Earth and the Celestial Sphere
As you know, Earth is actually an oblate spheroid, just as with chart
projections, we consider Earth a perfect sphere for celestial observations.
Simply stated, the celestial sphere is an imaginary sphere that stretches out
to the ends of the universe. At the very center of the celestial sphere is
Earth. As you know, Earth rotates on its axis from west to east. Because
of this rotation, celestial bodies appear to be in motion, rising in the east
then crossing over the observers meridian and going on to set in the west.
Figure 6-1. Celestial sphere revolving about Earth.
The celestial poles are located by extending Earths north and south poles
(fig. 6-1). The celestial equator (also known as the equinoctial) is
formed by extending Earths equator out onto the celestial sphere.
Meridians are formed by hour circles that are similar to great circles. The
celestial meridian is formed by extending the Greenwich meridian out
onto the celestial sphere.