The moons of Uranus

The planet Uranus, which is the seventh furthest from the Sun, is surrounded by moons, with 27 having been discovered to date. These range in size from 1578 km diameter (Titania) to around 20 km or even less. The sizes of some of the smallest moons, such as Ferdinand, Margaret, Mab, Cupid and Trinculo, are not known for certain and so the claim for which is the smallest moon of Uranus is still under debate.

The first two moons to be discovered, Titania and Oberon, were detected in 1787 by Sir William Herschel. He had been the original discoverer of Uranus in 1781. The names of the moons were given much later by Sir William’s son John in 1852. He defied the convention of naming bodies in the Solar System after characters from Greek and Roman mythology when he decided on names from Shakespeare, Titania and Oberon being respectively the queen and king of the fairies in A Midsummer Night’s Dream.

John Herschel had already named two other moons, these being the pair spotted in 1851 by William Lassell. Herschel chose names from Alexander Pope’s poem The Rape of the Lock, these being Ariel and Umbriel. As new Uranian moons have been discovered they have also been named after characters from English literature although only one – Belinda – owes its name to Alexander Pope. The rest are all Shakespearean names.

The name Ariel was possibly the clue that led later namers to consider Shakespeare’s The Tempest as a good source of names for moons. This is because Ariel is a character who appears in both works. Other “Tempest” moons are Caliban, Ferdinand, Francisco, Miranda, Prospero, Setebos, Stephano, Sycorax and Trinculo.

There is only one other “Dream” moon, this being Puck. The other names, which are almost all female ones, come from a wide variety of other plays: Bianca (The Taming of the Shrew or Othello – take your pick), Cordelia (King Lear), Cressida (Troilus and Cressida), Cupid (Timon of Athens – and therefore not a break of the “no mythology” rule) , Desdemona (Othello), Juliet and Mab (Romeo and Juliet), Margaret (Much Ado About Nothing), Perdita (The Winter’s Tale), Ophelia (Hamlet), Portia (The Merchant of Venice), and Rosalind (As You Like It).

Two of the Shakespeare names, namely Sycorax and Mab, are characters of whom reference is made in The Tempest but do not actually appear on stage.

As mentioned above, the earliest moon discoveries were made not long after Uranus itself was discovered, with two more following in the 19th century, but it was not until 1948 that a fifth moon was seen and named, this being Miranda which was discovered by Gerard Kuiper, a Dutch-American astronomer after whom the Kuiper Belt of trans-Neptunian objects was named.

The flypast of Uranus by Voyager 2 in 1985/6 led to a whole crop of smaller moons being discovered. Others have come to light in more recent years as a result of work on images provided by the Hubble Space Telescope, which has also added to the list of moons orbiting Saturn, Jupiter and Neptune.

The moons of Uranus can be divided into three main groups:

Major satellites. These are the five largest moons (Miranda, Ariel, Umbriel, Titania and Oberon) that were the earliest to be discovered and have sufficient mass to be spherical in shape (apart from some polar flattening and equatorial bulging). They orbit at distances from the parent planet ranging from 130,000 km to 584,000 km. They also share the extreme axial tilt of Uranus in that they rotate at virtually 90 degree to the perpendicular. They are composed of rock and ice, with Miranda having the largest proportion of ice.

Inner satellites. These 13 small moons are associated with the complex ring system that surrounds Uranus. The rings may well have originated from the break-up of other moons following collisions, and the remaining moons probably act as “shepherds” in determining the shape and behaviour of the rings.

Outer satellites. To date nine moons have been discovered that orbit beyond Oberon. The furthest out is Ferdinand, which lies at an average of distance of 21 million kilometres from Uranus. They are irregular in shape and several have extremely eccentric orbits. Of the nine, only Margaret orbits in the same direction as the rotation of Uranus, with the rest having “retrograde” orbits. It is quite likely that most of these distant bodies were “captured” by Uranus at some point in the distant past when the solar system contained many more “spare” objects.

It is always possible that more relatively sizable lumps of rock will be discovered in future to be in orbit around Uranus. Should this be the case, there are still plenty of Shakespeare characters to choose from as possible names!