# What Is Horizon

## Video: What Is Horizon

The word "horizon" literally translated from ancient Greek means "limiting". In everyday life, this word is called a curved line along which the sky is seen in contact with the earth's or water surface.

## Instructions

### Step 1

The horizon is called the apparent border, along which there is a clear separation of the heavenly and earthly surfaces, observed at a great distance. Distinguish between the concepts of the visible and true horizon.

### Step 2

The visible horizon is the line along which the sky borders the earth, and the celestial space above this line, and the terrestrial space in the observer's field of view.

### Step 3

The true (or mathematical) horizon is the mental circle of the celestial sphere in a plane perpendicular to the plumb line at the center of observation. The plumb line passes through the center of the celestial sphere and an observation point located on the surface of the Earth. In this case, the first point is called the zenith, and the second (the place where the observer stands) is called the nadir.

### Step 4

The line of the true horizon divides the imaginary celestial sphere into two parts: the visible hemisphere, the top of which is at the zenith, and the invisible hemisphere, the top of which is nadir. The true horizon is also called astronomical.

### Step 5

As a rule, the visible horizon is located below the true horizon, since it depends on the height of the observation site. In addition, the range of the visible horizon also depends on the state of the atmosphere.

### Step 6

The concept of the visible horizon is an important term in navigation. The apparent range relative to the horizon on ships is determined depending on the position of the observer, i.e. standing on the deck, at the helm, sitting, etc.

### Step 7

The concept of the true horizon is used both in geography and in navigation to determine the main directions of movement: north, south, west, east. These points of the true horizon are called the main points, and the intermediate directions, northeast, southwest, etc., are called quarter points. To determine the main directions of the horizon, navigators use an unmistakable landmark - the position of the Polar Star, which is part of the constellation Ursa Major, which is easy to find in the sky.