Of the imaginary coordinate lines that astronomers and navigators use in mapping the sky, perhaps the most important one is the ecliptic, the apparent path the sun appears to take through the sky as a result of the Earth's revolution around it.
Because of the Earth's yearly revolution around the sun, the sun appears to move in its annual journey through the heavens with the ecliptic as its path. Technically then, the ecliptic represents the extension or projection of the plane of the Earth's orbit out towards 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, these bodies, when visible in our sky, always stay relatively close to the ecliptic line. In other words, our solar system can be best defined as being somewhat flat, with the planets moving in very nearly the same plane.
