Horizon. There are two distinct meanings to the term. The first is the visible horizon, and may be defined as the line where earth and sky meet. This is irregular on land, and may be broken up considerably by hills, etc. On the open sea the visible horizon is a circle, depending in magnitude on the height of the observer, and depressed below him by an amount also depending on his elevation. This depression is called the dip of the visible horizon. The astronomical horizon signifies the great circle of the celestial sphere whose plane is at right angles to the line joining the zenith to the observer.