Uranus ... a tipped over planet

Introduction to Uranus

Greetings to a relatively boring planet with a rather interesting name. The politically correct pronounciaton is "Yoor' a nus" instead of the more comical "Your anus," and since we are living in such a time as this, you are encouraged to use the first pronounciation. If, however, you lapse into the other form of pronounciation, I will understand and you can have a few laughs when you learn that "Uranus is full of gas." Oh, and every astronomer's favorite little joke, "What do toilet paper and the starship Enterprise have in common? ... they both go around Uranus and pick up Kling-Ons." Ha ha ha! Okay, now that we have concluded the fun stuff, let's get down to what this blue-green planet is all about. In appearance, there is almost nothing to really distinguish this planet. It has barely perceptible weather bands, and I really mean barely. What IS unusual is that this planet is tipped on its side, so that it rotates sideways with the south pole presently aimed at the Sun. By sideways, I mean to describe the planet as being tipped completely over in such a way as if the Earth were tipped so that the north pole now points to the west instead of to the north. The ring system and even the moons are all rotating and revolving sideways. The reason for this odd tilt is unknown. In composition, Uranus is like Jupiter and Saturn ... a giant gas planet with rings and lots of moons. Finally, the position of this planet is seventh from the Sun, but only marks the halfway point between the Sun and Pluto! Only one spacecraft has visited Uranus, that being the Voyager 2 satellite in 1985.

Planetary data

Mass (kg), and mass relative to Earth

8.66x1025 kg = 14.54 earths

Equatorial diameter (km)


Mean density (gm/cm3)


Acceleration of gravity (m/s2)


Velocity of escape (km/s)


Period of rotation

17.9 hours

Period of revolution

84.0221 years

Aphelion (AU)


Aphelion (km)


Perihelion (AU)


Perihelion (km)


Mean orbital distance from the sun (AU)


Mean orbital distance from the sun (km)


Orbital velocity (km/s)






Inclination to the ecliptic

.774 degrees

Inclination of the equator to the orbit

97.92 degrees

Number of natural satellites


Names of natural satellites

Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, Puck, Miranda, Ariel, Umbriel, Titania, Oberon, Caliban, Stephano, Trinculo, Sycorax, Prospero, Setebos, and 6 other newly discovered moons that have not yet been named

More Information on the Planet Uranus from the Nine Planets Website

Much of the information below is direct from the Nine Planets website . Some material has been altered by me for this course, while other items and comments are directly copied. I hope to maintain a continuous update of this material to keep up with the findings from space satellites and telescopes.

Uranus is the seventh planet from the Sun and the third largest (by diameter). Uranus is larger in diameter but smaller in mass than Neptune.
orbit: 2,870,990,000 km (19.218 AU) from Sun
diameter: 51,118 km (equatorial)
mass: 8.683x1025 kg
Careful pronunciation may be necessary to avoid embarrassment; say "YOOR a nus" , not "your anus" or "urine us".

Uranus is the ancient Greek deity of the Heavens, the earliest supreme god. Uranus was the son and mate of Gaia the father of Cronus (Saturn) and of the Cyclopes and Titans (predecessors of the Olympian gods). This is interesting to think on. Uranus is a chief god, something you might not usually think of unless Uranus is sitting on the throne of your bathroom. Then you might get this picture more clearly.




Discovery of the planet, and satellite mission fly-by.

Uranus, the first planet discovered in modern times, was discovered by William Herschel while systematically searching the sky with his telescope on March 13, 1781. It had actually been seen many times before but ignored as simply another star (the earliest recorded sighting was in 1690 when John Flamsteed cataloged it as 34 Tauri). Herschel named it "the Georgium Sidus" (the Georgian Planet) in honor of his patron, the infamous (to Americans) King George III of England; others called it "Herschel". The name "Uranus" was first proposed by Bode in conformity with the other planetary names from classical mythology but didn't come into common use until 1850.

