The horizon (Ancient Greek ὁ ὁρίζων, /ho horídzôn/, from ὁρίζειν, "to limit")'s the apparent line that separates earth from sky. It's the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that don't. At many locations, the true horizon's obscured by trees, buildings, mountains, etc.,and the resulting intersection of earth and sky's called the visible… (More on Horizon)