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== Rotational characteristics ==

== Rotational characteristics ==

Uranus's sidereal day is 17.24 Earth hours. Uranus's rotation is ''retrograde'' with respect to its orbit around the Sun and the orbits of most of the other planets. Moreover its axis is tilted 97.86° with respect to its orbital plane, so that in a sense the planet is rolling on its side.

Uranus's sidereal day is 17.24 Earth hours. Uranus's rotation is ''retrograde'' with respect to its orbit around the Sun and the orbits of most of the other planets. Moreover, its axis is tilted 97.86° with respect to its orbital plane, so that in a sense the planet is rolling on its side.

== Physical characteristics ==

== Physical characteristics ==

=== Orientation of the Uranian system ===

=== Orientation of the Uranian system ===

According to evolutionary theories of how the solar system formed (i.e., the [[nebular hypothesis]]), Uranus's axis should not be tilted 98 degrees with respect to its orbit. Moreover its moons orbit Uranus around its equator, like the hands of a clock around a clock face. Uranus' thirteen rings also are around its equator, so they look vertical. The nebular hypothesis predicts none of these observations.

According to evolutionary theories of how the solar system formed (i.e., the [[nebular hypothesis]]), Uranus's axis should not be tilted 98 degrees with respect to its orbit. Moreover, its moons orbit Uranus around its equator, like the hands of a clock around a clock face. Uranus' thirteen rings also are around its equator, so they look vertical. The nebular hypothesis predicts none of these observations.

Naturalistic [[Astronomy|astronomers]] theorize that Uranus originally formed upright, but suffered a collision with another object, producing its severe tilt. This, however, raises more questions:

Naturalistic [[Astronomy|astronomers]] theorize that Uranus originally formed upright, but suffered a collision with another object, producing its severe tilt. This, however, raises more questions:

# Why is its orbit less eccentric than those of all the other planets except only those of [[Venus]], [[Neptune]], and [[Earth]]?

# Why is its orbit less eccentric than those of all the other planets except only those of [[Venus]], [[Neptune]], and Earth?

# Why is Uranus's orbit less inclined to the ecliptic than is that of any planet except Earth?

# Why is Uranus's orbit less inclined to the ecliptic than is that of any planet except Earth?

# Why are the orbits of Uranus's moons so slightly inclined with respect to Uranus's present equator?<ref name=AiG>Psarris, Spike. "[http://www.answersingenesis.org/creation/v24/i3/uranus.asp Entry for Uranus]." ''Creation'', 24(3):38-40, June 2002. Now hosted by [[Answers in Genesis]]. Accessed April 25, 2008.</ref> Indeed, the vertical orientation of Uranus's moons and of some of its rings was already well-known to astronomers before the Voyager 2 flyby. If the moons formed before the collision, then another series of precisely timed collisions would have been required to change the azimuths of all the orbits, ''and'' the rings&mdash;an absurd violation of [[Occam's razor]]. And if they formed after the collision, then in the absence of any condensing gas cloud, they ought to be inclined at random.

# Why are the orbits of Uranus's moons so slightly inclined with respect to Uranus's present equator?<ref name=AiG>Psarris, Spike. "[http://www.answersingenesis.org/creation/v24/i3/uranus.asp Entry for Uranus]." ''Creation'', 24(3):38-40, June 2002. Now hosted by [[Answers in Genesis]]. Accessed April 25, 2008.</ref> Indeed, the vertical orientation of Uranus's moons and of some of its rings was already well-known to astronomers before the Voyager 2 flyby. If the moons formed before the collision, then another series of precisely timed collisions would have been required to change the azimuths of all the orbits, ''and'' the rings&mdash;an absurd violation of [[Occam's razor]]. And if they formed after the collision, then in the absence of any condensing gas cloud, they ought to be inclined at random.

== Missions to Uranus ==

== Missions to Uranus ==

The only deep-space probe to visit Uranus thus far has been [[Voyager 2]], which flew by Uranus on January 24, 1986. Voyager 2 discovered ten of Uranus's twenty-seven satellites, and two previously unknown rings. Voyager 2 then continued onward to [[Neptune]] before heading out of the [[Solar System]].

The only deep-space probe to visit Uranus thus far has been [[Voyager 2]], which flew by Uranus on January 24, 1986. Voyager 2 discovered ten of Uranus's twenty-seven satellites, and two previously unknown rings. Voyager 2 then continued onward to Neptune before heading out of the Solar System.

== Gallery ==

== Gallery ==