Unlike all the other planets, whose spin axes are roughly vertical comparatively, Uranus’s axis of rotation lies almost 90 degrees off to one side. Relative to other planets, Uranus is tipped over on its side.

The north pole of Uranus, at some time in its orbit, points almost directly toward the Sun. Half a Uranus year later, its south pole faces the Sun.

When Voyager 2 encountered the planet in 1986, the north pole happened to be pointing nearly at the Sun, so it was midsummer in the northern hemisphere.

Due to the rotation axis, Uranus experiences some extreme seasonal effects.

If the north pole is facing the sun, that side is summer while the other side of the planet is winter. The planet experiences autumn and spring when the sun faces the equator of the planet.

No one knows why Uranus is tilted in this way – the other planets have rotation axes lying well out of the normal ecliptic plane. It is speculated that a catastrophic event, such as a glancing collision between the planet and another planet-sized body during the formative stages of the solar system may have altered Uranus’s spin axes. However, there is no direct evidence for such an occurrence, and no theory to tell us how we should seek to confirm it.

The Air is Weird Here

Studies of sunlight reflected from Uranus’s dense clouds indicate that the planet’s out atmosphere is quite similar to the atmospheres of Jupiter and Saturn.

The most abundant element is hydrogen, followed by helium and methane. Ammonia, which plays such an important role in the Jupiter and Saturn systems, however, is not present in any significant quantity in the atmosphere of Uranus. As the concentration of methane increases, the reflected light should appear bluer, which is why Uranus (which has less Methane than Neptune) appears to be a bluish-green when observed.

During its visit, Voyager 2 detected just a few cloud features in Uranus’s atmosphere, and even those became visible only after extensive enhancement. Like cirrus clouds on Earth, these Uranian clouds are made up predominantly of ice crystals, formed in the planet’s cold upper atmosphere.

Uranus lacks any significant internal heat source, and because the planet has a low surface temperature, its clouds are found only at low-lying, warmer levels in the atmosphere. The absence of high-level clouds means that we must look deep into the planet’s atmosphere to see any structure, so the bands and spots that characterize flow patterns on the other Jovian planets are largely invisible on Uranus because of the haze of the upper atmosphere

We Are Many

As of the year 2014 AD, 27 moons are known to orbit Uranus. William Herschel discovered and named Titania and Oberon, the two largest of Uranus’s five moons, in 1789.

British astronomer, William Lassel found Ariel and Umbriel, the next-largest moons, in 1851.

Gerard Kuiper found Miranda, the smallest, in 1948.

Ten smaller moons discovered by Voyager 2 all lie inside the orbit of Miranda. Many of them are intimately related to the Uranian ring system. All of these moons revolve in the planet’s skewed equator region.

Of the remaining 22 moons, one, orbiting close to the planet, was found after careful reanalysis of the Voyager 2 images.

All the rest were discovered via systematic ground-based searches made since 1997, with techniques similar to those that have been so successful in identifying new moons of Jupiter and Saturn.

Many of the moons’ names came from characters from William Shakespeare’s plays and some of Alexander Pope’s more famous works.

Miranda was named for the Princess of Milan in one of Shakespeare’s most famous works, The Tempest.

Titania and Oberon were both named for another of Shakespeare’s works, A Midsummer Night’s Dream.

Ariel and Umbriel both come from a poem by Alexander Pope.

Put a Ring On It

The ring system surrounding Uranus was first discovered in 1977, when astronomers observed a stellar “occultation”: The rings passed in front of a very bright star, momentarily dimming the star’s light. Such an alignment happens a few times per decade and allows astronomers to measure planet structures that are too small and faint to be detected directly.

The 1977 observation was actually aimed at studying the planet’s atmosphere by watching how it absorbed starlight. However, 40 minutes before and after Uranus itself occulted (passed in front of) the star, the flickering starlight revealed the presence of a set of rings. The discovery was particularly exciting because, at the time, only Saturn was known to have rings.

Jupiter’s rings went unseen until Voyager 1 arrived there in 1979, and those of Neptune were unambiguously detected only in 1989, by Voyager 2.

There are a total of nine thin rings around Uranus.

The main rings, in order of increasing size, are named Alpha, Beta, Gamma, Delta and Epsilon, and they range from 44,000 to 51,000 kilometers from the direct center of the planet. A fainter ring, known as the Eta ring, lies between the Beta and Gamma rings, and three other faint rings, known as 4, 5, and 6, lie between the Alpha ring and the planet itself. In 1986, Voyager 2 discovered two more even fainter rings, one between Delta and Epsilon and one between ring 6 and Uranus.

Saturn’s rings are bright and wide, with relatively narrow gaps between them, the rings of Uranus are dark, narrow and widely spaced. With the exception of the Epsilon ring and the tiny innermost ring, the rings of Uranus are all less than about 10 kilometers wide, and the spacing between them ranges from a few hundred to about 1000 kilometers. However, like Saturn’s rings, all Uranus’s rings are less than few tens of meters thick (that is, measured in ‘vertical’ thickness).

The density of particles within Uranus’s rings is comparable to that found in Saturn’s A and B rings. The particles that make up Saturn’s rings range in size from dust grains to large boulders, but in the case of Uranus, the particles show a much smaller spread – few, if any, are smaller than a centimeter or so in diameter. The ring particles are also considerably less reflective than Saturn’s ring particles, possibly because they are covered with the same dark material as Uranus’s moons.

Much like the F ring of Saturn, Uranus’s narrow rings require a ‘shepherd satellite’ to keep them from breaking apart and floating off either into space or back towards Uranus. The theory of shepherd satellites was first worked out to explain the rings of Uranus which had been detected before Voyager 2 ever encountered Saturn. Thus, the existence of the F ring did not come as quite such a surprise as it might have otherwise! Many of the small inner satellites of Uranus play some role in governing the appearance of the rings. Voyager 2 detected Cordelia and Ophelia, the shepherds of the Epsilon ring. Many other, undetected, shepherd satellites must also exist.