In late July 1609 the 45-year-old Padua professor of mathematics Galileo Galilei received a letter from a French correspondent in Paris reporting that a Dutch spectacle-maker — actually Hans Lippershey of Middelburg in Zeeland — had filed in October 1608 a patent for a tube containing two lenses that made distant objects appear closer. The patent had not been granted (the Dutch authorities decided the device was too simple to protect) but the device itself, and reasonable copies of it, were already circulating in Paris, London, and Frankfurt.
Galileo had seen no example. From the verbal description alone he reconstructed the device within forty-eight hours.
The first telescopes
Galileo’s first telescope — built at his Padua workshop in early August 1609 — was a 3x magnifier using a plano-convex objective lens and a plano-concave eyepiece. He immediately understood that the magnification could be increased by changing the lens curvatures. Within a week he had built an 8x telescope. Within a fortnight he had built a 9x telescope. By late August he had a working version that would prove suitable for naval observation.
The technical move was not the optical design — Lippershey already had that — but the production technique. Galileo had access to the Venetian glass-makers’ guild expertise on Murano. He could specify, commission, and accept or reject blanks of optical-quality glass that the Dutch makers could not match. His own grinding and polishing skills, learned during his Pisa years, allowed him to bring the curvature tolerance within the 1-in-50 accuracy that the higher magnifications required.
21 August 1609
The Venetian Senate convened a special demonstration session on the afternoon of 21 August 1609. The senators — accompanied by the Doge Leonardo Donato, the senior Venetian official Paolo Sarpi who had been Galileo’s principal patron, and Galileo himself — climbed the bell tower (campanile) of San Marco. Galileo set up the 8x telescope on a tripod and demonstrated.
The senators looked first at the island of Murano, about a mile across the lagoon, where Galileo’s glass had been ground. They then looked east toward the Lido, where the seaward channels approached the city. The eastern observation was the strategic one. With the telescope a Venetian fleet officer could identify approaching ships — friendly, neutral, or enemy — at approximately twice the distance the unaided eye permitted. For a maritime mercantile republic whose fleet was its survival, this was a substantial military capability.
The Doge thanked Galileo. The senators returned to the Doge’s Palace and voted, the same day, to double Galileo’s University of Padua mathematics-chair salary from 500 florins per year to 1,000 florins per year, in recognition of his contribution. They also confirmed his Padua appointment as a lifetime tenure.
The Senate’s gratitude was, however, conditional on a specific commitment that Galileo failed to make explicit at the time of the salary increase: that the device would remain a Venetian state secret. Other 8x telescopes from Galileo’s workshop were already circulating in Florence and Rome by October 1609. The Senate quietly suspended payment of the salary increase by 1610, having concluded that they had bought a non-exclusive product.
What he did next
In September 1609 Galileo built a 20x telescope. In November 1609 he aimed it at the Moon.
The lunar surface — which the contemporary Aristotelian-Ptolemaic cosmological orthodoxy insisted was perfectly spherical and smooth, being part of the supralunary celestial sphere — appeared in the eyepiece as mountainous, cratered, and shadowed. Galileo measured the heights of the lunar mountains by trigonometric analysis of the cast shadows. The heights were comparable to terrestrial mountains.
In January 1610 he aimed the same telescope at Jupiter and saw four small bright objects in a line. Over four consecutive nights — 7, 8, 9, 10 January 1610 — he watched them change position. The objects were satellites of Jupiter, the first non-lunar moons observed in human history. Jupiter had its own miniature solar system.
The lunar and Jovian observations were published in his Sidereus Nuncius (“The Sidereal Messenger”) at Venice in March 1610. The pamphlet — 60 pages, with hand-engraved lunar surface drawings — was the first publication in human history to present telescopic astronomical observations. It went through six editions in 1610 and made Galileo the most famous astronomer in Europe.
Galileo left Padua in September 1610 for a more lucrative court appointment under the Medici at Florence. The Venetian state’s attempt to retain him was unsuccessful. The bell-tower demonstration of 21 August 1609 had bought Venice approximately thirteen months of his presence.