In memoriam Hugo van Woerden (1926 – 2020)

By Felix Bettonvil

Abstract: On September 4, 2020, Prof. Dr. Hugo van Woerden, after a short illness, passed away at the age of 94. Hugo was both amateur and famous professional astronomer, and one of the founders of the Meteor Section of the Royal Dutch Association for Astronomy and Meteorology (KNVWS Werkgroep Meteoren), in 1946, and inventor of the use of star fields to determine the observer’s limiting magnitude.

Hugo van Woerden was professor in radio astronomy at the Kapteyn Institute in Groningen and among the first practicing radio astronomy in the Netherlands, and carried out important research with the Dwingeloo and Westerbork radio telescopes.

 

1 True amateur astronomer

Hugo van Woerden was born in 1926 and grew up in Arnhem. As many of us, already at a young age he got interested in astronomy, awakened by his father, being a chemistry teacher. Only 8 years old, his father taught him about the constellations and the planets their wandering on the sky, scintillation. He liked these evening walks very much.

 

His grandmother took him to the planetarium where he was fascinated by the show. With the Dutch astronomical almanac Sterrengids, written by the director Dr. J. J. Raimond Jr of the Zeiss planetarium in The Hague (owning one of the two very first Zeiss projectors), he observed everything he could. Already then, he was a motivated observer and analyst. He preferred the naked eye over an instrument, because it felt being closer to the stars. Two phenomena had his special interest: the zodiacal light and Mercury, both being difficult to observe from the Netherlands, but keeping his attention his entire life (I do remember at least two occasions being outside with him and where he looked at the evening sky and then pointing at the barely visible planet).

He started to write letters to Raimond about his observations, including brightness, color and timings, and also visited him in 1942. Raimond was in contact with a dozen active amateurs, with among them Sidney van den Bergh, also very young, who was looking for companions to set up a network of meteor observers. Together with Lammert Huizing they started a small society, the ‘Astro Club’. Hugo became Observing Director and treasurer, the other two chair and secretary. Communication was mainly by postal letters these days; they wrote to each other each week. It was WWII, but circumstances were ideal with the Netherlands blacked out, thus no artificial light allowed, being great times for amateur astronomy and one of the few activities considered harmless by the occupants.

In 1943 Hugo got his gymnasium diploma. Due to the war, continued study at a university was only possible when signing loyalty to the German occupation forces, which for Hugo was out of the question. Raimond had introduced him already earlier to Leiden Observatory, and it was Hugo’s physics teacher who brought him in contact, at the age of 17, to professor Oort (who discovered later the after him named Oort cloud, source of many of our comets). Oort invited him to become a volunteer assistant at Leiden Observatory. He could freely use the 6” refractor, library and other services. There, he followed some (illegal) lectures by Oort and was also present at the historic seminar (April 1944) where astronomer Henk van de Hulst predicted the observability of the 21 cm line of interstellar hydrogen.

The Astro Club grew steadily: in 1943–44 the club had 35 members, consisting of school friends, family members and some members of a local astronomy division in Arnhem as well as some amateurs elsewhere in the Netherlands.

Hugo liked the hunting for meteors: appealing was that they came by surprise, and in the meantime, he could study the constellations. He was himself one of the most active observers in these days.

As Observing Director Hugo set up meteor observing campaigns, in which observers were given observing instructions by mail, varying per location. The observing strategy was to plot meteors and to record time, duration, brightness, light curve and color. These first ones were not considered always a success, e.g. viewing angles were parallel instead of co-pointing to the same atmospheric spot.

 

2 ‘De Meteoor’

Many campaigns followed, with as example the ones in March and April 1944 being very successful. Some 200 Lyrids were observed and a few tens of Astro-club members observed later that year reported over 2000 Perseids. The instructions were always written by Hugo in the Astro Club’s own periodical, ‘De Meteoor’. De first issue appeared in November 1943 and was distributed by post. It was reproduced in small quantities with stencil machines at the University observatories at Leiden and Utrecht, operated by Hugo. The contribution was 1 Dutch Guilder (~40 Euro cent). The Meteor had soon also English content, with the Astro Club becoming in contact with observers in Belgium, France, Spain and Czechoslovakia.

From September 1944 coordination of observations started to hamper, with the war in its final phase. Hugo moved back from Leiden to his native city Arnhem. Communication was difficult with letters and unreliable, no radio, thus no time-signals to calibrate the clocks. In essence, until the liberation in mid-1945, meteor work came to a halt.

As soon as WWII had ended the Astro club resumed its activities, with 5 campaigns focused on Quadrantids, Lyrids, Aquariids, Perseids and Draconids. Results were always published in ‘De Meteoor’ but analysis nonetheless often lacked behind due to lack of experience, mentorship and leadership, as Hugo commented much later himself, but not strange given the age of the founding members.

Hugo kept creating instructions for the observers, and put already in the beginning emphasis on the importance of accuracy and quality and the need for calibration. He was critical when results remained missing and ambitious. He reported also that the analysis of the results turns out to take much time, and in practice started to conflict with Hugo’s study.

