The celestial sphere: its main points, circles and lines
The celestial sphere is an imaginary sphere of arbitrary radius centred on the observer; bodies are projected on it and only their directions matter. The plumb line meets it at the zenith Z above and the nadir Z′ below; the great circle 90° from the zenith is the mathematical horizon. The extension of the Earth’s axis meets the sphere at the north (P) and south (P′) celestial poles, and the circle 90° from the poles is the celestial equator. The celestial meridian passes through the poles and the zenith; it meets the horizon at the north (N) and south (S) points, while the celestial equator meets the horizon at the east (E) and west (W) points. A body’s altitude h is measured from the horizon, and its zenith distance is z = 90° – h.
With an adult, build a model of the celestial sphere from a ball and paper rings: mark the equator, meridian and horizon rings and glue on the zenith and poles. Only an adult should use scissors.