Ariel and Umbriel

William Lassell made a fortune selling beer to factory workers in Liverpool. He used that money to set up his own astronomical observatory, called Starfield, outside Liverpool. And he equipped it with telescopes he built himself – some of the best in the world. Using one of those instruments, Lassell made a major discovery 175 years ago this week. Make that two discoveries: two large moons of the planet Uranus. They were named Ariel and Umbriel, after characters from the works of William Shakespeare and Alexander Pope. The moons are almost exactly the same size – about 720 miles in diameter. And they have a similar makeup – mixtures of rock and frozen water and methane. But they look different. Umbriel is dark, and it’s covered by many large impact craters. That suggests the moon is quiet and still – not much happens there. Ariel is brighter, and it’s much more interesting. It’s crossed by giant canyons and cliffs, with few craters. Liquid water may bubble to the surface from an underground ocean. At the surface, it freezes. That may push the surrounding crust, causing it to crack. So Ariel may be an active world orbiting a giant planet. Uranus is an easy target for binoculars. It’s halfway between Aldebaran – the bright eye of the bull – and the Pleiades star cluster. They climb into good view in late evening. But the view will be better before dawn, after the Moon has dropped from sight. Script by Damond Benningfield