How accurate were seventeenth-century measurements of solar diameter?

Abstract

The assumption that the solar diameter is constant over periods that are short compared to the nuclear process has recently been questioned. In 1979 Eddy and Boornazian claimed that measurements of the solar diameter made at the Greenwich and US Naval observatories show a decrease of ∼2arcs, or 0.1%, per century over the period 1836–1953 (refs 1,2). Others have since extended this baseline by studying the evidence from Mercury transits³ and assessing the entire range of evidence from the beginning of the eighteenth century to the present⁴, and have found little or no evidence for a secular change in the solar diameter. Most recently, Ribes et al.⁵ have pushed the time baseline back to 1666, by considering the micrometer and transit measurements of the solar diameter made in Paris by Jean Picard (1666–82) and Philippe and Gabriel-Philippe de La Hire (1683–1719). Not only are these observations of importance for the investigation of solar changes at the end of the Maunder minimum, they can also be used for comparison with modern values. We discuss here the accuracy of Jean Picard's micrometer measurements of the solar diameter and his successors' transit measurements in light of the claim by Ribes et al.⁵ that these data provide evidence for a change in the apparent size of the Sun at the end of the Maunder minimum and a difference from the current size. Our evidence suggests that the necessary corrections to the measurement of sizes with early telescopes are larger than Ribes et al. assume. Therefore we call into question their conclusion that the Sun rapidly changed in size during this period.