Uranus has been visited by only one spacecraft, Voyager 2 on Jan 24 1986. The NASA chiefs had only planned the Voyager mission to go as far as Saturn, but since the planet alignment was unusually favorable for a potential visit to Uranus and Neptune, the JPL team members proposed altering the trajectory of Voyager 2 a bit and sending it out to Uranus. Now, there was a price to pay for this change in the mission trajectory. The amount of data able to be gathered from Saturn would be less, but the NASA bosses bought the idea.

There were two problems with this alteration of the Voyager 2 mission schedule. The scan platform upon which the telescope sat got stuck during the Saturn encounter. After a few years of trying to get the telescope to move again, the JPL engineers concluded that they had tried to move the telescope too often and too drastically during the Saturn flyby. Eventually, they were able to get the space telescope to move a little bit, but were forced to aim the satellite at targets instead of just the telescope, and also they had to constrain the overall motion of the telescope to as little as possible to keep the platform from locking up again.

The other problem was listening to the signal from Voyager 2 because Uranus is twice as far from the Sun as Saturn. By enlarging some radio telescope dishes on Earth and arraying sets of radio dishes at the VLA in New Mexico, they were able to get some wonderful infomation and images from the Uranus encounter in 1986. Here is some of what they learned.

What did Voyager 2 learn?

Most of the planets spin on an axis nearly perpendicular to the plane of the ecliptic but Uranus' axis is almost parallel to the ecliptic. At the time of Voyager 2's passage, Uranus' south pole was pointed almost directly at the Sun. This results in the odd fact that Uranus' polar regions receive more energy input from the Sun than do its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles. The mechanism underlying this is unknown.

Actually, there's an ongoing battle over which of Uranus' poles is its north pole! Either its axial inclination is a bit over 90 degrees and its rotation is direct, or it's a bit less than 90 degrees and the rotation is retrograde. The problem is that you need to draw a dividing line *somewhere*, because in a case like Venus there is little dispute that the rotation is indeed retrograde (not a direct rotation with an inclination of nearly 180).

The image to the left tells an interesting story. In 1977 prior to the Voyager launch 2, Earth-based astronomers had the opportunity to witness an occultation of a star by Uranus. This is an event when a foreground object passes in front of a more distant star, blocking the distant star's light. The purpose of the observation was to watch the star's light interact with the atmosphere of Uranus and see what happened to the star's light. This would give us clues to the composition of the atmosphere of Uranus. Much to the surprise of the telescopic observers, the star's light appeared to blink on and off ahead of the actual occulation and do the same blinking pattern after the occultation. Imagine the image to your left and a background star. The sideways orientation of the planet and its rings were the cause of the star's blinking. With each segment of the ring covering up the distant star's light, the occultation showed successive on-off blinks both before and after the planet covered the star. Astronomers knew before the Voyager 2 encounter that Uranus was tipped on its side!

As for why this planet is tipped over and its ring system and moons also, there is no explanation agreed upon by all members of the scientific community. Since the Voyager encounter, Uranus has moved in its orbit almost 1/4 of its total distance, so that the planet is now appearing edge-on relative to us. Below are a pair of image/image sequences that show this change from the picture above and left in 1986 to the view present in 2006. All of these pictures were taken with the Keck telescope atop Mauna Kea, Hawaii.

Composition and Structure

Uranus is composed primarily of rock and various ices, with only about 15% hydrogen and a little helium (in contrast to Jupiter and Saturn which are mostly hydrogen). Uranus (and Neptune) are in many ways similar to the cores of Jupiter and Saturn minus the massive liquid metallic hydrogen envelope. It appears that Uranus does not have a rocky core like Jupiter and Saturn but rather that its material is more or less uniformly distributed. This is a major change to the thinking of my in-class Astronomy students who like to open their oral reports with the statement, "Uranus is full of methane gas." While the sentance always gets its laughs, it is not very true.





Uranus' atmosphere is about 83% hydrogen, 15% helium and 2% methane.

