Uranus has 27 moons that we know of. All but a few of them are named for characters
from the plays of William Shakespeare. The five biggest moons of Uranus—Miranda, Ariel,
Umbriel, Titania and Oberon—are shown inFigure25.28.
Figure 25.28: These Voyager 2 photos have been resized to show the relative sizes of the five
main moons of Uranus. ( 14 )
Neptune
Neptune, shown inFigure25.29, is the eighth planet from the Sun. It is the only major
planet that can’t be seen from Earth without a telescope. Scientists predicted the existence
of Neptune before it was actually discovered. They noticed that Uranus did not always
appear exactly where it should appear. They knew there must be another planet beyond
Uranus whose gravity was affecting Uranus’ orbit. This planet was discovered in 1846, in
the position that had been predicted, and it was named Neptune for the Roman god of the
sea due to its blush color.
Neptune has slightly more mass than Uranus, but it is slightly smaller in size. In many
respects, it is similar to Uranus. Uranus and Neptune are often considered “sister planets.”
Neptune, which is nearly 4.5 billion kilometers (2.8 billion miles) from the Sun, is much
farther from the Sun than even distant Uranus. It moves very slowly in its orbit, taking 165
Earth years to complete one orbit around the Sun.
Extremes of Cold and Wind
Neptune is blue in color, with a few darker and lighter spots. The blue color is caused by
atmospheric gases, including methane (CH 4 ). When Voyager 2 visited Neptune in 1986,
there was a large dark-blue spot south of the equator. This spot was called the Great Dark
Spot. However, when the Hubble Space Telescope took pictures of Neptune in 1994, the
Great Dark Spot had disappeared. Instead, another dark spot had appeared north of the
equator. Astronomers believe both of these spots represent gaps in the methane clouds on
Neptune.
The changing appearance of Neptune is due to its turbulent atmosphere. The winds on
Neptune are stronger than on any other planet in the solar system, reaching speeds of 1,100