However, the scientific community strongly opposed to the theory. We know today that the Sun is not the center of the Universe. It is just one of the many stars in our Galaxy, and the latter in just one of the galaxies populating the Universe. Kepler enunciated three laws that rule the motion of the planets around the Sun. Kepler's three laws were deduced from the observations without any theoretical basis.
All planets move around the Sun on elliptical orbits.
The Sun occupies one of the two foci, the same focus for all ellipses. The ellipse is a plane figure, obtained by cutting a cone with a plane not orthogonal to its axis. The sum of the distances from two points, called foci, to any of of its points is constant. Since planets describe elliptical orbits, the Sun-planet distance varies as a function of time, and it has a maximum and a minimum values. A circumference can be imagined as a special ellipse, whose eccentricity is zero.