In landscape shots, the position of the horizon influences the mood of the photo. To imply a vast, wide open space, put the horizon along the lower third of the frame and show lots of sky. (This obviously works best when the sky is cooperating.) To imply a sense of closeness—or if the sky is a bland shade of gray—put the horizon along the upper third, showing little sky.
This rule, like others, is meant to be broken. For example, if you’re shooting a forlorn-looking desert landscape, you might want to have the horizon bisect the image to imply a sense of bleak monotony.