We dealt with the most intense magnetic storm of the 20th century, which took place on 13–15 May 1921. It was also observed at Stará Ďala (present-day Hurbanovo). However, the record was not complete. We estimated the missing sensitivity scale values and presented the resulting digitized Stará Ďala’s data for 13–15 May 1921. The data were compared with the records from other observatories. The analysis suggests that the auroral oval got close to Stará Ďala in the morning hours on 15 May 1921.

Historical observations are important, even for modern science (such as space research). The Clementinum in Prague was one of the first observatories where systematic measurements of the geomagnetic field commenced. We focus on the beginning of these Prague measurements, which were introduced by Karl Kreil in 1839. We describe archived data from that time, some details about the main instrument for observing magnetic storms and the first magnetic mapping in Bohemia.