- The plane mid-way between the zenith and the nadir which in a flat landscape divides earth from sky. Not to be confused with the event horizon, as found in black holes.
- As seen from a given location, the junction at which the earth, or the sea, appears to meet the sky. Also called apparent horizon, or visible horizon.
- A horizon (1) as seen from an antenna transmitting site. Such a horizon is the furthest distance that a direct wave may travel. It is the line that is formed by the points at which the direct waves from a radio transmitter become tangential to the surface of the earth. Due to atmospheric refraction, this horizon extends beyond the apparent horizon. Also called radio horizon.
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