You must know that the stars are huge balls of extremely hot gas which are very, very far away from us. Anyone who has seriously studied the solar system also knows that the planets, including our planet Earth, orbit the Sun which itself is just one of countless stars in the universe.However, that is not the way it looks to us. The sky above us seems to be a hemisphere with us at the center. Every observer on Earth has their own personal sky, slightly different for every location on the planet. It did not take long for human beings to imagine that there was the other half of the sphere under our feet. We became convinced of the concept of the celestial sphere.In order to understand how objects in the sky appear to move, it is worth studying the concept of the celestial sphere. The impression is so strong that a planetarium dome with a quality projector can make us believe we are under the actual night sky. We speak of the idea of a celestial sphere in order to model what we see in the sky. There several points defined on that sphere,Here is my video tour of the horizon coordinate system.