•If you “paint” the stars on a sphere much bigger than
the earth, then you can obtain
the motion of the stars by pretending that sphere rotates, rather than the earth.For most people that motion is easier to “see”.
•The sphere rotates once every day (actually once every
23h56mfor reasons we’ll see later)
•We will see later that the sun, moon, and planets move
slowly along the sphere
relative to the stars.You can think of
them (for a night or so) as “stars”
which move almost with the other fixed – but drift relative to them from night to night.
•The celestial sphere will also have marked on it a
projection of the earth’s
latitude and longitude system, as well as a few other special points and circles.