Diameter of the Sun in AD 1715

Abstract

Ribes et al.¹ have reopened the discussion about the variability of the Sun's diameter over the past few hundred years by asserting that observations made at the Paris Observatory by Picard and La Hire² during the late seventeenth and early part of the eighteenth century indicate that the diameter of the Sun was ˜4 arcs greater then than it is now. In two earlier papers^3,4 the conclusion drawn from analyses of solar eclipses and transits of Mercury was that there has been little or no discernible secular change in the Sun's diameter since 1715. Ribes et al.¹ dismissed this result and concluded that their result based on La Hire's observations, and ours^3,4 based on the 1715 eclipse, were not incompatible. Here we say that the two results are incompatible, and we give a new and careful discussion of the 1715 eclipse in support of our contention.