If you were to plot or photograph one of the outer planets for a few months, you would note that its movement against the background stars is not always in the same direction. This apparent reversal of direction (as illustrated below) is referred to as retrograde motion. The direction of movement and the period of time during which this would be detected varies by planet.

This is caused by the fact that the planets travel with different orbital velocities. Moving outward from the center of our solar system, each planet takes progressively longer to orbit the sun, from 88 days for Mercury to over 247 years for Pluto. In the following diagram of two planets (let's say Earth and Mars), as we view Mars from positions 1 through 5, it describes a path against the starry background that differs markedly from its actual motion in orbit. If we were to complete the drawing for one revolution of the Earth around the sun (one full year), we would see that Mars would appear to move in something like the zigzag motion shown above.

These orbital mechanics explain why the planets move among the various constellations along the ecliptic as seen from Earth. (You'd be surprised how many non-astronomers think that each of the planets always appears in the same spot in the sky at the same time each year.) The slower orbital speed also explains why the outer planets appear in the same constellation longer than do the closer-in planets.