The rings of Uranus were discovered on March 10, 1977, by a team of American astronomers aboard an airborne observatory. They were carrying out an occultation experiment – where an object – in this case the planet Uranus, passes in front of a star. By observing changes in the brightness of a star when it is hidden by a planet, the planet’s size and the temperature and pressure of any atmosphere it might have can be measured. Occultations are rare; to observe them one has to predict when the planet will pass in front of a star, and where on the Earth its shadow will be cast. In 1977 this meant taking the Kuiper Airborne Observatory to 39,000 feet above the Indian Ocean and using its 1 m telescope to observe the star. The equipment was turned on early – and the star flickered briefly from view five times both before and after it was eclipsed by the planet. This was interpreted as a series of narrow rings around the planet.
These rings are quite unlike the better known rings of Saturn – for one thing they are dark – really dark and they are really faint. They were photographed by the Voyager 2 spacecraft in 1986, when two more rings were discovered. Between 2003 and 2005 the Hubble Space Telescope detected more rings, bringing the total number known to thirteen. There are nine inner rings, two dust rings and two outer rings. The brightest inner ring is called the ε ring – it is constrained or “shepherded” by a pair of moons, Cordelia and Ophelia. The outermost ring, mu, discovered in 2005 is not visible in infra-red light, but can be seen in visible light – this means it must be blue. A tiny moon called Mab was discovered at the same time. Its orbit is the same as the outer ring – implying that this ring, like one of Saturn’s ring is linked to a moon. In the case of Saturn – the outermost blue ring shares an orbit with Encleladus – a moon that has icy plumes of material spurting from it and settling in the ring. Mab is much smaller than Enceladus – only about 25 km across, much too small to be producing plumes of anything, so some other mechanism must be at work.
The first two moons of Uranus were called Titania and Oberon from Shakespeare’s A Midsummer Night’s Dream. The next two found were called Ariel and Umbriel from Alexander Pope’s The Rape of the Lock. Since then, most of the moons have been named with reference to Shakespeare, although there is a moon Belinda –the source of the lock of hair in Pope’s poem. Features on the various moons take their names from characters or places in Shakespearian works or from the names of assorted mythological spirits. In contrast the rings of Uranus are merely Greek letters – from alpha to nu and the numbers 4, 5 and 6.
Listen live to “What is the Stars?” on RTÉ’s lyric Mondays and Fridays at 22:45. 