What is Retrograde Motion?

Retrograde motion simply describes an object that is moving in the opposite direction in relation to another moving object, and is a term that is commonly used in astronomy to describe the motion of various celestial bodies. In some areas relating to astronomy, objects in the sky seem to be moving backward in relation to Earth but really are not. There are also some planets that spin opposite, or retrograde, to the direction that they orbit the sun. The opposite of retrograde motion is prograde motion or direct motion, which describes an object that is moving in the same direction as another object.

Retrograde motion also applies to rotation and tilt, both of which are essential when describing the orbits of planets, moons, asteroids, and other objects in space. The angle at which an object rotates in relation to other objects is called the inclination of the object. The angle at which an object rotates in relation to a line drawn through the object's poles is known as the axial tilt. If either of these angles are between 90 and 270 degrees, retrograde motion occurs in the object's orbit.

The motion of celestial bodies has fascinated and bewildered people from the first glimpse of the stars, planets, and moons. One particular issue that fascinated stargazers was the apparent retrograde motion of some planets in the sky. All of the planets orbit the sun in the same direction, but some have smaller orbits while others have larger orbits. From the perspective of a planet with a small orbit, a planet with a large orbit appears to have retrograde motion as it moves across the sky. This occurs because planets with smaller orbits make smaller, faster trips around the sun than those with larger orbits.

The same appearance of retrograde motion appears when people pass cars on the road. The faster car passes the slower car, which seems to move backward in relation to it. Just as the slower car does not actually move backward, planets with larger orbits also do not move in a different direction than the rest of the planets.

Some objects in space actually do demonstrate real retrograde motion in their orbits. Some small moons orbit planets opposite to the direction that their planets orbits the sun. This means that they appear to move backward across the sky because they really are moving backward in relation to the orbit of the planet.