Description

Geographical position 

ETH-Sternwarte Zürich-Oberstrass, Schmelzbergstrasse 25, CH-8006 Zürich, Switzerland

Location 

Latitude 47°22’41’’ N, Longitude 08°33’02’’ E, Elevation 475m above mean sea level.

IAU observatory code 

---

Description of (scientific/cultural/natural) heritage 

 

The most important solar observatory in the 19th century was the ETH Observatory of the Eidgenössisches Polytechnikum (Confederate Polytechnical School, today "Swiss Federal Institute of Technology") in Zürich, Switzerland. The Observatory and the Polytechnikum were designed and built in 1861 to 1864 by the famous architect Gottfried Semper (1803--1879), professor of architecture and Head of the ETH architecture school Zürich.

Semper was appointed as first professor for architecture in 1854 in the just founded Eidgenössisches Polytechnikum (1855). Semper was admired already by his contemporaries as "Michelangelo of the 19th century" (Friedli et al. 1998). Semper is famous besides his buildings -- like the Semper Opera in Dresden und Frankfurt, buildings in London and Winterthur as well as monumental buildings like the Kaiserforum in Vienna -- for his theoretical and reformatoric work in architecture.

"For construction and equipment together 250,000 Francs were spent, of which around 70%
for the actual construction sum, 20 for the instruments and 10 for the official furniture, the gas and telegraph lines, etc." (Friedli 1998, p. 300)

The dome of the ETH Observatory was erected between 1861 and 1864 according to the ideas and specifications of Rudolf Wolf (1816--1893), mathematician and astronomer, and the well-known German professor of mechanical engineering Franz Reuleaux (1829--1905), constructed by the company Escher Wyss. The second floor was used by the meteorological central institute (until 1881).

The main instrument of the ETH observatory was a refractor in the large dome on the top of the building. In addition, meridian observations were made in the Southern meridian hall. Both instruments were delivered by Kern of Aarau, the optics by G. Merz & Sohn of Munich (1862/64).

Rudolf Wolf (1816--1893) and the sunspot cycle

Rudolf Wolf was very much impressed by the discovery of the sunspot cycle by Heinrich Schwabe (1789--1875). Then he started his own observations, and collected all the available data on sunspot activity back as far as 1610, carried out by many different astronomers using various instruments and observing techniques. By analyzing these sunspot observations, he calculated a period for the cycle of 11.1 years. In 1848, Rudolf Wolf defined the relative sunspot number in order to quantify the observations; this was accepted on an international level as Zurich Sunspot Number. Already in 1832, he found a relationship between the diurnal motions of the magnetic needle and the periodicity of the sunspot number. In 1852, Wolf was one out of four -- Johann von Lamont (1805--1879), Edward Sabine (1788--1883), Jean-Alfred Gautier (1793--1881) -- who discovered the link between the sunspot cycle and geomagnetic activity. In addition, Wolf made comprehensive aurora data available to his friend Hermann Fritz (1830--1893), who used them as basis for his "Verzeichnis beobachteter Polarlichter" (List of observed auroras), 1873. Thus, the correlation of auroral activity with solar and geomagnetic activity was derived (cf. Schröder 1984).

Wolf was a founding member of the Swiss Meteorological Commission in 1861, and was a popularizer of  meteorological knowledge. As President of the Geodetic Commission of Switzerland, he organized the surveys and published an important cartographical work about surveying in Switzerland "Geschichte der Vermessung in der Schweiz" (1879).

 

The ETH Observatory Zürich in the 20th century

Between 1910 and 1911, the so-called "Small Observatory" was built on the edge of the observation platform. Already since 1928 new buildings like the district heating plant (1933), the enlarged Chemical Institute (1937), and the University Hospital (since 1945) were added near the observatory and disturbed the observations.

In 1950/51, a solar tower was added, and between 1989 and 1993, the northern adjoining zone between the observatory and Kanton Pharmacy was redesigned.
The ETH Observatory in Zürich was in astronomical use until April 1, 1980.

From 1939 to 1980 the ETH Observatory also operated the astrophysical observatory on the Tschuggen in Arosa and from 1957 the still active Specola Solare in Locarno-Monti.  

