Francesco Fontana and his "astronomical" Telescope

TL;DR

Francesco Fontana was a pioneering 17th-century telescope maker and astronomer who made significant early observations of the Moon, planets, and moons of other planets, and contributed to the development of telescope technology and astronomical knowledge.

Contribution

This paper uncovers Fontana's early use of convex lenses, his detailed lunar and planetary observations, and his possible influence on art and telescope design, highlighting his overlooked role in early astronomy.

Findings

First to observe the sky with a self-made convex lens telescope.

Detailed lunar maps and observations of craters and their motions.

Suggested the existence of additional moons around Jupiter, Venus, and Saturn.

Abstract

In the late 1620s the Neapolitan telescope maker Francesco Fontana was the first to observe the sky using a telescope with two convex lenses, which he had manufactured himself. Fontana succeeded in drawing the most accurate maps of the Moon's surface of his time, which were to become popular through a number of publications spread all over Europe but without acknowledging the author. At the end of 1645, in a state of declining health and pressed by the need to defend his authorship, Fontana carried out an intense observational campaign, whose results he hurriedly collected in his Novae Coelestium Terrestriumque rerum Observationis (1646), the only book he left to posterity. Fontana observed the Moon's main craters, as the crater Tycho which he named Fons Major, their radial patterns and the change in their positions due to the Moon's motions. He observed the gibbosity of Mars at quadrature and, together with the Jesuit G.B. Zupus, he described the phases of Mercury. Fontana observed the two - and occasionally three - major bands of Jupiter, and inferred the rotation movement of the major planets Mars, Jupiter and Saturn, arguing that they could not be attached to an Aristotelian sky. He came close to revealing the ring structure of Saturn. He also suggested the presence of additional moons around Jupiter, Venus and Saturn, which prompted a debate that lasted more than a hundred years. In several places of his book Fontana repeatedly claimed to have conceived the first positive eyepiece in 1608, providing a declaration by Zupus to have used his telescope since 1614. This declaration is still the oldest record mentioning such a device. We finally suggest that the telescopes depicted in the two Allegory of Sight by J. Brueghel the Elder belonging to Albert VII might have been made by Fontana, and that he might have inspired the Sight by Jusepe Ribera (c. 1616).