 

3 ‘Werkgroep Meteoren’

Raimond in meantime was elected as chair of the Netherlands Association for Meteorology and Astronomy, NVWS, and thanks to the high level of amateur activity, decided to start – next to the local divisions in each major town- specialized sections, called ‘werkgroepen’. He proposed to form a ‘Werkgroep Meteoren’ based on the Astro Club. Hugo, Sidney and Lammert considered this was a good idea, bringing likely a new momentum, although they reported already in these days the frustrating bureaucratic process of a transition. In August 1946, the Astro Club became part of the NVWS and became named ‘Werkgroep Meteoren van de NVWS’. Hugo was again appointed Observing Director and treasurer. ‘De Meteoor’ became its periodical. Hugo may be considered as the architect of the new section; he wrote an observing manual which was accepted as the program of the werkgroep.

First observing activity of the new Werkgroep Meteoren was the return of the Draconids on October 9–10, 1946. A training session was organized at the planetarium in The Hague. Unfortunately, it was Full Moon, and the weather did not cooperate. Some reports were received but mainly from non-trained witnesses. But astronomers of the Kapteyn Institute in Groningen reported rates up to 60 per minute.

At that time Hugo was also accepted as student astronomy at Leiden University. The following years 1947 and 1948 the momentum in de werkgroep decreased. Sidney moved to Princeton and Hugo had to devote all his time to get his BsC degree, which created pressure and was to be followed by obligatory military service. ‘De Meteoor’ did not appear for two years.

Kees de Jager (Sonnenborgh Observatory, Utrecht) became president in 1948 so that Hugo could focus on his studies, and Kees could give the werkgroep the additional energy that was needed. Kees together with Hubenet observed already the Perseids the years before (under pseudonyms though, and Hugo discovered that only later). From 1949 ‘De Meteoor’ started to appear again.

End of the forties also the first photographic surveys started to appear, with help of sensitive Schmidt cameras. Also, the Werkgroep Meteoren made plans to build their own (1949), and some prototypes were actually made.

In 1950 Hugo returned to the werkgroep and would never leave anymore (and finally in 2002 he was elected as honorary member). He continued in his role as Observing Director, organized campaigns and observing instructions, made observations and analysis, but also at a somewhat larger distance as others started to take over. On April 7, 1953 a bright fireball appeared during photo-electric observations on variable stars. From the indirect flash he was able to derive the brightness of the fireball.

 

4 Determination of limiting magnitude

Already from the very beginning of the Astro Club Hugo pointed at the importance of accuracy and calibration, and he kept doing so, as well as stressing the importance of the link between observation and theory. In 1949 Hugo instructed, triggered by Whipple’s interest in meteor brightness, the observers to use specific stars as reference for their brightness estimations, and this led to –for us as meteor observers- maybe his most important achievement: he tested in 1956 (Sweden), and introduced in 1957 the use of star fields to determine the observer’s limiting magnitude (Roggemans, 2010). Today, these are still the standard and worldwide used.

The diversity of topics broadened, and apart from visual observing, also meteorites, photography (first Dutch meteor photograph in 1953), comets, fireballs, physics of streams, statistics were discussed. Analysis of observations went into deeper detail, and focused often on orbit determination. Remarkably, annual analysis of shower activity, not saying ZHR calculations, were largely missing.

End of the fifties, the scope of ‘De Meteoor’ broadened: it was not only used to report on meteor work anymore, and also other NVWS sections started making use of the magazine to report on their activities. Hugo got less time and soon after he finally gave up his function as Observing Director, he resigned in 1961 as editor. At that time photography was widely used, though difficult, spectroscopy, radar observation and space research started. Members of the werkgroep became routinely involved in satellite observation. There was close collaboration with Belgian observers.

In these years, we heard also for the first time of the uprising of the powerful Super Schmidt cameras (Harvard), so sensitive that they reached almost the sensitivity of the human eye, from then grew the belief that the role of the visual observer would lose importance soon, although visual reporting remained an ‘official’ research goal.

In the later years, Hugo still every now and then contributed, but it was clear that his focus changed.

 

5 Radio astronomy

The reason was evident, Hugo had started his PhD in 1955, on the structure of the interstellar clouds in the Orion region. He became an expert user of the then brand-new 25 m Dwingeloo radio telescope which was just completed (1956) and provided him with observations in the 21-cm line, one of the first major studies in the new exciting field. Soon, in 1957, an opportunity arose that would change his career. Adriaan Blaauw, the director of the Kapteyn Laboratory in Groningen was looking for excellent people who could support him in expanding radio astronomy in Groningen and offered Hugo a position as scientific research assistant in Groningen, which could help him to support his PhD work. He participated in the creation of the first radio map of the Milky way ever.

Hugo played an important role in the development of radio astronomy in the Netherlands, which started after WWII, which culminated with the realization of the Westerbork Radio Synthesis Telescope (WSRT) in 1966.

