Astronomy Notes

Discovery

• Uranus was discovered in 1781 accidentally by William Herschel when he was making a star map.
• Uranus had some irregular motions which indicated another planet farther out.
• Neptune was found in 1846 within an hour of beginning a search where Leverrier predicted.

Physical Characteristics

• Both planets are blue, because of methane gas.
• Both have thick atmospheres of hydrogen and helium gas.
• Uranus has no cloud bands; Neptune has cloud belts and a Great Dark Spot.
• Uranus is featureless because it is cold, Neptune is producing heat, like Jupiter.
• Both are believed to have rocky cores.

Uranus Weird Axis of Rotation

• Its axis is lying nearly in the plane of the ecliptic.
• Thus it has one season with sun on one pole, then one with sun above equator, then other pole.
• The moons revolve around the equator, so they have the same experience.

Rings

• Both planets have ring systems. Those of Uranus were discovered during an occultation of a star.
• Uranus main ring is extremely narrow, kept so thin by two shepherd satellites.
• Neptune has two "arc-rings," where only segments of the ring are there.

Uranus's Satellites

• Five moons were known before Voyager, now the total is fifteen.
• All of the new discoveries are near to the planet, near the ring system.
• All are named after characters in Shakespeare's plays.
• Miranda, the smallest of the five main moons, is the most fractured and resurfaced.
• It might have been smashed by an impact and reassembled.

Neptune's Satellites

• Two moons were known before Voyager, now the total is seven.
• The five new satellites are all very close to the planet, near the ring system.

Triton: Neptune's Erupting Moon

• Triton, the largest moon, has a retrograde orbit, inclined at a high angle to Neptune's equator.
• It is a creamy colored, icy moon.
• It has very few craters, and must be very geologically active to resurface it.
• It has active, erupting geysers or volcanoes, with dark smoke rising from them.
• It has a circular orbit, suggesting that it has been tidally circularized which could have heated it.

Discovery

• It was predicted because of perturbations of Neptune's orbit.
• It was discovered near where it had been predicted.
• When its mass was discovered to be less than our moon, we realized it's discovery had just been luck.
• It has a moon half of its own diameter.

Maybe not a Planet. The following are reasons not to classify it as a planet.

• It is much smaller than any other planet.
• It doesn't fit the trend of the giant planets.
• It doesn't follow Bode's rule, but actually comes inside of Neptune's orbit.
• Several other bodies that size are known to orbit the sun, such as Chiron.
• Thus, many think it should be classified as a double asteroid.