 

History 

 

 

In the middle of the 19th century a significant upturn in solar research started with the discovery of the 11 years cycle and later with the beginning of astrophysics, especially with photography and spectroscopy of the Sun. In the 19th century the Sun was observed in several private observatories, for example in Carrington’s Observatory Redhill (1852, no longer existing). Samuel Heinrich Schwabe (1789-1875), pharmacist, botanist and astronomer, started in 1825 with solar observations using a Fraunhofer telescope. He built an observatory on the top of his pharmacy in Dessau in 1829, which is well renovated.

The construction of the famous Zürich ETH Observatory is closely related to Rudolf Wolf (1816--1893), who was appointed professor of astronomy in 1855 to the newly founded Swiss Federal Polytechnic Institute (ETH) in Zürich, and Gottfried Semper (1803--1879), professor of architecture and Head of the ETH architecture school Zürich. Parallel to the construction of the ETH (1859--1864), the planning for the construction of an observatory was carried out on Wolf’s endeavors and according to plans by Semper.

In 1854, Rudolf Wolf, coming from Bern, applied to the commission for the organization of the Polytechnical Institute for an astronomical observatory in Zürich for a loan of CHF 10,500
"in order to buy a Repsold pendulum clock available from Horner’s estate, and to order an astronomical theodolite from Ertel in Munich, a meridian circle similar to the one delivered for Bern Observatory, and a six inch Merz refractor, in addition to the inventory of the old Feer observatory, a couple of sextants, etc.; this seemed to be enough, at least for the beginning ..."  (Friedli 1998, p. 299)

Concerning the meridian hall, Friedli et al. (1998) wrote:
"Separated by a light partition panel, the meridian circle, made by Kern of Aarau, was mounted in the eastern section on plinths separated from the rest of the building, and the meridian circle, made by Ertel of Munich, was placed in the western section. Above each instrument, the roof and the southern and northern walls of the building could be opened a little so that all stars from the northern horizon through the zenith to the southern horizon were accessible for observation. Both instruments could only be moved around a horizontal, east-west oriented axis and therefore only observe stars in the meridian. A "sunbed" was available for each observer, with which he could slide under the instrument on rails and thus in a relaxed way register the meridian transits of the stars culminating near the zenith. Graduated vertical circles were attached to both ends of the horizontal axes, from which the position of the sighting telescopes could be read using measuring microscopes.
The handling of the two meridian circles required years of practice and iron discipline in the systematic recording of the measured variables as well as in the tortuous mathematical reduction of the numerous systematic and random errors in observation and measurement."

The electric clocks of the city of Zurich were controlled by a standard city clock (städtische Normaluhr) set up in Ertel’s Meridian Hall via a telegraph line between the city and the observatory.
The meridian observations were stopped in 1950.
 

 

Observing with the Meridian Circle (Meridian-Circle-Beob.mp4)

 

 

Instruments of the ETH Observatory Zürich

• Nocturnal / night dial, Leonhard Hartmann (ca. 1650)
• Graphometer or semicircle / Halbkreisgerät [Astrolab], Michael Butterfield of Paris (ca. 1700)
• Ivory Diptych Sundial (Klappsonnenuhr), Georg Karner of Nuremberg (1711)
• Solar microscope and pocket microscope, John Cuff, ca. 1743
   
• Borda circle, William Cary, ca. 1780
• Reflecting Sextant (Spiegelsextant), Pistor & Schiek of Berlin (1824--1836)
• Astronomical Theodolite, made by Ertel of Munich
• Arithmographe, Mechanical add/subtract calculator (Zahlenschieber/Griffeladdierer), invented by Louis-J. Troncet (*1850) of Paris (1889), cf. "Addiator", made by Addiator Gesellschaft, Berlin, 1920 to 1982.
   
• Regulator (precision pendulum clock) with temperature compensation, Association Ouvrière of Locle
• Pendulum clock with mercury compensation, made by Repsold of Hamburg, used with the Ertel meridian circle
• Regulator, Louis Mairet (1812-1875) of Locle, mean sidereal time, used with the Kern meridian circle
• Pendulum clock, made by Hipp of Neuchâtel
• Cylinder chronograph (registering pendulum clock) in order to register the transit times on a paper strip
   
• Meridian circle, made by Ertel of Munich
   
• 11.7-cm-Meridian circle (1864), made by Kern of Aarau, Tubus A, 52’’’ = 11.7cm, focal length 72 Par. inch = 1.9m, optics made by G. Merz & Sohn of Munich (1856), with Double-Ring-Micrometer and Screw-Micrometer (740 Gulden)
  (Kost 2014, p. 420, Kost 2015, p. 487)
   
