In 1932 he learnt of K. G. Jansky’s detection of radio emission from beyond the Solar System, and by 1938 had built a paraboloidal dish 9.4 m in diameter, movable in declination like a transit instrument. From then until after World War II he was the world’s sole radio astronomer. He mapped the sky at radio wavelengths, and detected many sources (‘radio stars’), including Cassiopeia A and Cygnus A, which did not correspond to visible stars. He also found that the Sun and the Andromeda Galaxy emit radiation at radio wavelengths.