Imagine the Earth in space, and surrounded by a celestial sphere on which all the
heavenly bodies are moving. This is a quite simple representation of the universe,
but this is enough for our purpose.
The celestial sphere is centred on Earth with the celestial equator passing
through the Earth equator and the axis "Earth centre C to North Pole" defining
the axis of reference of the celestial sphere. A plan of reference defined on
Earth is also used on the celestial sphere: the Greenwich meridian.
On the celestial
sphere, we show a star S and its hour circle.
The star in the sky is like the lighthouse in the earlier example.