What Are the Types of Horizons in Astronomy? Definitions & Examples
The horizon is a visible line that separates all viewing lines depending on whether or not it intersects the Earth's surface. The study of the horizon is called horizon astronomy. The horizon definition in astronomy is defined as, the particular line, which can only be observed when it lies on the sea surface.
When looking at a sea from a seaside, then the part of the sea nearest to the horizon is called the offing. This line is obscured by terrain, towers, trees, houses, and mountains in many places, and the resultant convergence of earth and sky is referred to as the apparent horizon in astronomy.
The true horizon definition astronomy near the observer is generally assumed to be a circle, drawn on the surface of a spherical model of the Earth.
The horizon centre is under the observer and below sea level. Its distance from the observer differs from day to day due to atmospheric refraction, which is seriously affected by weather conditions.
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Also, the horizon is far away from the observer, the higher the observer's eyes are from sea level. For example, in standard atmosphere conditions, for an observer with eye-level above the sea level by 2 metres, the horizon is at a distance of about 6 kilometres.
Types of Horizon
When it is observed from high standpoints, like as a space station, then the horizon is far away and it surrounds a much larger area of Earth's surface. In this case, the horizon would no longer be an exact circle, not even a plane curve like an ellipse. Especially when the observer is above the equator, as the Earth's surface can be much better modelled as an ellipsoid than as a sphere.
There are two main types of horizons
1) Earth-sky horizons
2) Celestial horizons.
There are subtypes of earth-sky horizons and celestial horizons. The Local horizons, sea-level horizons and geographic horizons are the sub-parts of Earth-sky horizons.
While, the astronomical and true horizons are celestial horizons.
Earth-Sky Horizons
The local horizon is the visible boundary between the Earth and the sky. The local horizon includes trees, buildings, and mountains. The geographic horizon is the visible boundary between the Earth and the sky. Mountains, trees, buildings, and other elevated features are not considered part of the geographic horizon.
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The spatial horizon is the same as the sea-level horizon, but only at sea level. A beach is one of the best ways to see the sea-level horizon where the Earth seems to touch the horizon, the ocean and sky have a clean, flat surface. If the observer is standing on the beach and looking out at the sea, the offing refers to the part of the sea that "touches" the horizon.
Celestial Horizons
Celestial horizons are used by astronomers, for the measurements of the position of the Earth relative to the rest of the sky. The celestial horizon is a flat passing from the earth’s centre perpendicular to the zenith-nadir axis. The visual horizon approaches this plane at the earth’s surface.
The astronomy horizon is the type of celestial horizon whose imaginary horizontal plane is always at a 90-degree angle from the observer's zenith. Celestial horizons are perfect circles that surround the observer. The true horizon is the theoretical plane that passes through the centre of the Earth, its radius is perpendicular. From orbit, the true horizon is spherical, following the shape of the Earth. The altitude of a celestial body is calculated using the celestial horizon as a reference point. Celestial horizons are called rational horizon to compare sensible horizon.
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Important Points
The theory of the horizon is important to different types of work, including aviation, navigation, and art.
The horizon is used by pilots to keep the plane level when in the air. This method is called "attitude flying," Pilots can handle their aircraft by determining the relationship between the aircraft's front end or the aircraft’s nose, and the horizon. By shifting the horizon to be mostly sky or mostly earth, pilots may adjust their altitude or flight pattern.
Before the explanation of the latest tools such as Global Positioning System (GPS) devices, sailors depended on a perfect view of the horizons to navigate the ocean.
The sun's position to the horizon told sailors what time of day it was and what direction they were boat or crew.
You will see stars appearing on the eastern horizon astronomy (just as the Sun and Moon), spreading around the dome of the sky during the night, and settling on the western horizon if you lay down in an open field and watch the night sky for hours, as ancient shepherds and travellers did.
In astronomy, the horizon is the point where the sky seems to touch the earth or the sea. (In astronomy it is called the intersection on the celestial horizon sphere of a plane perpendicular to a straight line). The higher the observer, the lower and more distant is his visible horizon in astronomy.
Event Horizon Astronomy
In astronomy, event horizon definition is a partition beyond which events cannot affect an observer. The term was discovered by Wolfgang Rindler.
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The event horizon definition astronomy, is the circumstance defining the region of space around a black hole from which nothing can escape. And an object within the event horizon has a faster escape velocity than light. The name arises since it is impossible to observe any event taking place inside it is the horizon beyond which we cannot see. As a result, the event horizon essentially covers the singularity at the black hole's core, which is a relief for astrophysicists concerned about the collapse in physics at such a point.
All singularities are obscured by an event horizon, according to the cosmic mind regulation hypothesis, and this violation of the laws of physics is not visible. In theory, mass can be compressed sufficiently to form a black hole. The only stipulation is that it must be less than the radius of the event horizon. For example, our Sun would become a black hole if its mass was contained within a sphere about 3 km across.
FAQs on Horizon in Astronomy Explained: Definitions, Types & More
1. What does the term 'horizon' mean in astronomy?
In astronomy, the horizon is the flat plane that passes through the eyes of an observer. It is the fundamental plane of the horizontal coordinate system. This plane divides the sky into two halves: the hemisphere you can see and the hemisphere that is hidden from view by the Earth itself.
2. Are there different types of horizons used in physics and astronomy?
Yes, there are a few key types of horizons that are important to distinguish:
- Visible Horizon: This is the line you actually see where the sky appears to meet the land or sea. It is often irregular due to trees, mountains, or buildings.
- True or Geometrical Horizon: This is the theoretical line that would exist if the Earth were a perfect, smooth sphere with no air. It is a perfect circle.
- Celestial Horizon: This is an imaginary great circle on the celestial sphere. Its plane is perpendicular to the observer's local gravity, and it is used as the basis for locating stars and planets in the sky.
3. What are the zenith and nadir points in relation to the horizon?
The zenith is the imaginary point in the sky directly above an observer. The nadir is the point directly below the observer, on the opposite side of the celestial sphere. The celestial horizon is located exactly 90 degrees away from both the zenith and the nadir.
4. Why is understanding the horizon so important for astronomers?
The horizon is crucial because it forms the foundation of the horizontal coordinate system, which astronomers use to pinpoint the location of objects in the sky at any given moment. An object's position is described by its altitude (its angle above the horizon) and its azimuth (its direction along the horizon, like N, S, E, W). This system helps in tracking celestial bodies and predicting their rise and set times.
5. How is the astronomical or true horizon different from the horizon we see every day?
The main difference is that the astronomical horizon is a perfect, theoretical plane used for calculations, assuming a perfectly round Earth. The visible horizon we see daily is almost always imperfect. It's affected by the landscape (hills, buildings), and the Earth's atmosphere bends light, a phenomenon called atmospheric refraction, which can make objects near the horizon appear slightly higher than they actually are.
6. Is there a simple way to calculate the distance to the horizon?
Yes, you can estimate the distance to the true horizon using a simple formula based on your height. The distance (d) in kilometres is approximately 3.57 times the square root of your height (h) in metres. So, if you are standing at the beach with your eyes 2 metres above the water, the horizon would be about 5 kilometres away.
7. If an astronaut stood on the Moon or Mars, would the horizon look the same as on Earth?
The concept of a horizon would be the same, but its distance would change. The distance to the horizon depends on the radius of the planet or moon you are standing on. Since Mars is smaller than Earth, the horizon would appear noticeably closer. On the much smaller Moon, the horizon would seem very close, and the curvature of the surface would be much more obvious.
