Uranus

Young Astronomers Blog, Volume 29, Number 5.

For thousands of years, humans looked up at the night sky and observed the stars. They found that stars moved with a predictable pattern from night to night and year to year. However, they notice five objects that behaved differently. These objects “wandered” against the background of stars. The Greeks called them asters planetai (wandering stars) or planetes (wanderers). Over time, the terms evolved into the English word planet. There were other objects, such as comets, that also moved against the background of stars, but in ancient times they were thought to be singular (non-repeating) events, clearly not the same as planets.

There were five of these wandering planets in all, okay six if you count the Earth, but folks hadn’t figured that out yet. These five planets received names, from the mythology of the time, that are still with us today: Mercury, Venus, Mars, Jupiter, and Saturn. After Copernicus and others determined that the Earth was just a planet, it was added to the list.

With the invention of the telescope, astronomers could start to see some detail. It was clear that these planets were different from the stars. They were disk shaped. Two, Mercury and Venus, would go through phases. Mars has a reddish tint to it and appeared to have white areas near its poles. Jupiter had four large moons and a large red spot. Saturn, of course, had rings and several moons.

Still there were only six. Then on March 13, 1781, things changed. William Herschel was searching the night sky and he found something moving. He initially thought it was a comet. However, he soon discovered that he had found a planet, something no one had done in modern or ancient times. By 1783, this new object was generally accepted as a new planet. By the way, this discovery was 240 years ago in March 2021.

After waiting thousands of years for the discovery of a new planet, it took only twenty more years before more “potential” planets were discovered. They were given names: Ceres, Pallas, Juno, and Vesta. Herschel measured the diameter of Ceres and Pallas soon after their discovery and found they were on the small side, much smaller than Uranus. They were also located between the orbit of Mars and Jupiter and not further out in the Solar System like Uranus. However, most astronomers considered them to be planets, at least for a while.

Herschel wasn’t too happy that these new small objects were considered to be on par with his discovery Uranus, so he suggested they be called asteroids (starlike). He was immediately criticized by many of the astronomers of the day, who thought the proposed classification was a poor idea. However, by the mid-1800s, more and more of these objects were found. They were eventually removed from the list of planets and reclassified as asteroids. To complicate things, in 1846, a new planet Neptune was discovered, and in 1930, a 9^th planet (for a while), Pluto, was found. See Discovering Pluto for a discussion of the asteroids and Pluto.

Naming the new planet proved to be a challenge. Herschel suggested Georgium Sidus (Georgian Star) after England’s King George III. This was a bit of a mouthful and naming something after a British monarch did not go over very well in some parts of the world, especially the Americas shortly after the Revolution. A couple years later, Johann Bode suggested Uranus, god of the sky and partner to Gaia, god of the Earth, in Greek mythology from Hesiod’s Theogony, c700 BCE. Although, it wasn’t until around 1850 for the name to become fully accepted.

The new planet had a name, but today we struggle with the pronunciation. Many of us were taught to say you-RAIN-us (U-RAIN-us). However, most astronomers now use YUR-ah-nus (Ur-a-nus). Emily Lakdawalla of the Planetary Society has a great way to remember this. Look at the person next to you and call them a “nuss.” (“You’re a nuss”).

Uranus turned out to be a rather large planet, some four times the diameter of the Earth. It is similar to Jupiter and Saturn with a gassy exterior composed of hydrogen and helium. It does have some methane, which gives it a bluish/green color. Uranus is classified as an “ice giant” because of icy material (water, methane, and ammonia) in its interior.

Its most interesting characteristic is its spin. Uranus spins on its side, once every 17 hours. It can be thought of as a ball rolling along its orbit, while most planets spin like a top along their orbits. This results in extreme seasons when one hemisphere faces directly toward the Sun while the other faces away from the Sun during summer and winter.

Like the other gas and ice giants, Uranus has a series of rings. One of my favorite astronomy posters is published by Astronomy magazine called Ring around the Planets. It provides a nice compassion of the rings around the four outer planets.

At last count, Uranus has 27 moons. Five of these moons are large enough to be spherical (Miranda, Ariel, Umbriel, Titania, and Oberon). They are thought to be between ¼ to ½ the size of the Earth’s moon and composed of a combination of water ice and rock.

Of the moons, Miranda is the most interesting. It has huge faults and canyons. Astronomers think it might have been smash apart and reformed, or it was hit by meteorites, partially melted, and refroze. There is a large chevron shape on its surface. If you look at it upside down, it becomes a seven, maybe identifying the 7^th planet Uranus. Who knows? 😊

For most planets, moons are names after Greek or Roman mythology in a way to tie them back to the name of the planet. Not for Uranus. John Herschel, William’s son, proposed that the moons be named for characters in English literature, particularly works by William Shakespeare. This tradition continues to this day and is supported by the International Astronomical Union’s naming conventions.

• Miranda is the heroine from Shakespeare’s The Tempest.
• Ariel and Umbriel are characters from a poem by Alexander Pope.
• Oberon and Titania are the king and queen, respectively, of the fairies from A Midsummers Night’s Dream.

You might recognize a few of the smaller moons, such as Puck, Juliet, Ophelia, and Cupid, all from Shakespeare.