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An animation explaining why the planet Mercury may appear to move "backwards", or retrograde across Earth's sky. Apparent retrograde motion is the apparent motion of a planet in a direction opposite to that of other bodies within its system, as observed from a particular vantage point.
A celestial object's axial tilt indicates whether the object's rotation is prograde or retrograde. Axial tilt is the angle between an object's rotation axis and a line perpendicular to its orbital plane passing through the object's centre. An object with an axial tilt up to 90 degrees is rotating in the same direction as its primary.
Since obliquity is the angle between the axis of rotation and the direction perpendicular to the orbital plane, it changes as the orbital plane changes due to the influence of other planets. But the axis of rotation can also move (axial precession), due to torque exerted by the Sun on a planet's equatorial bulge. Like Earth, all of the rocky ...
Neptune's mass of 1.0243 × 10 26 kg [8] is intermediate between Earth and the larger gas giants: it is 17 times that of Earth but just 1/19th that of Jupiter. [g] Its gravity at 1 bar is 11.15 m/s 2, 1.14 times the surface gravity of Earth, [71] and surpassed only by Jupiter. [72] Neptune's equatorial radius of 24,764 km [11] is nearly four ...
Triton orbits Neptune in a retrograde orbit—revolving in the opposite direction to the parent planet's rotation—the only large moon in the Solar System to do so. [ 3 ] [ 13 ] Triton is thought to have once been a dwarf planet from the Kuiper belt , captured into Neptune's orbit by the latter's gravity .
The equatorial plane is the plane perpendicular to the axis of rotation of the central body. An inclination of 30° could also be described using an angle of 150°. The convention is that the normal orbit is prograde, an orbit in the same direction as the planet rotates. Inclinations greater than 90° describe retrograde orbits (backward). Thus:
Earth's rotation axis moves with respect to the fixed stars (inertial space); the components of this motion are precession and nutation. It also moves with respect to Earth's crust; this is called polar motion. Precession is a rotation of Earth's rotation axis, caused primarily by external torques from the gravity of the Sun, Moon and other bodies.
True polar wander is a solid-body rotation (or reorientation) of a planet or moon with respect to its spin axis, causing the geographic locations of the north and south poles to change, or "wander". In rotational equilibrium, a planetary body has the largest moment of inertia axis aligned with the spin axis, with the smaller two moments of ...