The Unusual Inclination of Uranus

Remember way back in June when we talked about the strange inclination of Venus? Believe it or not, there is one planet that has a stranger inclination. Here, the Sun does not rise in the west and set in the east. But then, it doesn't really rise in the east or set in the west all year long. For a portion of its year, Uranus has a hemisphere completely illuminated and another hemisphere completely in the dark.

Uranus inclination is about 98°, which means that its rotational axis is nearly perpendicular to its orbital axis. Another way of saying this, is that Uranus rotates on its side. This means that for a portion of its year, the north pole of Uranus is almost pointed straight at the Sun and the northern hemisphere is almost completely sunlight while the south pole is pointed away from the Sun and therefore, the southern hemisphere is dark. Vice versa, halfway around its orbit, the south pole is pointed towards the Sun and the north pole away.

This phenomena happens on Earth, but not as extreme. There are two latitudes, one north of the equator and one south of the equator, where in the northern summer, at any location north of the northern latitude the Sun will never set and in the northern winter, the Sun will never rise. The same thing happens in the southern latitudes near Antarctica. These latitudes are called the Arctic Circle and the Antarctic Circle. If you have heard of "The Land of the Midnight Sun", this is in reference to the Arctic Circle. On Earth, these circles are at 66.5° North for the Arctic Circle and 66.5° South for the Antarctic Circle. If you recall from science class, that the Earth is tilted 23.5° with respect to the Earth's orbital plane, those two angles equal 90°. On Uranus, it is a little more complicated than that.

When the north pole of Uranus is pointed towards the Sun (or the northern "summer" on Uranus), any latitude north of 8°N will never see the Sun set and any latitude south of 8°S on Uranus will never see the Sun rise. Vice versa, in the southern "summer" on Uranus, any latitude south of 8°S will never see the Sun set and any latitude north of 8°N will never see the Sun rise.

Why does Uranus have such an extreme inclination? Much like Venus, it is believed that soon after formation, Uranus was hit by a large object that knocked it on its side, but because it has a much larger mass than Venus, it did not flip upside down, but rather just onto its side.

The Moons of Uranus

The Moons of Uranus from closest to farthest starting from the left

Image Credit:

NASA/Voyager

Uranus has 27 known moons which are mostly icy bodies similar to comets. Most of them might be captured comets from the Kuiper Belt or the Oort Cloud. However, there are a few that are considered planetary mass bodies, which means they have enough mass to be differentiated and spherical in shape. Most likely, these five major moons were probably formed at the same time as Uranus.

Titania

Image Credit:

NOAA

Titania has the largest diameter of all of the moons of Uranus, but it is still smaller than our Moon. Its diameter is 1600 km, compared to 3500 km for the Earth. Even though it has the largest diameter, it is not the most massive. That honor goes to Oberon which will be discussed later. Of the five major moons, it is the fourth out from Uranus and has an inclination of 0.34°, meaning it orbits along the equatorial plane of Uranus. William Herschel discovered Titania along with Oberon in January of 1787. Observations of Titania with telescopes and with the Voyager probes tell us a lot of Titania. We know that the moon has two main layers, and icy crust/mantle and a rocky core. The surface is heavily cratered, though not as much as Oberon which also tells us that it was more active in the past than Oberon. Titania was named after the Queen of the Fairies in William Shakespeare's A Midsummer's Night Dream.

Oberon

Image Credit:

NOAA

Oberon has the largest mass, and is second to Titania in diameter. It is also the farthest of the major moons. Discovered at the same time as Titania, Oberon was named after the King of the Fairies in A Midsummer's Night Dream. It is the most heavily cratered moon orbiting Uranus and much like Titania, it has a icy crust/mantle with a rocky core. With an inclination of only 0.058°, it is almost completely aligned with Uranus' equator.

Umbriel

Image Credit:

NOAA

Umbriel is the third largest in mass and diameter of the moons, but was not discovered until 1851 by William Lassell. Like Titania and Oberon, it is an icy and rocky body with an inclination of 0.128°. It is the second most cratered body but went under internal geologic processes in its past to give it the surface it has now. It was named after a character in Alexander Pope's The Rape of the Lock.

Ariel

Image Credit:

NOAA

Ariel is the fourth largest moon and second closest. It was discovered with Umbriel by William Lassell. Again, it is an icy and rocky body that formed in the accretion disk around Uranus based on its inclination of 0.260°. It is also tidally locked in the same way the Moon is tidally locked to Earth. Its name comes from two sources: Pope's The Rape of the Lock and Shakespeare's The Tempest.

Miranda
Image Credit:
NOAA
The last and smallest major moon is Miranda, which is also the closest to Uranus. It is also tidally locked to Uranus and was the last one discovered. Gerard Kuiper (after whom the Kuiper Belt is named - we'll discuss the Kuiper Belt later) found Miranda in 1948. Voyager imaged Miranda as it passed by the Uranus system and discovered that it is the most geologically active because it is the closest and tidal forces keep the interior warm enough. It has the highest inclination at 4.232° and was named after a character in Shakespeare's The Tempest.

The Rings of Uranus

Image Credit:

NASA

 

 

Image Credit:

NASA

 

Uranus has a faint ring system around the planet. William Herschel thought that he had discovered rings around the planet when he first spotted Uranus, but based on how faint they are, it seems unlikely that what he saw were the rings.

The rings are oriented parallel to the equator of Uranus and much like those of Jupiter and Saturn, are continually replenished by collisions by objects colliding with the moons of Uranus. They are very dark, though they contain icy particles because they are covered with dust from the moons and the lack of sunlight reaching the region around Uranus.

