The horizon is the apparent line separating the sky from Earth, or the boundary that makes the sky and Earth appear to intersect. In Astronomy, it is also defined as the intersection of a plane perpendicular to a vertical line with the celestial sphere—a hypothetical sphere of infinite radius and concentric with Earth, encompassing all celestial bodies. However, do you know that there are several types of horizons? What are they? What is the difference between them? What is the importance of horizons in navigation? How far is the horizon?
Horizons have two main types: the first is the "geoidal celestial horizon", which is the horizon that an ordinary person can see by the naked eye, because it is completely visible no matter how far it is; it is, in turn, divided into several types. First, the nautical horizon is the geographic horizon at the sea surface and is the most common type of horizons; it can be clearly seen at sea shores when the sea intersects with the sky within sight. If you are standing at the sea shore on a flat location, the intersecting point between the sea and the horizon is called “offing”. Second, the local or geometric horizon is less common than the nautical horizon, as it is not observed by many people despite being visible; it includes buildings, trees, and highlands. Third, the geographic horizon is the visible line between the sky and the Earth; it does not include what is previously mentioned.
The second main type of horizon is the "celestial horizon"; it is divided in turn into two types. The first is the astronomical horizon, which is a hypothetical horizon tilted at a 90-degree angle; the second is the true horizon. Both types are used as reference to astronomers, to identify a specific location, whether on the surface of the Earth or on the surface of any other celestial body, or to identify the location of the Earth itself from the sky.
From this perspective, we realize that the horizon is of great importance; as mentioned before, the celestial horizons are used in positioning, but it is not just about that. Horizons have a significant importance on several fronts; such as in aerial and maritime navigation. Pilots sometimes turn to horizon to maintain the level of the plane during flight; through simple calculations to distinguish the correlation between the front of the plane or the front end, the pilot can control the plane's altitude more accurately.
It is known that ancient sailors resorted to the stars and celestial constellations to set the direction of their ships; however, most stars and constellations only appear at night, so they depend more on the horizon to set directions in daylight. With the tremendous development in the use of the Global Positioning System (GPS) in defining directions, the use of the horizon is currently limited in this field.
How far is the horizon?
It is not difficult to identify the distance between us and the horizon, because it is not an estimation as some people think; it must be borne in mind that the higher the location of detecting the horizon is from the sea, the farther the distance of the horizon is. For example, if a person stands at sea level, the horizon is about five kilometers away; whenever the person rises 1.5 meters above sea level, the distance between the person and the horizon increases by 4.5 kilometers, so, if a person stands on the summit of Everest, which rises above sea level 8,848 meters, then the horizon will be 370 kilometers away.
All the previously mentioned numbers are an approximation, except the height of the summit of Everest; this is due to the presence of many factors that may influence the accuracy of identifying the distance of the horizon, including the weather, the clouds, and light refraction. Experiments proved that cold weather can make the horizon look farther, and the clouds obscure observing the horizon clearly.
The horizon is not just a breathtaking natural scene we experience on seashores, and it is not a mere inspiration for artists, but a scientific term of many types and major astronomical significance. It has helped ancient sailors set their directions, and helps modern-day scientists in many fields.
References
britannica.com
livescience.com
nationalgeographic.org