Ann. Geophys., 39, 439–454, 2021 https://doi.org/10.5194/angeo-39-439-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. The geomagnetic data of the Clementinum observatory in Prague since 1839 Pavel Hejda1, Fridrich Valach2, and Miloš Revallo3 1Institute of Geophysics, Academy of Sciences of the Czech Republic, Bocníˇ II/1401, 141 00 Prague, Czech Republic 2Geomagnetic Observatory, Earth Science Institute, Slovak Academy of Sciences, Komárnanskᡠ108, 947 01 Hurbanovo, Slovakia 3Geophysical Division, Earth Science Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 840 05 Bratislava, Slovakia Correspondence: Pavel Hejda ([email protected]) Received: 29 January 2021 – Discussion started: 19 February 2021 Revised: 8 April 2021 – Accepted: 9 April 2021 – Published: 17 May 2021 Abstract. The historical magnetic observatory Clementinum 1 Introduction operated in Prague from 1839 to 1926. The data from the yearbooks that recorded the observations at Clementinum have recently been digitized and were subsequently con- Reliable long time series of geomagnetic records are needed verted, in this work, into the physical units of the Interna- for studying the long-term behaviour of the Earth’s mag- tional System of Units (SI). Introducing a database of ge- netic field, for instance its secular variation (Cafarella et omagnetic data from this historical source is a part of our al., 1992). At present, high-resolution direct geomagnetic paper. Some controversial data are also analysed here. In the measurements provide extremely useful information about original historical sources, we identified an error in using the changes in the geomagnetic field. Magnetic and auroral physical units. It was probably introduced by the observers records are available from a large number of stations sit- determining the temperature coefficient of the bifilar appara- uated at auroral and lower latitudes and are supplemented tus. By recalculating the values in the records, some missing with satellite data and solar observations. However, the time values are added; for instance, the temperature coefficients span of these high-resolution data is limited to the past few for the bifilar magnetometer, the baselines, and the annual av- decades. On the other hand, palaeomagnetism provides low- erages for the horizontal intensity in the first years of obser- resolution data on the field over timescales of thousands vations were redetermined. The values of absolute measure- of years to hundreds of millennia. To reconstruct the sec- ments of the declination in 1852, which could not be found ular variation, it is desirable to have high-quality measure- in the original sources, were also estimated. The main contri- ments of the geomagnetic field covering as long a time span bution of this article rests in critically reviewed information as possible. Historical geomagnetic records can be useful about the magnetic observations in Prague, which is, so far, for dealing with this task; however, they can be heteroge- more complete than any other. The work also contributes to neous, incomplete, or consist of corrupted data. Their pro- the space weather topic by revealing a record of the now al- cessing should involve several steps like digitization, conver- most forgotten magnetic disturbance of 3 September 1839. sion from the originally adopted scale units (i.e. divisions of the instrument scale) to proper physical units, dealing with inaccuracies or missing data, searching for accompanying in- formation like auroral records, etc. To eliminate possible in- strumental and observational errors, an approach based on the comparison of old geomagnetic records with archaeo- magnetic and volcanic data was proposed in Arneitz et al. (2017a). Also, the knowledge on development of the his- torical magnetic instrumentation and observatory methods is Published by Copernicus Publications on behalf of the European Geosciences Union. 440 P. Hejda et al.: Geomagnetic data of Clementinum necessary for proper understanding and interpretation of old magnetometers. The Göttingen observatory became the pro- geomagnetic data (Schröder and Wiederkehr, 2000; Reay et totype for many other observatories worldwide. al., 2011; Matzka et al., 2010). Improvement of observatory practice was not a goal of This work presents the first comprehensive review of the Gauss’s work but just a tool for understanding the nature of historical magnetic data recorded at the Clementinum obser- the Earth’s magnetic field. Gauss and Weber therefore joined vatory in Prague. There are other, previous studies of this the activity of Alexander von Humboldt in establishing a kind that refer to the past registrations at European geomag- worldwide network of observatories, known as the Göttin- netic observatories (Malin and Bullard, 1981; Alexandrescu gen Magnetic Union (GMU), that made simultaneous mea- et al., 1996; Korte et al., 2009; Pro et al., 2018) which aim surements at specific intervals (called term days). The co- to study the secular variation in, and search for, interesting ordinated measurements started with nine European obser- geomagnetic disturbances. To build complete data sets of ge- vatories (six of them in Germany) in 1836, and the number omagnetic data, compiling archive records from several ge- increased to 31 observatories in 1841. omagnetic observatories is necessary. In previous works by, The Prague observatory joined the GMU in 1839. The ob- e.g., Jackson et al.