an accomplished astronomer, ultimately named these and the other Uranian moons, choosing characters from the works of William Shakespeare and Alexander Pope. He named the brighter of the two moons that his father discovered Titania, after the queen of the fairies in Shakespeare’s A Midsummer Night’s Dream. The other was named Oberon, after the fairy king.

Voyager 2’s highest-resolution photo of Oberon, showing the moon’s natural, slightly reddish color.

The Voyager 2 encounter with Uranus in 1986 revealed Titania and Oberon to be heavily cratered worlds, each about half the size of Earth’s Moon. Their similar densities imply rocky and icy interiors. Spectroscopy since then has revealed surfaces with water and CO₂ ices, as well as simple organic molecules like those produced from the irradiation of methane or other carbon-bearing ices.

Voyager 2’s highest-resolution photo of Titania, taken on January 24, 1986. The large crater on the right is called Gertrude, and the 930-mile-long (1,500-kilometer) Messina Chasma is the bright streak at the center.