什么是 Horizontal coordinates？

The altitude $h$ of a star is its angle above the horizon, from $-90{}^{\circ}$ to $90{}^{\circ}$; its zenith distance is $z = 90{}^{\circ} - h$. Its azimuth $\mathrm{A}$ is the angle, along the horizon, from the north point to the foot of the star’s vertical circle, counted from $0{}^{\circ}$ to $360{}^{\circ}$ through the east.