The Great Aurora of January 1770 observed in Spain

An analysis is made of the records made by Spanish observers of a notable aurora on 18 January 1770 in order to study the characteristics of this event. The records indicate that the phenomenon was observed in both continental and insular territories of Spain, in particular at San Cristóbal de la Laguna, Cádiz, Córdoba, Badajoz, Valencia, Castellón, Madrid, Barcelona, and Gerri de la Sal. The most equatorward observational site was San Cristóbal de la Laguna (28.48° N, 16.32° W) in the Canary Islands. In general, the descriptions put its duration from sunset to midnight, but the observers from Córdoba and Madrid report the aurora as being visible during the last hours of the night, and it was even observed the following day at Castellón. All the observers described the aurora as red in colour, while white and ash colours were also reported at Córdoba and Gerri de la Sal. The brightness and shape of auroral display changed over time. Calculations of the geomagnetic latitudes of the observation locations gave San Cristóbal de la Laguna as the southernmost (26° N) and Gerri de la Sal the northernmost (35° N) and indicate this aurora was observed over a wide range of abnormally low latitudes for such a phenomenon. Solar activity around the event was high, with the astronomer Horrebow registering 10 sunspot groups on that date (18 January 1770).