Because of a phenomenon called stellar aberration. If you carefully measure the location of stars in the sky during the course of a year, you will see them move in tiny ellipses with a size of about 20 seconds of arc. This is a relativistic effect which can be related to your speed relative to these stars of (20/206265)x speed of light = 30 kilometers/second. The shapes of these ellipses changes with your distance above or below the plane of the ecliptic, and the only conclusion you can draw from this is that the Earth is orbiting the Sun in the plane of the ecliptic at a speed of 30 kilometers/sec. This is the only observational evidence I know about that absolutely shows that the Sun is at the center of the Solar System, and that the Earth orbits the Sun at roughly constant speed.