What role does the visible horizon play in Celestial Navigation?

The visible horizon plays a crucial role in celestial navigation, particularly in measuring the altitude of celestial bodies, such as stars, planets, the sun, and the moon. The altitude is defined as the angle between the celestial body and the observer's local horizontal plane, which is referenced to the visible horizon.

When navigators take observations of celestial bodies, they align their sextant or another measuring instrument to sight the body and determine its angle above the visible horizon. This altitude measurement is essential for calculating the position of the vessel on the Earth's surface. By knowing the altitude of a celestial body and referencing it to the horizon, navigators can further apply mathematical formulas to determine their latitude and, when combined with time, their longitude.

In essence, the visible horizon acts as a critical reference point from which these measurements are made, enabling navigators to pinpoint their location at sea accurately.