1. Great Dark Spot A hurricane-like storm on Neptune. It is Earth-size and similar to Jupiter’s Great Red Spot.

2. perturbation theory The mathematics that explains how the orbit of one celestial body would be affected by the attraction of a third celestial body.

3. radiation darkening High-energy bodies striking the surface of a moon can break up molecules; leading to chemical reactions whose products are dark, organic materials. This might explain the low albedo of the moons of Uranus and Neptune.

4. stellar occultation - When an object blocks the light of a bright star, dimming the star’s light. A stellar occultation by Uranus in 1977 resulted in the discovery of its rings (the first seen since Saturn’s).

1. How was Uranus discovered? Uranus was discovered by William Herschel in He first thought it was a comet, but it moved too slowly. It was the first new planet discovered in over 2000 years.

2. Why did astronomers suspect an eighth planet beyond Uranus? No elliptical orbit fit the orbit of Uranus. Astronomers concluded the only explanation was that another object was attracting Uranus.

3. What is the most unusual feature about the rotation of Uranus? Uranus’ rotation axis lies IN the ecliptic plane. It appears as though Uranus is lying on its side. Uranus exhibits differential rotation, but it rotates faster at the poles than at the equator.

4. Why do Uranus and Neptune have a bluish color? There is more methane in the atmosphere. Methane absorbs red light.

5. How are the interiors of Uranus and Neptune thought to differ from those of Jupiter and Saturn? Uranus and Neptune are denser than Saturn and Jupiter, so their rocky cores must make up a greater percentage of the planet. The rocky cores of all the Jovian planets are about the size of Earth.

6. Name an unusual feature of the winds on Neptune. The zonal wind flow is retrograde to the planet’s rotation. The surface winds have speeds of over 2000 km/hr.

7. What is so unusual about the magnetic fields of Uranus and Neptune? °The magnetic fields are tilted from 60 to 46 degrees from the rotational axis. They are also offset from the center of the planets.

8. What observation weakens the idea that the sideways rotation of Uranus was caused by a catastrophic event? The moons are all similar, indicating they were not captured. But they orbit in the rotational plane; what catastrophic event could have also tilted the moon’s orbits without expelling them from their orbits?

9. What is unique about Miranda? A wide range of surface terrains. It appears that Miranda has been catastrophically disrupted several times, with gravity pulling the pieces together in a jumbled, chaotic way.

10. How does Neptune’s system of moons differ from those of other Jovian worlds? Neptune is the only Jovian planet without a regular moon system. Triton is the only large moon in the solar system with a retrograde orbit. Nereid has a tremendously elongated orbit (1.4 million km to 9.7 million km).

11. What is the predicted fate of Triton? Triton’s tidal bulge and retrograde orbit cause it to spiral toward Neptune. In about 100 million years Triton will move within Neptune’s Roche Limit and it will be torn apart by gravitational forces and the remains will form a ring.

12. How do the rings of Uranus differ from the rings of Saturn? They are much darker. They are about the same thickness. They have shepherd satellites.

13. How do the rings of Neptune differ from those of Uranus and Saturn? Neptune has four primary rings. They are darker than Saturn’s.

14. How was Pluto discovered? Neptune and Uranus seemed to be further perturbed by another object. Scientists calculated where this object should be in the sky and concentrated their observations there. It was discovered by Clyde Tombaugh in 1930.

15. How were the masses and radii of Pluto and Charon determined? By observing a series of Pluto-Charon eclipses from 1985 to 1991.

16. In what respect is Pluto more like a moon than like a Jovian or terrestrial planet? Pluto is much smaller than any other planet. It is barely larger than half the size of the United States. Pluto is similar in mass and size to Triton (Neptune’s moon).