Measuring the position of the center of the Sun at the Clementine Gnomon of Santa Maria degli Angeli in Rome

TL;DR

This study analyzes historical and recent measurements of the Clementine Gnomon in Rome to determine the necessary corrections for restoring its original alignment, aiding precise solar position tracking over centuries.

Contribution

It provides a method to assess and correct the gnomon's alignment by comparing historical and recent measurements with ephemerides data.

Findings

Determined the North-South correction needed for the gnomon's pinhole.

Validated the gnomon's measurements against modern ephemerides.

Provided guidelines for restoring the gnomon's original accuracy.

Abstract

The Clementine Gnomon in the Basilica of Santa Maria degli Angeli in Rome has been realized in 1702 with the aim to measure the variation of the obliquity of the Earth axis along the forthcoming centuries. Since then the church and the instrument undergone several restorations and the original conditions of the pinhole changed. The measurements of the position of the image in the days before and of the 2011 winter solstice with respect to the original markers compared with the ephemerides gives us the North-South correction for the position of the pinhole to be restored.