什么是 The celestial sphere？

The celestial sphere is an imaginary sphere centred on the observer, on which every star is placed in its direction; only directions matter, not distances. The extensions of the Earth’s axis meet it at the north and south celestial poles; the plane of the Earth’s equator cuts it along the celestial equator. The zenith is the point straight overhead, along the local vertical; the horizon is the great circle $90{}^{\circ}$ from it. The local meridian is the great circle through the zenith and the celestial poles; it meets the horizon at the north and south points.