Prograde and retrograde motionRetrograde motion is the orbital
motion of a body in a direction opposite that which is normal to
spatial bodies within a given system. 'Retrograde' derives from
the Latin words retro, backwards, and gradus, step. In the Solar
system, mostly everything rotates in the same sense: all major
planets orbit the Sun counterclockwise as seen from the pole
star (Polaris). Most planets spin in the same sense, including
Earth. The same happens with the orbital motions of the Moon,
Mars' moons, and the biggest moons of Jupiter and Saturn around
their planets. All these motions are called "direct" or
"prograde." Rotation in the opposite sense is called retrograde.
Venus and Uranus spin clockwise, so they have a retrograde
rotation. Some small moons orbit clockwise around their planet,
and are called retrograde satellites. Some comets and small
asteroids orbit the Sun in retrograde orbits. All these are the
exception rather than the rule. When we observe the sky, we
expect most objects to app