The Horizon Coordinate System

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  • 0:01 Finding Objects on…
  • 0:37 The Horizon System
  • 1:52 Altitude & Azimuth
  • 3:58 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will describe a celestial coordinate system known as the horizon system or altazimuth system, as well as important terms such as altitude, azimuth, zenith, and angular distance.

Finding Objects on Earth and The Sky

If some poor chap - let's call him little Timmy - is floating on a life raft in the middle of the ocean, he better hope he has an emergency beacon on board that can send out the coordinates of his location so he can get rescued. This is because if you ever want to find precisely where someone is on Earth, you would need to get their coordinates in latitude and longitude. The lines of longitude appear vertically on a map, and the lines of latitude are the ones that are horizontal on the map.

But if you want to find stuff on the sky, you can use another system, called the horizon system. This lesson will teach you this system's definition and major components, as well as its major shortcoming.

The Horizon System

The horizon system is a type of coordinate system that can be used to locate the position of objects in the sky. So, if little Timmy is on a life raft in the middle of the ocean awaiting rescue as his emergency beacon sends out his Earthly coordinates, he can pass his time using the horizon system to find objects in the sky.

The horizon system is based on several important factors. One of these is called the celestial horizon, a circular boundary between the earth and sky, located on the celestial sphere, which divides the celestial sphere into an upper visible half and a lower half that is not visible.

Luckily, little Timmy happens to be in the middle of the vast ocean, one of two scenarios where the celestial horizon can be clearly seen. The other scenario would be some sort of vast, flat plane on land.

It's important to know that the celestial horizon is measured with respect to the zenith. Meaning, any point on the celestial horizon is 90 degrees from the zenith. The zenith is a point in the sky that's located directly above the observer. If little Timmy looks up, straight up, from his life raft as he prays and looks for a plane that may or may not ever come to his rescue, he'll be looking up at the zenith.

Altitude & Azimuth

The horizon system doesn't use latitude and longitude as coordinates. Instead, it uses altitude and azimuth. Altitude is the angular distance above the celestial horizon. The angular distance is the angle that is formed by two imaginary lines starting at an observer's eye and ending at two objects. All this is saying is that the altitude is the height of a celestial object on the sky, measured as an angle from the horizon.

Altitude is like the latitude coordinate little Timmy's rescuers are receiving from his emergency beacon. To remember altitude and latitude are sort of the same, just look at how if you switch the 'a' and the 'l' in altitude, you spell out latitude. On our celestial sphere, the horizon will have an altitude of 0 while the zenith will have one of 90 degrees.

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