Monitoring the Solar Radius from the Royal Observatory of the Spanish Navy during the Last Quarter-Millennium

TL;DR

This study presents a comprehensive historical dataset of solar semidiameter measurements from 1773 to 2006, confirming the solar radius's constancy over this period and finding no correlation with sunspot activity.

Contribution

It provides an extensive new database of solar radius measurements spanning over two centuries, derived from historical transit data and modern observations, with analysis confirming long-term stability.

Findings

No significant long-term trends in solar radius detected.

The average solar semidiameter is 958.87 arcseconds with a standard deviation of 1.77 arcseconds.

No correlation found between solar radius and sunspot number index.

Abstract

The solar diameter has been monitored at the Royal Observatory of the Spanish Navy (today the Real Instituto y Observatorio de la Armada: ROA) almost continuously since its creation in 1753 (i.e. during the last quarter of a millennium). After a painstaking effort to collect data in the historical archive of this institution, we present here the data of the solar semidiameter from 1773 to 2006, making up an extensive new database for solar-radius measurements can be considered. We have calculated the solar semidiameter from the transit times registered by the observers (except values of the solar radius from the modern Danjon astrolabe, which were published by ROA). These data were analysed to reveal any significant long-term trends, but no such trends were found. Therefore, the data sample confirms the constancy of the solar diameter during the last quarter of a millennium (approximately) within instrumental and methodological limits. Moreover, no relationship between solar radius and the new sunspot-number index has been found from measurements of the ROA. Finally, the mean value for solar semidiameter (with one standard deviation) calculated from the observations made in the ROA (1773-2006), after applying corrections by refraction and diffraction, is equal to 958.87" \pm 1.77"