Transcript: In the 1940s Grote Reber used amateur astronomy radio equipment in his backyard to discover the first cosmic sources of radio radiation. The first three sources he discovered were in the constellation of Sagittarius from the center of our own galaxy, and the constellation Cassiopeia from a supernova remnant, and in the constellation Cygnus which was a radio galaxy at a distance of seven hundred and fifty million lightyears. By the 1950s hundreds of radio sources were known due to a series of large radio telescopes being built in the United States and in England. But there were no accurate positions because these large telescopes used long wavelength radiation, and so their diffraction limits were a few arcminutes. The positional accuracy was no better than optical astronomy before the invention of the telescope. By the early 1960s astronomers used the clever technique of occultation by the Moon to pinpoint the location of several radio sources from the third Cambridge catalog. With these accurate positions it was possible to identify optical counterparts. In some cases the counterparts looked to be a distant galaxy, but in other cases it appeared to be a stellar source.