The rings themselves are very narrow, made up of meter-sized particles and smaller. The majority of the rings are no more than 10km wide, with the widest ring only 100km (less than 0.2% the diameter of Uranus). Much like Pandora and Prometheus keeping the F-Ring around Saturn narrow, the rings of Uranus are kept narrow because of shepherd satellites, like Cordelia and Ophelia maintain the ε-Ring (epsilon ring). The ε-Ring is actually very eccentric because of the highly eccentric orbits of Cordelia and Ophelia.

The ring around Uranus were first indirectly discovered in 1977 when Uranus occulted a star, i.e. passed in front of the star. Just before the disk of Uranus passed in front of the star, the star dimmed a little, and just after the Uranus passed in front, the star did not return to its original brightness. Based on the dimming, astronomers were able to conclude, correctly, that Uranus possibly had a ring system. It wasn't unitl Voyager 2 passed Uranus that we were able to image the rings directly, and show that Uranus did have rings.

The Discovery of Uranus

Uranus is the first planet that was not known to ancient astronomers and was not discovered until more than 180 years after Galileo first turned his telescope to the sky and discovered the Galilean satellites. Uranus is bright enough to have been seen by Galileo with his telescope, but if he had seen it (and there is evidence that he might have), he would have only assumed it was a star as his telescope did not have enough resolution to see Uranus as a disk rather than a pinpoint of light. Also, that pinpoint of light would not have moved fast enough to distinguish itself as a planet or some other object rather than a star.

In 1690, John Flamsteed actually saw Uranus with his telescope. But much like Galileo, he did not see it moving fast enough that in the six times that he observed it, he thought it was a star in Taurus and even labeled it as 34 Tauri. The 34 means that to him, Uranus was the 34th star discovered in Taurus. Since he did not observe it long enough, he was not able to see it moving faster compared to the actual stars in Taurus.

Pierre Lemonnier also spotted Uranus between 1750 and 1769. However, he also was not able to distinguish it as a planet as he did not see it moving among the stars.

Finally, in 1781, William Herschel made careful observations of Uranus and was able to tell that it moved faster than the farther stars. Not only that, his telescope had good enough resolution to see Uranus was a fuzzy disk rather than a pinpoint like a star would appear in a telescope. By watching Uranus over time, he was able to tell whether or not it was a comet or a planet. A comet would have been moving much faster than a planet as a comet would start to come in towards the inner solar system, but Uranus did not. By following the trajectory of Uranus, he was able to determine how far away it was.

Herschel wanted to name the planet after the King George III of his adopted country, England. But the name was argued back and forth, British astronomers decided to name it Uranus, who was Zeus' grandfather in Greek mythology. It became the first planet, other than Earth, not be named after a Roman god.

Uranus

Image Credit:

NASA/Voyager 2

 

No planet or celestial object causes as much snickering as the seventh planet from the Sun. Depending on whom you ask, it can be pronounced one of two ways. The first way is the one that causes the snickering, especially among high school and college students (I know, I taught college astronomy). The other way, the way that I prefer to use, does not make it sound so humorous. Of course, I am talking about the planet, Uranus.

I pronounce Uranus as if it sounds like "You're A Nus" or "You're a Nis". Trust me, it helps people from giggling when you say its name. The name Uranus itself comes from Greek mythology as Uranus was the father of the first Titans (including Cronus) and the grandfather of the Olympians, the Greek gods. It is the first planet (other than Earth) not named for a Roman god. If the tradition had held using Roman names, Uranus should have been called Caelus, the father of Saturn and in turn, the grandfather of Jupiter.

Uranus is also the first planet discovered with a telescope. Up to its discovery in 1781 by William Herschel, only the six planets were known (Mercury, Venus, Earth, Mars, Jupiter, and Saturn). Of course, before the heliocentric model of the Solar System, Earth was not considered a planet. There is evidence that Galileo saw Uranus, but mistook it for a star. Two other astronomers also observed Uranus, but did not identify it as a planet.

Like Jupiter and Saturn, Uranus is a Jovian planet, i.e. a gas giant planet. It does not have a solid surface, but contains hydrogen, helium, methane, ammonia, and water in the outer layers. Its "mantle" is a mixture of ice and rock and has a heavy element core. The density of Uranus is 1.29 g/cm³, making it the seventh densest planet, only ahead of Saturn.

Uranus is the third largest planet in terms of diameter, but the fourth largest in mass (Neptune is larger in mass, but has a smaller diameter). It orbits 19.18 AU from the Sun, taking just over 84 years to orbit the Sun. Since its discovery in 1781, it has only completed two orbits, its third orbit won't be complete until 2033.

Since it is a Jovian planet, it also exhibits two properties that Jupiter and Saturn display: a ring system and multiple moons. The rings are more similar to Jupiter's rings than Saturn's rings as they are very faint and were not confirmed until Voyager 2 imaged them directly. Uranus has 27 confirmed moons which are all icy bodies. Five are considered to have planetary mass which means that they are spherical. Titania and Oberon were actually first discovered by William Herschel in 1787, after his discovery of Uranus.

Image Credit:

W. M. Keck Observatory

Lastly, the most amazing thing about Uranus is its day. Recall that Venus has an inclination of almost 180°. Uranus' inclination is not as severe, but may considered stranger. Its inclination is 98° which means that its axis of rotation is almost parallel to Uranus' orbital plane.