As the Earth revolves around the sun, the position of the sun relative to the background stars changes. Because of this, over the course of a year, the sun appears to move through the heavens along a path called the ecliptic. Technically then, the ecliptic represents the extension or projection of the plane of the Earth's orbit out toward the sky.

But since the moon and planets also move in orbits whose planes do not differ greatly from that of the Earth's orbit, those bodies, when visible in our sky, also travel relatively close to the ecliptic. In fact, in this way, our solar system is really quite flat, with all of the planets moving in very nearly the same plane.

It is for this reason that even the most ancient sky charts usually plot the position of the ecliptic; it is something of a warning to skywatchers that the moon, not to mention those strange wandering stars once revered by the ancients (the planets) will often appear near or along this path and possibly interfere with the observation of the fixed stars.

Since the moon and the planets are not located in exactly the same orbital plane as Earth, they are seldom located exactly on the ecliptic, but rather within a narrow strip of sky several degrees wide that we call the Zodiac.

The ecliptic runs exactly along the middle of the Zodiac.