| In May 1951, Oort wrote to van de Hulst: "I've just received a telephone call from Muller at Kootwijk saying that he has tonight observed the 22 cm line [sic] in the vicinity of Sagittarius". Thus the true beginning of Dutch radio astronomy. The first hints of detection in Kootwijk were obtained by SRZM engineer Lex Muller on May 11, 1951, today exactly 60 years ago. In the following days, Muller obtained definitive confirmation of the detection and informed Oort who, in turn, informs van de Hulst, the person who had predicted in 1944, instigated by Oort, that the 21-cm line of neutral hydrogen (for some odd reason in the early days referred to as the 22-cm line) could be detectable. It must have been an incredibly exciting period for all involved. The line had been detected for the first time ever by Ewen and Purcell at Harvard six weeks earlier on March 25, 1951. But with the detection in Kootwijk the initiative was back in the Netherlands. This is reflected in the papers both teams published in Nature (both in the same issue, together with a note by Christiansen and Hindman who had later also had detected the line in Australia). The Ewen & Purcell paper is mainly technical, describing the detection, while the Muller & Oort paper already derives some parameters of Galactic structure.
The story of the 21-cm line is a perfect example of how simple scientific shrewdness leads to a major breakthrough in science. During the war, Oort, inspired by the results of Reber, realised that if there would be a detectable spectral line in the radio, it would give an unobscured view (unobscured by interstellar dust but also by the Dutch weather!!!) of the structure and kinematics of the Galaxy. This turned out to be a brilliant idea. Without the 21-cm studies done over the years, our understanding of the structure and kinematics of galaxies would be nowhere near where it is now. And with SKA (originally called the 'hydrogen telescope') and its pathfinders, 21-cm work will continue to play this prominent role in astronomy for many decades to come.
The figure shows a spectrum of one of the first detections of the 21-cm line in Kootwijk by Lex Muller. The 'positive-negative' shape of the profile is due to the frequency switching technique used in detecting the line. One of the main factors which had hindered earlier detection was the limited stability of the receiver. Ewen and Purcell had found that frequency switching (a kind of differential measurement between two nearby frequencies) was the solution to this problem. When Muller heard about this, he quickly implemented it in Kootwijk and soon he detected the line.
Today a ceremony will be held at the location of the former Kootwijk antenna, and a panel commemorating the detection will be inaugurated.
More information on the story of the 21-cm line can be found in chapter 16 of the wonderful book "Cosmic Noise" by Woody Sullivan (see http://www.astro.washington.edu/users/woody/hra.html )