Vertkov states that the first mentions of the Gusli date back to 591 AD to a treatise by the Greek historian Theophylact Simocatta which describes the instrument being used by Slavs from the area of the later Kievan Rus' kingdom.
Copernicus, born in 1473 and already well over 60 years old, had never published any astronomical work, as his only publication had been his translation of poems of Theophylact Simocatta, printed in 1509 by Johann Haller.
Theophylact Simocatta gives a similar account with additional information.