A Celestial Prodigy. Leopoldo de’ Medici and the Lunar Eclipse of 16 June 1666

On 16 June 1666 a rare celestial event occurred: a horizontal eclipse, also known as a selenelion, in which the Sun and Moon are aligned at 180 degrees on opposite sides of the Earth and are both briefly visible at the same time. The eyes of all Europe’s astronomers were gazing skywards on that day but adverse weather conditions prevented nearly everybody from observing the phenomenon. The only one who succeeded was the young mathematician Donato Rossetti, sent by Leopoldo de’ Medici to the isle of Gorgona in the Tuscan archipelago. Drawing on a new source, this article retraces the history of that observation and highlights the pivotal role Leopoldo played, initially as promoter of observations in different places and later as reference point of the subsequent debate.