(2000); Jonkers et al.(2003); Arneitz et servatory had its seat in the Clementinum college situated al.(2017b), historical geomagnetic data were collected, pro- in the Old Town close to Charles Bridge. The college was cessed, and analysed with the aim of preparing data sets ac- established by Jesuits in 1566. Since 1622, Jesuits have ad- cessible to the broader geoscience community. Historical ge- ministered Charles University and transferred the University omagnetic records have received growing attention in con- Library to Clementinum. At the beginning of the 18th cen- nection with the space climate studies (Lockwood et al., tury, the astronomical tower was build there, and in 1752, 2017, 2018) and can also provide a basis for the study of the astronomical observatory was established. The first di- past changes in solar quiet magnetic variations (Cnossen and rector, Joseph Stepling, started meteorological measurements Matzka, 2016). Combining historical geomagnetic data with as well. An uninterrupted series of high-quality tempera- auroral records can be beneficial for reconstructing the past ture measurements dates back to 1 January 1775 and is well solar–terrestrial relations (Ptitsyna et al., 2018). In particu- known to climatologists all around the world. Thanks to Karl lar, compiled historical declination data can serve to detect Kreil, the importance of magnetic observations does not lag geomagnetic jerks (Korte et al., 2009; Dobrica et al., 2018). behind the meteorological ones. Historical geomagnetic observations are thus understood Kreil came to the Prague observatory from Milan in 1838. to be an important source of information about the structure In Milan, he was visited by Gauss’s assistants and became and time behaviour of the geomagnetic field. The magnetic enthused by this new research discipline. After he was in- needle had already been commonly used in naval naviga- formed about his transfer to Prague, he took care of the ac- tion since the 13th century; however, at first it was supposed quisition of magnetic instrumentation. The equipment of the that the needle was oriented towards true north. The fact that observatory corresponded to the prototypes used in Göttin- the orientation of the magnetic needle depended on the posi- gen. The variation observations were installed in a large cor- tion was documented in the 16th century by explorers who ridor of the astronomical observatory. The building was not sailed the Atlantic and Indian oceans. On the other hand, completely free of iron; however, all iron objects were re- the first sustained series of measurements at a single site in moved from the vicinity of the instruments, and test obser- Greenwich showed that the geomagnetic field was subject to vations did not indicate any magnetic contamination. The time-dependent change. The magnetic needle moved about instruments were arranged in such a way that they did not 7◦ west over the period 1580–1634. The first magnetic incli- interfere and that one observer was able to perform the eye nometer (a magnetized needle rotating on a horizontal axis in observations of declination, horizontal intensity, and inclina- the vertical plane of the magnetic meridian) was constructed tion in time intervals of 5 min during the term days or even by London compass maker, Robert Norman, in about 1580. more frequent observations of declination and horizontal in- Relative magnetic intensity data began to be compiled tensity during the periods of disturbed magnetic field. Com- from 1790s by comparing the time it took a magnetic nee- pared with modern observatory variometers, the instruments dle displaced from its preferred orientation to return to it or were quite massive. The weight of the declination needle (a by the duration of a given number of such oscillations. magnetic rod in the form of a parallelepiped) was 1682 g, and The method of absolute determination of magnetic inten- the weight of rod in the bifilar magnetometer was 2780 g. The sity was developed by Carl Friedrich Gauss, in cooperation magnetic rods in the