Astronomers base their measurement of positions of
objects on the concept of the celestial sphere upon
which all objects are assumed to lie regardless of
their true distances.
 The meridian is the circle running from one pole to the other through a point directly overhead for an observer. The point directly overhead is called the zenith |
 The celestial poles and equator are the projections of the Earth's poles and equator onto the sky. |
What stars you see passing through your zenith, depends on your
location on the Earth. Consider standing on
the top of the earth, the North Pole. At zenith, you will always
see Polaris!
|
On the equator, then Polaris will be your furthest
northern horizon point. Stars pass over you like you were inside
a rolling barrel.
At our latitude, 39o, the celestial sphere rolls around us like a tilted barrel. |