An Etymological Dictionary of Astronomy and Astrophysics
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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.

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