Observer Coordinate Systems

观察者坐标系统

From an observer's position on the Earth, one can define a coordinate system based on the horizon circle and the compass directions.

从地球上的观察者位置出发，可以基于地平线圆和罗盘方向定义一个坐标系。

From a horizon plane coordinate system anywhere on the Earth's, we can construct an ordinary observer's reference frame with a zenith overhead and compass directions in the horizontal plane. From the surface observer's point of view, the zenith and Polaris (defining the celestial north pole) are convenient reference points.

从地球上的任何水平坐标系出发，我们可以构建一个普通观察者的参考系，其中天顶位于上方，罗盘方向位于水平平面内。从地表观察者的角度看，天顶和北极星（定义天球北极）都是方便的参考点。

The plane through the north celestial pole and the zenith is called the celestial meridian or commonly just the meridian, understood to be this specific meridian. When an object crosses this meridian, it is called its meridian transit and it will be highest in the sky at that point.

通过北天极和天顶的平面称为天赤道子午圈，或简称为子午线，指的就是这条特定的子午线。当一个天体经过这条子午线时，称为它的过天顶，此时该天体在天空中位置最高。

For pointing a telescope, it is convenient to use the Azimuth angle Az from North toward East and the Altitude angle Alt from the horizon plane. Also used is the zenith angle z, which is just 90° - Alt.

为对准望远镜，使用从正北向正东方向的方位角Az和从水平面垂直向上的高度角Alt会比较方便。此外还使用了天顶角z，它仅仅是90° - Alt。

The problem in finding a celestial object is that the Az and Alt are different for different observers and change continually as the object progresses from east to west across the sky. The location of a celestial object with respect to the celestial sphere is expressed in terms of its right ascension α and declination δ. Calculating the Az and Alt from α and δ requires the hour angle H and the latitude of the observer.

寻找天体的问题在于，方位角Az和高度角Alt因不同观测者而异，并且随着天体从东向西穿过天空而不断变化。天体相对于天球的位置用其赤经α和赤纬δ来表示。由α和δ计算Az和Alt需要知道时角H和观测者的纬度。