• 16-cm-Refractor (Tubus B, 6 Paris inch, focal length 96 Par. inch = 2.6m), made by Kern of Aarau, optics by G. Merz & Sohn of Munich (1862), in 1982 restored by Kern of Aarau, since 2009 in Studien-Sammlung Kern in Stadtmuseum Aarau.
  Cf. Preis-Courant (1878), similar refractor, Nr. 245, p. 15, plate X.
  (Kost 2014, p. 420, Kost 2015, p. 487)
   
• ZEISS-Coudé-Refractor, made by Zeiss of Jena
  (since 1997 in the School and Public Observatory in Bülach)
   
• Cassegrain Reflecting Teleskope

Information about the historical units:
1 Pied de Paris (1’) Par. ft. = 0.32484 m,
1’  = 12’’ (pouce), 1’’ (Par. inch) = 2.7cm,
1’’ = 12’’’ (ligne), 1’’’ = 12’’’’ (point).

 

Directors

During the course of its history (1863 to 1980) the observatory was run by four directors: Rudolf Wolf (1864--1893), Alfred Wolfer (1894--1926), William Otto Brunner (1926--1945) and Max Waldmeier (1945--1979).

• Rudolf Wolf (1864--1893), 1864 to 1893
  
• Alfred Wolfer (1894--1926), 1894 to 1926 - Habilitation ETH 1883
  
• William Otto Brunner (1926--1945), 1926 to 1945
  
• Max Waldmeier (1945--1979), 1945 to 1979.
  

 

 

 

State of preservation 

The ETH Observatory in Zürich was used until 1980 for astronomy, and was declared as a building of national importance (7918), put under monument protection in 1981 (Liste der Kulturgüter in Zürich/Kreis 6), and restored from 1995 to 1997.
The most famous, well preserved solar observatory of the 19th century is the ETH Observatory in Zürich, built by Gottfried Semper, a remarkable astronomical heritage.

The historical Coronograph was sold (March 20, 2021) to the Astronomische Gesellschaft Graubünden (AGG) for astronomical use.
In May 2021, a restoration of the Zeiss-Kern-Coronograph of Arosa Observatory (1938) was started by the Arbeitsgruppe der Sammlung Kern (AGK) of Aarau in cooperation with the Astronomischen Arbeitsgemeinschaft Aalen e.V. (AAAA).

 

Comparison with related/similar sites 

The ETH Observatory in Zürich, built by Gottfried Semper (1803--1879) in 1864, belongs to the few observatories of 19th century, built by famous architects like Karl Friedrich Schinkel (1781--1841) (Berlin and Bonn observatories), Christian Heinrich Grosch (1801--1865), influenced by Christian Frederik Hansen (1756--1845) and Schinkel (Copenhagen Observatory, 1833) and Theophil von Hansen (1813-1891) (Athens Observatory, 1842).
The shape of the ETH Observatory in Zürich is unique, not comparable to the standard layout of the 19th century with three domes like Pulkovo Observatory, St. Petersburg (or a Greek cross like in the beginning of 19th century).

The Zürich 6 Paris inch refractor (16-cm, focal length 2.6m), made by Kern of Aarau, optics by G. Merz & Sohn of Munich (5000 Gulden/guilder), has an equatorial mounting with clock drive, a position micrometer (1859) and a Merz photometer was added (295 Gulden) (Kost 2015, p. 429-430).
A similar 16-cm-refractor was delivered to Neuchâtel Observatory in 1859.
 

Threats or potential threats 

no threats

Present use 

After the closure of the observatory in 1980, the building was used by the Department of Microtechnological Wood Research.
After an elaborate restoration (1995--1997), in 1997, the observatory building was taken over by the "Collegium Helveticum", an interdisciplinary research institute -- a joint Institute for Advanced Studies (IAS) of the ETH Zuerich, the University of Zuerich, and the Zuerich University of the Arts -- with the aim to provide a meeting place and forum for dialogue between the humanities, social sciences, physical sciences, engineering, medical science and the arts.

Astronomical relevance today 

The ETH Observatory in Zürich is since 1980 no longer used for solar physics or astronomy.

From 1939 to 1980 the ETH Observatory also operated the Astrophysical Observatory (46°46’52’’ N, 9°40’2’’ E) on the Tschuggen in Arosa (2050m); The corona observatoions started in Arosa-Prätschli (1900m) in 1938, since December 14, 1939, in Arosa-Tschuggen.
The coronagraph, made by Kern & Co AG of Aarau (1938), was used until 1965.
A Zeiss instrument was installed in a new dome in 1965, and used until 1980.
 

