Reading Errors in Sundials with Incorrect Hour Lines: The Seventeenth-Century Sundial in Lerma

Abstract

The restoration project of a seventeenth century sundial at Lerma (Spain) with incorrect engraved marks for the hour lines and devoid of its gnomon raised the issue of whether it is possible to predict what might be the reading errors when providing a new gnomon for it. Moreover, the analysis of possible slight changes in gnomon’s orientation and/or geometrical features to get better precisions was an appealing challenge. We must consider that Lerma’s case is not unique: templates were used in the past to engrave marks in numerous dials, and small differences in orientation were not taken into account. Differences in latitude under 0.5° were also neglected, by rounding off to integer values. Although Lerma’s sundial has been our guiding thread and its results are presented in detail, the method can be extrapolated to calculate reading errors for any dials with wrong marks for orientation or latitude different form the actual ones.