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Reference system effects detected with meridian astronomy on the Clementine Gnomon (Rome, 1702)
1  ICRA/Sapienza and ITIS Galileo Ferraris, Rome


The Clementine Gnomon was built in 1700-1702 by Francesco Bianchini upon the will of the Pope Clement XI. The aim of such 45 meters' meridian line was to measure the secular variation of the obliquity of the ecliptic, epsilon. This is measurable as well the much smaller nutation effect, on the solar positions near the solstices.
I will analyze the observational campaing 2018-2021 with respect to the following keywords: ephemerides, aberration, nutation, proper motion, precession, refraction anomalies, apsides, solstices, equinoxes, tropical year, Sun rotation, equation of time, Earth rotation and DUT1, obliquity, eccentricity, air turbulence, time accuracy and metrological precision.
This meridian line is located in the Basilica of santa Maria degli Angeli e dei Martiri in Rome, and is visited by thousands of students each year.
Special attention is dedicated at the December's solstices during with the position of both limbs and its meridian transit can reach the maximum relative accuracy, allowing also the vision and the timing of the major sunspots.
The quadratic fitting of these solstice's data obtain a general resolution on the epsilon parameter of one arcsecond, unaffected by the field curvature for optical aberration because the objective is a 25 mm lensless pinhole.

The knowledge of the instants of "anni cardines" i.e. solstices and equinoxes allows to formulate, within a uniform circular motion approach, the eccentric theory of the Earth orbit, obtaining a precise determination of the current apsides dates (4-6 january and 4-6 July) and of the Earth's orbital eccentricity. This is the double of the keplerian eccentricity -simply because of ellipse's definition-.

All those considerations pave the way to modern astrometry, where the sub-arcsecond accuracy is required in order to detect clearly the relativistic effect.

Keywords: ephemerides; aberration; nutation; proper motion; precession; refraction anomalies; apsides; solstices; equinoxes; tropical year; Sun rotation; equation of time; Earth rotation; DUT1; obliquity; eccentricity; air turbulence; time accuracy; metrological pr