In order to track the Sun, Moon and Planets in their orbits with telescopes, astronomers have defined an imaginary line across the sky.  This line is called the ecliptic.  The ecliptic runs east to west in an arc shape.  The Sun, Moon and Planets follow this line because our solar system orbits on a flat plane.  

     The ecliptic goes through 13 constellations.  The constellations that touch the ecliptic are called the zodiac.  The zodiac means nothing more than this.

     The ecliptic got its name because eclipses occur when the Moon is closest to the line.