The first topics encountered during the study of astronomy are the appearance and the motions of the Moon, the planets, and the stars on the Celestial Sphere.  Through careful observation of this set of objects that are visible to the unaided eye, you can construct a three dimensional model for the Earth's place in the Solar System and the Solar System's relationship to the distant stars.  The force of gravity is what binds the planets to the Sun and moons to planets, and it predicts properties like the orbital period, separation, and velocity of any two objects bound to each other.