Nouvelle theorie des taches du Soleil, by E. Pezenas (1692-1776). A fully annoted edition of a manuscript

TL;DR

This paper presents a fully annotated edition of Esprit Pezenas's 18th-century manuscript on sunspot observation methods, revealing a geometric approach to deducing the Sun's equator inclination and rotation period.

Contribution

It provides the first detailed analysis and digital annotation of Pezenas's unpublished manuscript, highlighting his geometric methods for solar observation and rotation estimation.

Findings

Pezenas's method allows inclination of the Sun's equator to be deduced from three observations.

The manuscript estimates the Sun's rotation period as approximately 26 days and 9 hours.

The work corrects and comments on previous observational data by Le Monnier.

Abstract

The manuscript New Theory of Sunspots was identified and attributed to the Jesuit astronomer and hydrographer from Marseille Esprit Pezenas (1692-1776) during the year 2002, during the systematic search for papers concerning him in the various French funds. The manuscript, in large part unpublished, New Theory of Sunspots was written by astronomer and professor of Marseilles Jesuit hydrography, Father Esprit Pezenas (1692-1776). The text was composed and revised between the years 1766 and 1772. In this manuscript, of which we are going to study the conditions of its composition, P. Pezenas gives one of the last geometric methods, related graphical methods, allowing the inclination of the Sun's equator to be deduced on ecliptic, using three observations of a sunspot, at given times, and in taking into account the movement of the Earth with respect to the Sun during the duration of observations. This text contains all the digital elements allowing to follow and understand the application of geometric processing methods of observations and observation techniques mentioned above in the introduction: passages of edges and Sun spots at the meridian, use of objective micrometers and wire micrometers. At detour of his calculations, Father Pezenas announces a duration of about 26 days and 9 hours for the rotation of the Sun around its axis, comments on and corrects some observation data published in Le Monnier's Heavenly History.