William Herschel was the first person ever to discover a planet. In 1781 he discovered the planet given the name Uranus for the ancient Greek sky god. Although Uranus has at least 27 moons, most of them weren't discovered until the Space Age.
After Uranus was discovered, it took 167 years to find its five large moons.
- William Herschel discovered Titania and Oberon in 1787.
- English astronomer William Lassell discovered Ariel and Umbriel in 1851.
- Dutch-American astronomer Gerard Kuiper discovered Miranda in 1948.
- All the others were discovered after 1985, most of them by the Voyager 2 mission that visited Uranus.
Oberon and Titania are the king and queen of the fairies in Shakespeare's A Midsummer Night's Dream, and Ariel and Umbriel were spirits of the air in a popular poem of the time.
Gerard Kuiper followed the Shakespearian theme by naming his discovery Miranda after a character in The Tempest.
Uranus has at least 13 inner moons.
These tiny moons formed at the same time as the planet itself. They're all dark and lumpy, and orbit Uranus in less than a day. They move in circular orbits in the same direction as the planet does.
Some of the inner moons are shepherd moons.
The gravity of ring shepherds keeps a ring in place. There is evidence that two more shepherd moons may exist that haven't yet been seen.
Uranus has five large moons that formed out of a disk of material circling the equator.
This may have happened when the planet itself formed. But we also know that a giant collision later knocked the planet onto its side. So the moons could have been made from material thrown out from that impact.
In order of size, the five large moons are Titania (the biggest), Oberon, Umbriel, Ariel and Miranda.
The sizes of the large moons vary from Titania's 1500 km across (930 miles) to Miranda's 470 km (290 miles). They all have enough mass to be made round by gravity.
The large moons are dark, heavily cratered and airless.
Except for Miranda, they're made of about half and half rock and ice. And except for Umbriel, they show signs of geological activity that has created deep canyons and cracks, and covered over craters. Umbriel has the oldest surface, and Ariel has the youngest surface with the fewest craters.
We don't know much about the moons, but they may resemble the moons of Saturn studied by the Cassini mission.
It's possible that the two biggest Uranian moons, Titania and Oberon, are also layered, have rocky cores and ice mantles, and even contain liquid water oceans.
Miranda is the odd one out it's one of the weirdest bodies in the Solar System.
Miranda is made mostly of ice, and has a rugged surface criss-crossed by huge canyons. This shows that there has been intense geological activity in the past. And since some of the surface has fewer craters than we'd expect, Miranda may still be active. The cliff Verona Rupes is over 5 km (3 miles) high, the biggest one known in the Solar System.
Nine irregular outer moons have been discovered.
All of these moons have been discovered since 1999, and they're little and lumpy like the inner moons. Their sizes vary from Trinculo's 20 km across (12 miles) to Sycorax's 120200 km (75-125 miles).
The outer moons are a long way away from Uranus.
The farthest large moon from Uranus is Oberon. The closest outer moon to Uranus is seven times farther out than Oberon, and the most distant one is over 35 times farther away.
The outer moons have very eccentric orbits and except for Margaret, orbit in the opposite direction to Uranus's other moons.
A circle is not eccentric, but the more squashed a circle is, the more eccentric it is. Margaret has one of the most eccentric orbits of any known moon.
Unlike the other moons, the irregular moons are captured objects.
The Uranian moon system is less massive than that of any of the other giant planets.
Neptune's moon Triton is the seventh largest moon in the Solar System. If you added up the mass of Uranus's five largest moons, the total would be less than half that of Triton.