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.

In 1702 with the inauguration in Rome of the great Clementine Gnomon in St. Maria degli Angeli, a 45 m pinhole-meridian line, built upon the will of pope Clemens XI, Francesco Bianchini started the Roman tradition of solar astrometry. Bianchini was inspired by the Heliometer of Giandomenico Cassini, a 67 m pinhole-meridian line realized in Bologna and in operation since 1655. In 1736 Eustachio Manfredi collected the measures of the solar meridian diameters made at the Heliometer since 1655, they encompassed the Maunder minimum of the Sun (1645-1715) and the first 20 years after. The result of a recent reanalysis of these data of Bologna, is that the average solar angular diameter at astronomical unit distance did not change between the Maunder minimum and the 20 years after, but this analysis did not include realistic errorbars, since Manfredi provided only the reduced data. Dedicated observations realized at the Clementine Gnomon in Rome between 2018 and 2025, in various meteorological conditions, contribute to this debate on the solar diameter evolution during and after the Maunder minimum. We compare the meridian diameters measured by Bianchini (raw and reduced) in the winter solstices of 1701-1702 with the ones measured by Sigismondi in 2018-2025 at the same Gnomon and in winter solstices. We provide the observational errorbars, not smaller than ±4” per single measure and systematic diminutions of the observed diameters of -10 mm with respect to the ephemerides, due to the light contrast of the solar image projected onto a white marble. After reducing the data of Rome Bianchini in 1701-1702 measured an angular diameter reduced at 1 AU of θ_Sun=1926”±12”, while us in 2018-2025, with similar observing conditions, obtained θ_Sun=1920”±18”. We can model the errorbars for the measures along the whole year, also for the Heliometer of Bologna, and we find that these data do not meaningfully constrain our current understanding of the Sun’s evolution during and after the Maunder minimum.

The transits of Venus occur in couples every 105/122 years: the observed ones were in 1639; 1761–1769; 1874–1882; and 2004–2012. The next couple will occur in the years 2117 and 2125. We need all four contacts to determine the solar diameter accurately. The black-drop phenomenon blurs internal contacts, so we developed a parabolic analysis of the chords drawn by the disk of Venus on the solar limb. The extrapolation of the zeroes gives the contact timings. We tested this method with some high-quality images obtained in 2004 and 2012, and we applied it to the observations of 2012 in a visual band (Huairou Solar Observing Station, hazy weather) and H-alpha (Shen Zen Astronomical Observatory). To exclude a reduction in the measured diameter by the haze, we made two series of measures at the Clementine Gnomon (Rome) and at the PHYSIS telescope (Rome), under various sky transparencies and with diffraction-limited instruments. The haze and the low altitudes above the horizon reduced accuracy at all first contacts examined, without changing the solar diameter. Our measures obtained in China during the transit of 2012 yielded a photospheric radius R_⊙P = 959.33″ ± 0.06″, based on 76 + 75 diffraction-limited images; this is compatible with the chromospheric radius measured at the base of the spiculae, which is R_⊙C = 959.78″ ± 0.11″, relying on 7 + 5 diffraction-limited series of images.