19 November

1918: On 19 November 1918, Hendrik Christoffel van de Hulst was born.

Van de Hulst was a Dutch astronomer who predicted theoretically that 21-centimetre radio waves were produced by interstellar hydrogen atoms.

His calculations later proved valuable in mapping the Milky Way and were the basis for radio astronomy during its early development. Shortly after the Second World War, several groups set about to test the prediction. The 21-centimetre line of atomic hydrogen was detected in 1951, first at Harvard University then followed within a few weeks by others.

The discovery demonstrated that astronomical research, which at that time was limited to conventional light, could be complemented with observations at radio wavelengths, revealing a range of new physical processes.