Instruments of the Astrophysical Observatory Arosa-Tschuggen of the Eidgenössische Sternwarte Zürich

• 12-cm-Coronagraph (focal length 30m) in NE dome (5.5m diameter), made by Kern & Co AG of Aarau (1938), 2.5m long, mounting by Carl Zeiss of Jena, objective, made by Andre Couder, Optical Laboratory of Paris Observatory, used until 1965,
  since 2021 Astronomische Gesellschaft Graubünden (AGG)
• Large Spectrograph, used with the coronagraph
   
• 30-m Horizontal telescope, 25-cm-Objective, long focal length 30m (1947)  
• 30-cm-Coelostat mirrors (quartz, aluminized) (1947) in the Coelostat house
• Photographic objectives of short focal length for photographing the Sun in projection
• Spectroscope with 3 Amici prisms, Carl Zeiss of Jena, collimator- and camera objective (1.5cm aperture, 15cm focal length, dispersion 29{AA}/mm at 5303{AA} and 68{AA}/mm at 6374{AA}) for observing the strong Corona lines at 5303{AA} green (first light 1939) and 6374{AA} red (first light 1940)
• Large Spectroheliograph
• Laboratory in the cellar, connected by a 30-m long corridor with the Coelostat house:
  Measuring Microscope, photoelectric self-registering Photometer
• Zeiss Coronagraph in a new dome (1965)

 

 

 

The corona observations in Arosa Observatory were supplemented by twenty solar eclipse expeditions from 1952 to 1980 and led to significant results: studying the shape of the corona and its magnetic field, its variation in the 11-year cycle and the analysis of the coronal structures -- published in numerous publications. The Arosa Observatory, astrophysical observatory of the ETH Zürich, was decommissioned in 2002.

From 1957, the ETH Observatory operated in addition the still active Specola Solare in Locarno-Monti.

References

Bibliography (books and published articles) 

• Altermatt, Peter: Astrophysikalisches Observatorium Arosa "Tschuggen". In: Orion - Zeitschrift der Schweizerischen Astronomischen Gesellschaft 39 (1981), Heft 187, p. 180-182. (https://doi.org/10.5169/seals-899386).
   
• Baertschi, Christian: Max Waldmeier. In: Historisches Lexikon der Schweiz (HLS) 18.12.2012. (https://hls-dhs-dss.ch/de/articles/031702/2012-12-18)
   
• Erfurth, H.: Samuel Heinrich Schwabe: Apotheker, Astronom, Botaniker. Dessau: Museum für Naturkunde und Vorgeschichte 1989.
   
• Folkers, Gerd & Martin Schmid: Under the Sign of the Sun. In: Folkers, Gerd & Martin Schmid (ed.): Ideas at ETH Zurich that shaped the world. Zürich: Chronos 2016, p. 103-114.
   
• Franz, Axel, Zeiss in Oberkochen, Astronomischen Arbeitsgesellschaft Aalen e.V., personal communication (2022)
   
• Friedli, T.K.; Fröhlich, M.; Muschg, A.; Rebsamen, Hp. & B. Schnitter: Sempers ehemalige Eidgenössische Sternwarte in Zürich. Schweizerische Kunstführer GSK. Bern: Schweizerische Gesellschaft für Kunstgeschichte 1998.
   
• Günther, Siegmund: Wolf, Rudolf. In: Allgemeine Deutsche Biographie (ADB), Band 43. Leipzig: Duncker & Humblot 1898, p. 785-788.
   
• Haupt, Hermann: Max Waldmeier. Nachruf. In: Almanach der ├ûsterreichischen Akademie der Wissenschaften, Bd. 151 (2001), S. 459-469.
   
• Keller, Hans-Ulrich: Rudolf Wolf und die ehem. Eidgenössische Sterwarte in Zürich. In: Orion - Zeitschrift der Schweizerischen Astronomischen Gesellschaft 51 (1993), Heft 254, p. 4-11 (https://doi.org/10.5169/seals-898165).
   
• Kost, Jürgen: Wissenschaftlicher Instrumentenbau der Firma Merz in München (1838--1932). Scientific Instrument Making of the Company Merz in Munich (1838--1932). Bearbeitet und herausgegeben von Gudrun Wolfschmidt. Hamburg: tredition (Nuncius Hamburgensis; Band 40) 2015.
   
