horizon, apparent horizon, visible horizon, sensible horizon, skyline (noun)

the line at which the sky and Earth appear to meet

horizon, view, purview (noun)

the range of interest or activity that can be anticipated

"It is beyond the horizon of present knowledge"

horizon (noun)

a specific layer or stratum of soil or subsoil in a vertical cross section of land

horizon, celestial horizon (noun)

the great circle on the celestial sphere whose plane passes through the sensible horizon and the center of the Earth

horizon (Noun)

The horizontal line that appears to separate the Earth from the sky.

horizon (Noun)

The range or limit of one's knowledge, experience or interest.

horizon (Noun)

A specific layer of soil or strata

horizon (Noun)

A cultural sub-period or level within a more encompassing time period.

Horizon

The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the visible horizon. When looking at a sea from a shore, the part of the sea closest to the horizon is called the offing. The word horizon derives from the Greek "ὁρίζων κύκλος" horizōn kyklos, "separating circle", from the verb ὁρίζω horizō, "to divide", "to separate", and that from "ὅρος", "boundary, landmark".

Horizon

The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whether it intersects the relevant body's surface or not. The true horizon is a theoretical line, which can only be observed to any degree of accuracy when it lies along a relatively smooth surface such as that of Earth's oceans. At many locations, this line is obscured by terrain, and on Earth it can also be obscured by life forms such as trees and/or human constructs such as buildings. The resulting intersection of such obstructions with the sky is called the visible horizon. On Earth, when looking at a sea from a shore, the part of the sea closest to the horizon is called the offing.The true horizon surrounds the observer and it is typically assumed to be a circle, drawn on the surface of a perfectly spherical model of the relevant celestial body, i.e., a small circle of the local osculating sphere. With respect to Earth, the center of the true horizon is below the observer and below sea level. Its radius or horizontal distance from the observer varies slightly from day to day due to atmospheric refraction, which is greatly affected by weather conditions. Also, the higher the observer's eyes are from sea level, the farther away the horizon is from the observer. For instance, in standard atmospheric conditions, for an observer with eye level above sea level by 1.70 metres (5 ft 7 in), the horizon is at a distance of about 5 kilometres (3.1 mi). When observed from very high standpoints, such as a space station, the horizon is much farther away and it encompasses a much larger area of Earth's surface. In this case, the horizon would no longer be a perfect circle, not even a plane curve such as an ellipse, especially when the observer is above the equator, as the Earth's surface can be better modeled as an oblate ellipsoid than as a sphere.