celestial pole, zenith, meridian
Did you notice that the tail of the Little Dipper (the star Polaris) remained fixed while the other stars just rotated around it? If not, go back and check.
What fraction of a rotation did the sky undergo in the 9 hours between 9 p.m. and 6 a.m.? go back and check!
The points of rotation are called celestial poles. There is one at the north end of the celestial sphere (near the star Polaris) and one at the south end (not near anything in particular).
The below picture shows where the north celestial pole is located in our sky. Here at CSB/SJU the north celestial pole sits about 45° directly above the north point on the horizon. The line that starts at the north point, goes through the north celestial pole through the point directly overhead and back to the south point on the horizon is called the meridian. We'll also need a name for "the point directly overhead"; it's called the zenith. |
