Which of the following statements about the celestial sphere is not true? a) the "celestial sphere" is just another name for our universe. b) from any location on earth, we can see only half the celestial sphere at any one time. c) when we look in the sky, the stars all appear to be located on the celestial sphere. d) the earth is placed at the center of the celestial sphere. e) the celestial sphere does not exist physically.