词条 方位角
描述: 在水平坐标系中，方位角指的是物体所在的方向（沿地平线的角度）。方位角以度为单位，从正北向正东方向测量。方位角的数值范围覆盖完整的圆周，从0度到360度。换句话说，如果在天球上画一条从天体到地平线并垂直于地平线的假想弧线，方位角会告诉你这条弧线与地平线相交的位置。一个位于正北的天体，其方位角为0度；位于正东的天体，其方位角为90度，依此类推。在一些国家使用的较早的教科书中，方位角的测量方式是从正南开始，向西测量。因此，在这些教科书中的方位角数值会比现代标准的方位角值偏移180度。
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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
