Is it possible to carry out eco-sustainable rehabilitations on specially protected buildings? This is the main question and starting point for this research. We will use the tower of the Church of “Santiago Apóstol” in Montilla as a case study; with its most remote antecedents in the 15th century, it is an emblematic building of one of the most important cities in the Cordovan countryside and is listed as an Asset of Cultural Interest (Bien de Interés Cultural or BIC) as of 2001. The application of eco-efficiency criteria in the rehabilitation of this type of building might stimulate the reactivation of the construction sector in the rural area, positively impacting the promotion of a circular economy. To this end, a general methodology has been established for carrying out eco-sustainable renovations on this type of building, which defines indicators for evaluating the eco-sustainability of such interventions. This methodology is applied to the case study of this important building in Montilla to ensure that a feasible intervention has been proposed, aligned with three basic pillars of sustainability that considers its environmental, economic, and social impact.

The calculation of the angle between the day-night line on the Earth’s surface, a terminator, and the meridian at a given point, at dawn or at sunset, can easily be performed either by means of flat geometry or, for those familiarized with astronomy, through formulae for the Sun’s azimuth and elevation, both derived from spherical trigonometry. The two procedures are compared. An auxiliary angle obtained in the process of derivation via planar geometry becomes useful to represent the daylight map on a terrestrial planisphere.

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.