Like the other gas planets, Uranus has bands of clouds that blow around rapidly. But they are extremely faint, visible only with radical image enhancement of the Voyager 2 pictures. Recent observations with HST (left) show larger and more pronounced streaks. Further HST observations show even more activity. Uranus is no longer the bland boring planet that Voyager saw! It now seems clear that the differences are due to seasonal effects since the Sun is now at a lower Uranian latitude which may cause more pronounced day/night weather effects. By 2007 the Sun will be directly over Uranus's equator.

Uranus' blue color is the result of absorption of red light by methane in the upper atmosphere. There may be colored bands like Jupiter's but they are hidden from view by the overlaying methane layer.


Rings and Moons

Like the other gas planets, Uranus has rings (left). Like Jupiter's, they are very dark but like Saturn's they are composed of fairly large particles ranging up to 10 meters in diameter in addition to fine dust. There are 11 known rings, all very faint; the brightest is known as the Epsilon ring. The Uranian rings were the first after Saturn's to be discovered. This was of considerable importance since we now know that rings are a common feature of planets, not a peculiarity of Saturn alone.




Voyager 2 discovered 10 small moons in addition to the 5 large ones already known. Since the Voyager passage, 10 more moons have been discovered, bringing the total to 25. The 5 major moons are shown below, and you will learn more about them in the Uranian Moons page.

Uranus' magnetic field is odd in that it is not centered on the center of the planet and is tilted almost 60 degrees with respect to the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus. Planetary scientists determine the length of a rotation period by measuring the spin of the magnetic field. Watching clouds go by relative to the magnetic field tells us about the speeds of the wind in the atmosphere. What is so unusual about Uranus, and even more pronounced in Neptune, is the displacement of the center of the magnetic field relative to the core of the planet. Apparently, the two centers are not even near each other, and everyone I have asked about this has offered an unsatisfactory answer.


Perhaps the most interesting aspect of Uranus are its seasons and days. Since this world looks so bland, it is nice that there is something interesting about it for me students. At present, the south pole is facing the Sun. As the planet spins counterclockwise, the Sun will appear to move in a really interesting manner. Overhead, if you were on the clouds of the south pole, the Sun would either stay motionless in the sky, or move in circles every 17 hour day. Those living on the equator would see the Sun move in a vast circular path exactly on the horizon. Twenty one years later, those same people on the equator would see the Sun rise in the East and set in the West, while south pole residents would see the sun move counterclockwise on their horizon while those on the north pole would see the Sun move clockwise on their horizon. Twenty one years after that, north pole residents would see the Sun moving in little circles above their heads and equatorial residents would see the Sun moving along the horizon clockwise. Twenty one years later, north pole residents would see the Sun on the horizon moving counterclockwise while south pole residents would notice a clockwise motion, and those on the equator would see the Sun rising in the West and setting in the East. Finally, after another twenty one years, or eighty four total Earth years of time, all residents on the planet would see the same solar motion. This means that no two 17 hour days are the same over an 84 year cycle!

Uranus is sometimes just barely visible with the unaided eye on a very clear night; it is fairly easy to spot with binoculars (if you know exactly where to look). A small astronomical telescope will show a small disk. There are several Web sites that show the current position of Uranus (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program such as Starry Night.

Uranus' Satellites

Uranus has 21 named moons plus 6 recently discovered ones which as yet have not been given an official name.
Unlike the other bodies in the solar system which have names from classical mythology, Uranus' moons take their names from the writings of Shakespeare and Pope.
They form three distinct classes: the 13 small very dark inner ones discovered by Voyager 2, the 5 large ones (right), and the newly discovered much more distant ones.
Most have nearly circular orbits in the plane of Uranus' equator (and hence at a large angle to the plane of the ecliptic); the outer 7 are much more elliptical. To learn more about the new ones, go to Sam Sheppard's website about these new moons of Uranus.

