A theoretical exploration of a sundial of the franciscan friary in Dubrovnik (Croatia)

Abstract

The Franciscan friary in Dubrovnik (Croatia) has two old wall sundials mounted in the small and large monastery cloisters. Both indicate equinoctial hours according to the apparent solar time. It is known for one who made it and the year it was made in - Pasko Baletin in 1770, and for the other, there are no data. The second sundial has two scales; on the upper scale, the direction of the shadow indicates hours counted from midnight (Ore francese), whereas on the lower scale, the hours counted from the previous sunset are indicated by the tip of the shadow i.e. its "sunlet" (Ore italiche) as it has been believed so far. In order to check this, we calculated both scales, made drafts and compared them with the existing sundial scales: hour lines on the upper scale coincide, while on the lower scale all calculated hour lines deviate from the existing lines towards the west. However, when the calculations for the lower scale are done in accordance to the hours counted from the end of the civil twilight (Ore italiche da campanile) there is complete compatibility. This was confirmed by on-site observation of the sundial. It remains to search through Dubrovnik archives to discover when the two-scale sundial was made. The last-mentioned variant could have been brought to Dubrovnik in the second half of the 18th century, and since then there was a need to construct a two-scale sundial.