• Lardelli, Aldo, Arbeitsgruppe Studiensammlung Kern im Stadtmuseum Aarau, personal communication (2022)
   
• Meadows, A.J.: Early Solar Physics. Oxford: Pergamon Press 1970.
   
• Neuenschwander, Erwin: Wolf, Rudolf. In: Historisches Lexikon der Schweiz (HLS). 2013. (https://hls-dhs-dss.ch/de/articles/028984/2015-01-11/ (11.01.2015).
   
• Schröder, Wilfried: Das Phänomen des Polarlichts.  Darmstadt: Wissenschaftliche Buchgesellschaft 1984.
   
• Schütz, Stephan: 200 Jahre Kern-Aarau - Von der Präzessionsmechanik und -optik zum High-Tech-Systemhaus. Aarau 2019.
   
• Waldmeier, Max aus Hellikon (Aargau): Neue Eigenschaften der Sonnenfleckenkurve. Eidgenössische Technische Hochschule in Zürich zur Erlangung der Würde eines Doktors der Naturwissenschaften genehmigte Promotionsarbeit. Zürich: Schulthess & Co. 1936. (https://doi.org/10.3929/ethz-a-000090931)
   
• Waldmeier, Max: Leitfaden der astronomischen Orts- und Zeitbestimmung. Aarau: Sauerländer 1946 (3. Auflage) 1968.
   
• Waldmeier, Max: Einführung in die Astrophysik. Basel: Birkhaeuser (Lehrbücher und Monographien aus dem Gebiete der exakten Wissenschaften 18 = Astronomisch-geophysikalische Reihe Band 3) 1948 (381 pages).
   
• Waldmeier, Max: Die Sonnenkorona: Beobachtungen der Korona von 1939--1949. Heidelberg, New York: Springer 1951 (books.google.ch). Basel: Birkhäuser 1957.
   
• Waldmeier, Max: Nachruf für William Brunner (1878--1958) und seine Publikationen. In: Verhandlungen der Schweizerischen Naturforschenden Gesellschaft = Actes de la Société Helvétique des Sciences Naturelles = Atti della Societ├á Elvetica di Scienze Naturali, Bd. 139 (1959), S. 379-392.
   
• Waldmeier, Max: Die Eidgenössische Sternwarte 1863--1980. Turicum 1981.
   
• Wolf, Rudolf: Neue Untersuchungen über die Periode der Sonnenflecken und ihre Bedeutung. [New investigations regarding the period of sunspots and its significance]. In: Mittheilungen der Naturforschenden Gesellschaft in Bern 255 (1852), p. 249-270, esp. p. 250-251.
   
• Wolf, Rudolf: Sonnenflecken-Beobachtungen in der ersten Hälfte des Jahres 1852. Entdeckung des Zusammenhanges zwischen den Declinationsvariationen der Magnetnadel und den Sonnenflecken. [Sunspot observations in the first half of the year 1852; discovery of a relation between the variations of the declination of the magnetic needle and sunspots]. In: Mittheilungen der Naturforschenden Gesellschaft in Bern 245 (1852), p. 179-184.
   
• Wolf, Rudolf: Astronomische Mittheilungen von Dr. Rudolf Wolf. In: Vierteljahrsschrift der Astronomischen Gesellschaft 1 (1866), p. 218-222.
   
• Wolf, Rudolf: Handbuch der Mathematik, Physik, Geodäsie und Astronomie. Zürich: Friedrich Schulthess 1870-1872.
   
• Wolf, Rudolf: Geschichte der Astronomie. Oldenbourg, München (Geschichte der Wissenschaften in Deutschland, Neuere Zeit, Band 16) 1877.
   
• Wolf, Rudolf: Geschichte der Vermessung der Schweiz als Historische Einleitung zu dem Arbeiten der schweiz. geodätischen Commission. Zürich: S. Höhr 1879.
   
• Wolf, Rudolf: Handbuch der Astronomie, ihrer Geschichte und Literatur. 2 Bände. Zürich: F. Schulthess 1890-1893. Reprint: Hildeheim: Olms 1973.
   
• Wolfer, Alfred: Todes-Anzeige [R. Wolf]. In: Astronomische Nachrichten, Bd. 134 (1893), Nr. 3203, p. 183.
  (1893AN....134..183W).
   
