What is defined as the line at which the sky and Earth appear to meet?

The horizon is defined as the line where the sky appears to meet the Earth. It represents the furthest point at which an observer can see outward, grounded in their viewpoint. The horizon is influenced by the observer's position and elevation, and serves as an essential reference point in surveying and navigation.

Understanding the horizon is important for surveyors, as it helps them determine visible landmarks, align surveying instruments, and establish sight lines. Other terms, like zenith, meridian, and equatorial plane, refer to different astronomical or geographical concepts, but none represents the same idea of the boundary line between the Earth and the sky as the horizon does. The zenith is the point directly above an observer, the meridian refers to a north-south line of longitude, and the equatorial plane is the imaginary plane that extends out from the equator of the Earth. Each of these terms has its own significance, but they do not capture the fundamental concept of the horizon.