Hugo started to focus on radio astronomy, but was and remained interested in optical astronomy as well. After obtaining his PhD, he left for two years to Mount Wilson and Palomar Observatories in Pasadena (now the Observatories of the Carnegie Institution of Washington), supplementing the radio data with optical data.

In 1965 he was back in Groningen and appointed associate professor. After Adriaan Blaauw left Groningen, much of the managing work at the Kapteyn Laboratory was left to him, and later he became director. The institute flourished as was the case with all universities in the late 1960s and 1970s when they underwent an enormous expansion. New staff positions became available almost every year, and Hugo made excellent use of these positions, attracting many international guests and staff. Hugo laid the Groningen foundation for radio astronomy and extragalactic research and he was a key person behind the huge success of the WRST in the 1970s and 1980s.

With the WRST he worked on neutral hydrogen in Spiral Galaxies and later produced important work on neutral hydrogen in lenticular galaxies and in galaxies in the Virgo cluster in the 1980s and 1990s and on mapping and understanding and finding the distances to high-velocity hydrogen clouds.

Hugo became full professor in 1980 and was chair of the Astronomy Department from 1985 until his retirement. In that function he played significant roles in many national and international committees and boards.

 

6 Netherlands Association for Meteorology and Astronomy

In 1991 Hugo retired. As expected, he remained very active. He kept visiting his office, in Groningen, first on a daily basis, later once per week. He was one of the main organizers of the XXII^nd General Assembly of the International Astronomical Union held in 1994 in The Hague.

Soon after his retirement, he was asked to join the board of the NVWS as board member, but from 1992 until 2002 as chair. Back to popularizing astronomy and a leader of the amateur society which was what he started 50 years earlier.

It turned out to be a very good choice. Under his guidance the NVWS celebrated its 100^th anniversary, which was attended by the Dutch queen Beatrix. NVWS changed its name to Royal Netherlands Association for Meteorology and Astronomy, KNVWS.

This period is the time when most of us have their memories of Hugo. He was interested in everything and everyone. He maintained close contact with all sections, including his beloved Werkgroep Meteoren. He participated in almost all annual meetings, and if he did not contribute himself, he came with a reaction after almost every talk, stimulating and participating in debate and discussion. He valued your work, showed enthusiasm, gratitude and courage, hinted to next steps. If you made a mistake in your presentation, he would let you know, though, but always in a kind way. For many of us he was like a father or coach. He was a strong supporter of amateur work to assist in our scientific understanding of the universe. This put him apart from many others.

It was more than obvious that Hugo had an enormous drive, he was very precise, and had a fantastic memory. He was decorated as ‘Ridder in de Orde van de Nederlandse Leeuw’ in 1992, and asteroid 10429 van Woerden was named after him. In 2016 a symposium was organized to honor his 90^th birthday.

 

7 International Meteor Conference

Hugo gave acte de présence on two International Meteor Conferences, in 1996 and in 2006, both organized in The Netherlands. At both conferences he gave oral presentations, the first on his experience with meteors in three different periods of his life: as amateur meteor hunter, professional astronomer and as president of the KNVWS, for which he used three different hats. At the International Meteor Conference of 2006, he put the observation of meteors in broad perspective, from comets to planetary systems and exoplanets. Indeed, also today, many of us look at meteor science that way, now 17 years later. Again, he stressed the importance of calibration once, identical to what he did in his juvenile years.

During his period as KNVWS chair, Hugo handed out three times the Van der Bilt award to meteor observers (Ten Haaf, Koning, Van Leverink), a prestigious national award for extraordinary achievements in their field. In 2011, the KNVWS council also installed a similar award for youngsters, the Hugo van Woerden award.

Dutch astronomers may be proud to have had Hugo van Woerden among them, being a passionate scientist and true ambassador for astronomy, both for amateurs and professionals. He has inspired many of us.

 

Huug will always have a special place in the hearts of many, and he will be missed.

I want to express my gratitude to Urijan Poerink, Kees de Jager, Herman ten Haaf and Niek de Kort, who helped in giving additional insight in Hugo’s life and career, providing answers to questions, and/or helped in studying and checking reference material. More information can be found on the websites: KNVWS Werkgroep Meteoren, KNVWS and Kapteyn Institute.

 

 

References

De Meteoor, periodic of the Astro-Club (1943–1946) and NVWS Werkgroep Meteoren (from 1946), in Dutch.

Hemel and Dampkring, periodical of the Netherlands Association for Astronomy and Meteorology (NVWS), in Dutch (1943–1973).

De Meteoor, special issue Werkgroep Meteoren NVWS 50 Years (1996).

Terugblik, Kees de Jager, Stip Media, Alkmaar (2014). In Dutch.

Knöfel A., Roggemans P., editors, Proceedings of the International Meteor Conferences, Apeldoorn – the Netherlands, 19 – 22 September 1996, published by IMO.

Bettonvil F., Kac J., editors, Proceedings of the International Meteor Conferences, Roden – the Netherlands, 14 – 17 September 2006, published by IMO.

Roggemans P. (2010). “Origin of limiting magnitude counting triangles and squares”. WGN, Journal of the International Meteor Organization, 38, 115–117.