The movement of a plane in an opposing direction across the sky a Kepler's second law b Retrograde motion c Planetismals d Solar nebula

## Step 1<br />The problem is asking us to identify the term that describes the movement of a planet in an opposing direction across the sky. This is a concept in astronomy known as "retrograde motion".<br /><br />## Step 2<br />Retrograde motion is the apparent backward (eastward) motion of a planet as viewed from another planet. This is a common occurrence in our solar system, where planets appear to move in a direction opposite to their normal path.<br /><br />## Step 3<br />The other options provided, Kepler's second law, planetismals, and solar nebula, do not describe this type of motion. Kepler's second law refers to the speed at which a planet orbits the sun, planetismals are small bodies that eventually form planets, and a solar nebula is a cloud of gas and dust that forms a solar system.