backwardrotating

Backward rotating, or backward rotation, describes motion where a body spins opposite to a chosen reference direction. In astronomy this is known as retrograde rotation: the spin is opposite the orbital motion around the Sun. The angular velocity vector points opposite to the orbital angular momentum.

Venus is the canonical example among Solar System planets; its rotation is retrograde, so it rotates clockwise when viewed from above the Sun. It completes one rotation in about 243 Earth days, while its orbit around the Sun takes about 224.7 days.

Retrograde rotation can arise from collisions, tidal forces, or gravitational interactions that impart angular momentum opposite to the original spin. Some exoplanets are suspected or modeled to have retrograde spins; measurements are difficult and uncertainties remain.

In Newtonian mechanics, backward rotation is simply a sign convention: the rotor or wheel spins in the negative direction relative to the defined positive orientation. In engineering, backward-rotating components can occur intentionally (e.g., counter-rotation systems) or due to faults; control systems must account for direction to ensure proper operation.

Observationally, determining spin direction requires precise timing, spectroscopy, or imaging, and is sensitive to the orientation of the rotation axis (axial tilt) and the viewing geometry.

See also prograde, retrograde, tidal locking, axial tilt, spin–orbit resonance.