In 1729 he published his discovery of the aberration of light (also called astronomical aberration). This was he first observational proof of the Copernican Hypothesis that the Sun is at the centre of the Solar System and not the Earth. He noticed a motion in the stars that did not fit the accepted pattern of the time. What he discovered was stellar aberration. In 1748 he published his discovery. That same year he was awarded the Copley Medal from the Royal Society for his discovery.