An Etymological Dictionary of Astronomy and Astrophysics

The → emission line of → neutral hydrogen in → interstellar clouds. It lies in the → radio spectrum at → wavelength of about 21 cm, → frequency 1420 MHz. Its existence was predicted by Hendrik van de Hulst in 1944 and discovered by Harold Ewen and Edward Purcell in 1951.

See also: Twenty-one (21); → twenty + → one; → centimeter; → line.