Heidegger uses the term "horizon" as a structural metaphor, a way of expressing the ecastic nature of time. Just as the power of time seems to ensure that the perceivable present is always open, always already unfolding beyond itself, so the distant horizon seems to hold open the perceivable landscape, binding it always to the that which lies beyond it.
The visible horizon, that is, a kind of gateway or threshold, joining the presence of the surrounding terrain to that which exceeds this open presence, to that which is hidden beyond the horizon. The horizon carries the promise of something more, something other.
David Abrams, The Spell of the Sensuous.