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The two innermost moons, Cordelia and Ophelia, are shepherds of Uranus's ε ring, whereas the small moon Mab is a source of Uranus's outermost μ ring. [12] There may be two additional small (2–7 km in radius) undiscovered shepherd moons located about 100 km exterior to Uranus's α and β rings .
The poles of astronomical bodies are determined based on their axis of rotation in relation to the celestial poles of the celestial sphere. Astronomical bodies include stars, planets, dwarf planets and small Solar System bodies such as comets and minor planets (e.g., asteroids), as well as natural satellites and minor-planet moons.
Typically, the stated rotation period for a giant planet (such as Jupiter, Saturn, Uranus, Neptune) is its internal rotation period, as determined from the rotation of the planet's magnetic field. For objects that are not spherically symmetrical, the rotation period is, in general, not fixed, even in the absence of gravitational or tidal forces
The exceptions – the planets with retrograde rotation – are Venus and Uranus. Venus's axial tilt is 177°, which means it is rotating almost exactly in the opposite direction to its orbit. Uranus has an axial tilt of 97.77°, so its axis of rotation is approximately parallel with the plane of the Solar System.
Uranus has retrograde rotation when defined this way. Alternatively, the convention in which a body's north and south poles are defined according to the right-hand rule in relation to the direction of rotation, Uranus's axial tilt may be given instead as 97.8°, which reverses which pole is considered north and which is considered south and ...
Regardless of which convention is used, Uranus has a retrograde rotation relative to its orbit. [79] Comparison of the rotation period (sped up 10 000 times, negative values denoting retrograde), flattening and axial tilt of the planets and the Moon (SVG animation) The rotation of a planet can be induced by several factors during formation.
Six of the eight solar planets have prograde rotation—that is, they rotate more than once per year in the same direction as they orbit the Sun, so the Sun rises in the east. [17] Venus and Uranus, however, have retrograde rotation. For prograde rotation, the formula relating the lengths of the sidereal and solar days is:
The angles for Earth, Uranus, and Venus are approximately 23°, 97°, and 177° respectively. In astronomy , axial tilt , also known as obliquity , is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane ; equivalently, it is the angle between its equatorial plane and orbital ...