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

📝 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"

💡 Analysis

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"

📄 Content

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Monitoring the Solar Radius from the Royal Observatory of the Spanish Navy during the Last Quarter–Millennium J.M. Vaquero1,2 • M.C. Gallego2,3 • J.J. Ruiz-Lorenzo2,3 • T. López-Moratalla4 • V.M.S. Carrasco2,3 • A.J.P. Aparicio2,3 • F.J. González-González4 • E. Hernández-García5 1 Departamento de Física, Universidad de Extremadura, Mérida (Badajoz), Spain, jvaquero@unex.es 2 Instituto Universitario de Investigación del Agua, Cambio Climático y Sostenibilidad (IACYS), Universidad de Extremadura, Badajoz, Spain 3 Departamento de Física, Universidad de Extremadura, Badajoz, Spain 4 Real Instituto y Observatorio de la Armada, San Fernando (Cádiz), Spain 5 Departamento de Matemáticas, Universidad de Extremadura, Mérida (Badajoz), Spain

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”±1.77”. Keyword: Sun, solar observations, solar radius. 2

1. Introduction The Sun is the driver of numerous processes on Earth and our near-space environment. In addition, it is the nearest and best observed star. Therefore, the long-term evolution of the Sun is of great interest to astrophysicists and geophysicists. Furthermore, variations in the solar diameter would have effects on the climate of our planet. For instance, during a solar cycle, the solar-radius variations are very small (Bush, Emilio, and Kuhn, 2010; Meftah et al., 2015). Despite this fact, small changes in the solar luminosity due to a possible secular decrease or increase of the solar radius could affect the Earth’s climate (Sofia et al., 1979; Haigh, 1996, Foukal et al., 2006). Long time series that provide information about solar activity come from, among others, isotopic production due to cosmic rays (Usoskin, 2013), covering the last few millennia. However, time series related to direct observations of the Sun are shorter. The naked- eye sunspot observations cover the last two millennia (Vaquero, Gallego, and García, 2002; Vaquero and Vázquez, 2009), and the telescopic observation of sunspots covers the last four centuries (Vaquero, 2007; Clette et al., 2014). Another important set of direct solar observations is related to measurements of the solar diameter. The first telescopic devices to measure the solar diameter were conceived in the 17th century. Thus, we have today a long series of measurements of the solar diameter during the last four centuries (Rozelot and Damiani, 2012). However, the apparent solar-radius variations differ according to the instruments and methods used, even for the most recent determinations. The origin of these differences might be the Sun, the Earth’s atmosphere (in the case of ground-based observations), the instrument, or even the observer (especially in historical observations). Therefore, the recovery of historical solar-radius measurements could shed more light on and suggest new approaches to this old problem (Sigismondi and Fraschetti, 2001; Meftah et al., 2014). In addition, the long-term trend of the solar diameter is still an open and controversial issue. We can cite that Eddy and Boornazian (1979) showed from measurements of the solar diameter made at Greenwich during the period 1836–1953 a decrease of 2.25” per century. Furthermore, Gilliland (1981), after analyzing five different data sets, concluded that a secular decrease of ≈0.1” per century over the last 265 years is likely, 3

and Ribes, Ribes, and Barthalot (1987) suggest from observations recorded by French astronomers Picard and La Hire that solar diameter was about 4” greater around 1700 than now. In contrast, other investigations conclude that no long-term trend in the solar diameter is detected. For example, Parkinson, Morrison, and Stephenson (1980) find

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