The Sun is not at the center of the ellipse, but is instead at one focus (generally there is nothing at the other focus of the ellipse). The planet then follows the ellipse in its orbit, which means that the Earth-Sun distance is constantly changing as the planet goes around its orbit. For purpose of illustration we have shown the orbit as rather eccentric; remember that the actual orbits are much less eccentric than this.
Kepler's First Law is about the eccentricity of orbit.
Large eccentricity.
Medium eccentricity.
Circle = 0 (zero) eccentricity.
