Glossary term: 天頂
Description: 天頂定義為觀測者正上方的一點，因此這個點隨著觀測者位置的變化而變化。例如，對位於倫敦、北京與開普敦的觀測者來說，他們的天頂是各不相同的。若用角度來度量，則天頂與觀測者的地平線成90度。天頂角定義為天體與天頂之間的角距離。位於天頂的天體的天頂角為0度，而位於地平線上的天體的天頂角為90度。與天頂相對的一點，即觀測者的正下方，稱為觀測者的天底。如果地球是一個完美的球體，那麼觀測者的天頂和天底之間的連線就會穿過地心，但實際上地球只是近似於球體，因此地心與這條連線通常有一定的距離。
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Term and definition status: The original definition of this term in English have been approved by a research astronomer and a teacher The translation of this term and its definition is still awaiting approval
This is an automated transliteration of the simplified Chinese translation of this term
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Related Diagrams
Horizontal Coordinate System
Caption: This image shows the horizontal coordinate system of an observer on Earth. On the right we see the coordinate system in the local context of the observer. The observer appears here as a point surrounded by the celestial sphere. The ground appears as a plane, while the Earth is round, we can approximate the ground in the area around the observer as a plane. The line where this plane intersects with the celestial sphere is the horizon. The point directly above the observer is the zenith, the point directly below the observer is the nadir which is hidden by the ground.
Two coordinates define this coordinate system, altitude and azimuth, hence the reason this sometimes called an alt-az coordinate system. The altitude is zero at the horizon, maximum (90°) at the zenith and minimum (-90°) at the nadir. Azimuth is the angle around the horizon. Most commonly this is defined to be zero in the direction of north. Any point on the celestial sphere can be defined by these two coordinates. However what objects appear on these position will depend on the time and the location of the observer.
On the left-hand part of the diagram, we can see that when we put the observer and their local horizontal coordinate system in a global context, that the parts of the sky the observer can see depend on their position on the Earth and on the rotation of the Earth. The zenith points perpendicular to the Earth's surface so the position on the sky the zenith and nadir correspond to is dependent on the observer's latitude. The horizon also depends on the observer's position on the Earth.
Credit: Maria Cristina Fortuna/IAU OAE
License: CC-BY-4.0 Creative Commons 姓名標示 4.0 國際 (CC BY 4.0) icons
