The astrometric recognition of the solar Clementine gnomon (1702)

TL;DR

This study uses astrometric methods to accurately determine the original position of the pinhole in the 1702 Clementine gnomon, revealing historical modifications and confirming its original design for precise solar measurements.

Contribution

The paper introduces an astrometric approach to recover the original pinhole position and assess historical modifications of the Clementine gnomon, enhancing understanding of its historical accuracy.

Findings

Original pinhole was 4.5 mm north of its current position.

The actual pinhole shape is oval, not circular.

Some reference star positions are incorrectly documented after restoration.

Abstract

The Clementine gnomon has been built in 1702 to measure the Earth's obliquity variation. For this reason the pinhole was located in the walls of Diocletian's times (305 a. D.) in order to remain stable along the centuries, but its original form and position have been modified. We used an astrometric method to recover the original position of the pinhole: reshaping the pinhole to a circle of 1.5 cm of diameter, the positions of the Northern and Southern limbs have been compared with the ephemerides. A sistematic shift of 4.5 mm Southward of the whole solar image shows that the original pinhole was 4.5 mm North of the actual position, as the images in the Bianchini's book (1703) suggest. The oval shape of the actual pinhole is also wrong. Using a circle the larger solar spots are clearly visible. Some reference stars of the catalogue of Philippe de la Hire (1702), used originally for measuring the ecliptic latitude of the Sun, are written next to the meridian line, but after the last restauration (2000), four of them are wrongly located. Finally the deviation from the true North, of the meridian line's azimuth confirms the value recovered in 1750. This, with the local deviations of a true line, will remain as systematic error, like for all these historical instruments.