Let's put an observer on the Earth. The point Z
on the celestial sphere, which is directly above the head of the observer, is
called his zenith. The distance on the celestial sphere between the zenith Z
and the star S is the zenith distance z. The zenith distance is like
the distance d from the lighthouse of the previous example. We can draw a circle
centred on the star S with a radius z. We are somewhere on the projection
of this circle on the Earth. This is our new circle of position.
The position of the projection of the star on the Earth (latitude, longitude)
and the position of the star S itself on the celestial sphere are identical
(same angles, same reference plans: the celestial equator and the Greenwich
meridian).
