Solar Astrometry in Rome at the End of the Maunder Minimum

With the great Clementine Gnomon in St. Maria degli Angeli, a 45 m pinhole meridian line, built in 1700–1702 upon the will of Pope Clemens XI, Francesco Bianchini inaugurated the Roman tradition of solar astrometry. We analyze two thousand dedicated observations at the Clementine Gnomon between 2018 and 2025, with solar altitudes from 20° to 71° and in various meteorological conditions, in order to assess the observational uncertainties on the solar diameter and their causes. We compare the meridian diameters measured by Bianchini near the winter solstices of 1701–1702 with the ones measured by Sigismondi in 2018–2025, reporting the observational errorbars per single measure and the systematic diminutions of the observed diameters with respect to the ephemerides, due to the turbulence and image contrast loss. Simulated datasets based on our measured uncertainties show that pinhole meridian lines cannot resolve solar diameter variations smaller than 1″ over 80 years. These limitations prevent tighter constraints on solar evolution across centuries using such instruments.