Transcript: The celestial sphere is an imaginary sphere surrounding the Earth onto which are projected the objects of the night sky.  There are several fixed points on the celestial sphere that are important.  The Zenith is the point directly over your head.  The Nadir is the point directly below your feet.  The line drawn across the sky that represents the highest elevation of the sun or any other object as it traverses the sky is called the Meridian.  The sun rises through to its highest point and then starts setting as it passes the Meridian.  The other important points are to define two angles anywhere on the celestial sphere that give uniquely the position of any star or object.  Altitude and Azimuth are two angles that can do this.  Altitude is the angle with reference to the horizon, and Azimuth is the angle along the horizon.  Astronomers use a different coordinate system based on Right Ascension and Declination.  The other important track on the sky is the Ecliptic, the path traveled by the sun. This is also the region of the sky within which you will see the motions of the moon and the planets.