词条 高度角
描述: 高度有两种含义：一是指在某种坐标系中的特定角度，如“恒星的高度角”，二是指相对于给定参考水平的垂直距离，如“海拔高度5000米”。
在天文学（以及测量学）中，高度角是所谓地平坐标系中的一个角度。这个角度衡量的是天体距离地平线有多高——如果你用手指指向一个天体，然后将手指直下移动到地平线处，那么你的手臂的方向就会发生变化，变化的角度就是高度角。高度角可以用度或弧度来测量。位于地平线的天体，其高度角为0°；而位于头顶正上方（即所谓“天顶”）的天体，其高度角为90°。负的高度角表示当前位于地平线以下的天体——此处，高度角测量的是天体在地平线以下的距离。位于你脚底正下方（即所谓“天底”）的天体，其高度角为-90°。
在航空或大气物理学等其他领域，海拔高度是指一个位置比某个给定的参考水平面高出多少。在地球上，海拔高度通常被定义为平均海平面以上的高度。根据这一含义，海拔高度是以长度单位（例如如米）来衡量的。
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相关图表
Horizontal Coordinate System
图注： 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.
来源： Maria Cristina Fortuna/IAU OAE
