Stigler's law of eponymy
Examples include Hubble's law , which was derived by Georges Lemaître two years before Edwin Hubble , the Pythagorean theorem , which was known to Babylonian mathematicians before Pythagoras , and Halley's Comet , which was observed by astronomers since at least 240 BC ( although its official designation is due to the first ever mathematical prediction of such astronomical phenomenon in the sky , not to its discovery ) . 
<triplet> Hubble's law <subj> Georges Lemaître <obj> named after