• Wolfschmidt, Gudrun: B1. Astronomical Heritage - Solar Observatories and Instruments, 17th to 19th Century. In: Astronomy and World Heritage: Across Time and Continents. Proceedings of the UNESCO Conference Kazan, August 2009. Ed. by Mikhail Ya. Marov. Associate Scientific Editors: Olga B. Dluzhnevskaya, Jarita C. Holbrook, Viktor K. Abalakin, Yuri A. Nefedyev, Tamila M. Potyomkina. Kazan: Publishing House of Kazan University 2016, p. 172-177.
   
• Zeiss: Astronomische Instrumente. Werkausgabe 1970.

 

Links to external sites 

• Semper Observatory, Collegium Helveticum
   
• Eidgenössische Sternwarte Zürich (German Wikipedia)
   
• Sammlung Sternwarte - Eidgenössische Sternwarte Zürich, Collections (ca. 300 items): Geodetic surveying instruments, time measuring devices, astronomical and optical instruments, mathematical models
   
• Videos of Waldmeier, 1946 to 1958, Prominences and solar eclipses
   
• Johann Rudolf Wolf, MacTutor
   
• Rudolf Wolf, erster Direktor der Eidgenössischen Sternwarte 1864-1893, Biographie der ETH Zürich
   
• Lutstorf, Heinz Theo: Professor Rudolf Wolf und seine Zeit 1816--1893, Schriftenreihe der ETH-Bibliothek, Nr. 31. Zürich: ETH-Bibliothek 1993 (57 pages).
   
• Alfred Wolfer (1894--1926)
   
• William Otto Brunner (1926--1945)
   
• Max Waldmeier (1945--1979)
   
• Quadri, Agnese: Freier Zugang zum Kulturerbe - Die Objekte der Sammlung der Eidgenössischen Sternwarte Zürich sind digitalisiert, B.I.T. Online
   
• Rudolf Wolf: Verzeichniss der Sammlungen der Zürcher-Sternwarte. Zürich: Zürcher & Furrer 1873.
  (Scientifically and culturally important collection: geodetic surveying instruments, astronomical, optical and time measuring devices, as well as mathematical models -- Rudolf Wolf: 120 objects (acquired before 1873) and approx. 130 objects archive documentation, plus 47 objects from the instruments of the Swiss Federal Observatory in Zurich, which was closed in 1984).
   
• Wolf, Rudolf: Verzeichniss der Sammlungen der Zürcher-Sternwarte. In: Handschriften und Autographen der ETH-Bibliothek, 121. Zürich: Wissenschaftshistorische Sammlungen der ETH-Bibliothek 1989.
   
• Wiederkehr, Stefan: Die Strategie 2015 bis 2020 für die Sammlungen und Archive der ETH Zürich. In: Rafael Ball / Stefan Wiederkehr (ed.): Vernetztes Wissen. Online. Die Bibliothek als Managementaufgabe. Festschrift für Wolfram Neubauer zum 65. Geburtstag, Berlin 2015, p. 181-188.
   
• Dr. Heinz Aeschlimann (Text) & Aldo Lardelli (Illustrations): Meridiankreis für die Sternwarte Zürich
   
• Studiensammlung Kern Aarau, Kern & Co. AG - 1819 bis 1991 in Aarau, Werke für Präzisionsmechanik, Optik und Elektronik - Studiensammlung ab 2009 im Stadtmuseum Aarau
   
• Stadtmuseum Aarau
   
• Stadtmuseum Aarau: Der Kreis der Kerns. Ein Rundgang durch die Sammlung Kern
   
• Exhibitions about Kern, Aarau, in Stadtmuseum Aarau
   
• Wolfscher Refraktor für die Sternwarte Zürich
   
• ETH, Eastern meridian hall, observation with the meridian circle
   
• Dr. Heinz Aeschlimann (text) & Aldo Lardelli (illustrations): Der Meridiankreis von Kern
   
• Meridiankreis-Infos, Stadt Museum Aarau
   
• Muhr, Robert: ETH Zuerich - Neuer Glanz fuer geschichtstraechtiges Bauwerk - ETH Chemistry Building
   
• Arosa Astrophysical Observatory,     Tschuggen, Graubünden
   
• Coronagraph, made by Kern & Co AG of Aarau (1938)
   
• Die Korona-Forschung der Eidgenössischen Sternwarte, ETH-Bibliothek
   
• Specola Solare Ticinese, Locarno
   
• Feusi, Alois (Arosa): Das versteckte Labor von Arosa (Ozon Observatory, 1921), Neue Züricher Zeitung (18.01.2016)