Uranian Moons








50,000 km

13 km


Voyager 2



54,000 km

16 km


Voyager 2



59,000 km

22 k m


Voyager 2



62,,000 km

33 km


Voyager 2



63,000 km

29 km


Voyager 2



64,000 km

42 km


Voyager 2



66,000 km

55 km


Voyager 2



70,000 km

27 km


Voyager 2


S/2003 U2

74,800 km

5 km





75,300 km

40 km


Voyager 2


S/1986 U 10

76,420 km

10 km





86,000 km

81 km


Voyager 2


S/2003 U1

97,734 km

5 km





129,900 km

236 km





190,900 km

579 km





266,000 km

585 km





436,300 km

789 km





583,500 km

761 km





7,169,000 km

49 km





7,948,000 km

10 km





8,578,000 km

5 km





12,213,000 km

80 km





16,568,000 km

15 km





17,681,000 km

15 km




S/2001 U2

21,000,000 km

6 km




In the in-classroom Astronomy class, we always have fun with the names of these moons. As if Simon and Garfunkle were singing, "Ophelia, you're breaking my heart, you're tearing my confidence daily." The Bianca Blast is an old-fashioned mouth spray to freshen your breath. Some people like to drive the Toyota Cressida, perhaps on their way to the Shakesperean play, "Romeo and Juliet." Kirby Puck, of Minnesota bassball fame is not the 34th moon, but is still a pretty object. If you commit a crime and are caught by the cops, they are supposed to Miranda you (read you your rights). King Triton's rebellious daughter Ariel sings beautifully in "The Little Mermaid." If it is raining outside, you might want to bring your Umbriel to stay dry ... something that did not happen to Leonard DiCaprio when he sunk with the Titania. Oberon Won Kenobi sucessfully trained Luke Skywalker, but failed with Anikan Skywalker. The Caliban sounds like a potential terrorist group. And we have to mention Stephano, who was one of the evil characters plaed by Count Olaf in one of the Lemony Snickets books about unfortunate events in the lives of three orphaned children. As for S/2003 U2 ... "it still hasn't found what it's looking for."

Uranus' Rings


Distance *


1986 U2R

38,000 km

2,500 km


41,840 km

1-3 km


42,230 km

2-3 km


42,580 km

2-3 km


44,720 km

7-12 km


45,670 km

7-12 km


47,190 km

0-2 km


47,630 km

1-4 km


48,290 km

3-9 km


50,020 km



51,140 km

20-100 km

*Distance measures from center of Uranus to the ring's inner edge*

If you want to learn more about the planet Uranus, go to the JPL webpage devoted to this strange planet.

If you have not already done so, you are asked to move first to the Uranian Moons page, and then through the Uranian Rings page to complete your reading on this planet. While I agree with most of the astronomers whom I have talked to about the boring nature of this planet, it is still cool because it is tipped over on its side. When you can finished reading about the moons and rings, then consider these questions about the planet that I have compiled for you:)

1) Give a brief description of the appearance of planet Uranus as you might expect to see it from a spacecraft.

2) What is the attribute of the planet Uranus that distinguishes it from other planets?

3) What is the name of the only spacecraft ever to visit this planet, and when did it arrive?

4) Who discovered the planet Uranus and when was this discovery made?

5) How many moons are currently believed to be orbiting the planet Uranus?

6) What do scientists believe caused the surface of Miranda to appears as it does?

7) In what ways are the rings of the planet Uranus similar to those of Jupiter and Saturn?

8) What is unusual about the composition of the planet Uranus compared to Jupiter and Saturn?

9) What is the epsilon ring composed of?

10) What might be a challenge to a manned mission to the planet Uranus that is different from what you may have listed for any of the other planets?

Good ... so you think you know the answers to these 10 questions? Well, go ahead and submit them at the Uranus Quiz page and afterward, go forward to planet Neptune, or return to an Introduction to the Gas Giants, or the Planet Introduction, or to the Syllabus.

If you want to look at an alternative to my pages, connect to the JPL site about Uranus.

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