Solar Astrometry in Rome at the End of the Maunder Minimum

The meridian astrometry in the XVIII century was performed both in Rome on the Clementine Gnomon (1702, Francesco Bianchini) and in Bologna at the Heliometer in S. Petronio Basilica (1655, Giovan Domenico Cassini). These two pinhole-meridian lines are cornerstones in the his-tory of the solar astrometry. The “solar theory” is the celestial mechanics of the Earth’s orbit combined with the multiperiodical motions of the Earth’s axis; it must include the atmospheric refraction, from 20° to 67° of altitude, the lowest and the highest meridian altitude of the Sun in Bologna, which Cassini published in 1656. Dedicated observations realized at the Clementine gnomon between 2018 and 2025, in various meteorological conditions, contribute to the debate on the solar diameter during and after the Maunder minimum, recently based on the analysis of data collected by Eustachio Manfredi (1736) on the Cassini's meridian line since 1655. The stellar meridian transits, once observed in S. Maria degli Angeli, were ancillary data to the solar transits. Two decades before the discovery of stellar aberration in 1727, Francesco Bianchini measured its effect on the Polaris. A comparison between the meridian diameters measured by Bianchini in the winter solstices of 1701-2 and the ones of the present observational campaign 2018-25 is made to detect differences in the solar diameter from the end of Maunder minimum to the present maximum of solar cycle XXV. The first repetition of the stellar transits at the meridian line is presented.