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The declination of the Sun along the year varies according to a sinusoid.Around the solstices this curve is approximated by a parabola. In kinematics aparabola is obtained with a constant acceleration. This acceleration has beenestimated in the days 21-29 December 2017, from the measurements taken at themeridian line in the Basilica of Santa Maria degli Angeli in Rome made byFrancesco Bianchini in 1702 with purpose of measuring the variation of theobliquity of the ecliptic. The parabola equation is fitted to the data toobtain the solstice's instant with an accuracy of one hour. The departure ofthe measures is within 4 arcsec (the daytime seeing during these solartransits) from the ephemerides of IMCCE. The pipeline of the algorithm used toobtain the angular data of the center of the Sun, starting from the groundmeasurements affected by the atmospheric refraction, and corrected by theCassini equation, is described. Bianchini in 1703 reduced the error on thesolstices timings by using the difference in right ascension between the Sunand a star observed at the same meridian line even in daytime (as he did withSirius in June-July 1703). The present one is an absolute measurement, withoutstellar references. The meridian diameter is averagely measured 24 arcsec lessthan the true value, with 20 arcsec of standard deviation, because of differentluminosity contrasts in the sky and in the Basilica. Conversely the center ofthe image is much better defined (the contrast acts symmetrically withoutmoving the center of the image), allowing an accuracy to the nearest arcsecondin the determination of the true obliquity.

Differential refraction, 2017 winter solstice timing and true ecliptic obliquity measured at the meridian line of Santa Maria degli Angeli in Rome