A mass of innumerable stars

And especially considering that, in those same days, another instrument-maker, Sacharias Janssen (1588-1630), he too a spectacles-maker in Middelburg, indicated by Pierre Borel (c. 1620-1671) a few decades later as the true inventor of the telescope – declared that he knew how to build the instrument.

News of the invention spread rapidly throughout Europe, and already by April 1609 little telescopes about thirty centimetres long were to be found on sale, at the shops of spectacle-makers, in Paris and presumably in London.

In Italy, the new instrument made its appearance at Milan in May of the same year, and two or three months later in Rome, Naples, Padua and Venice, where Fra Paolo Sarpi (1552-1623), a friend of Galileo, had heard news of it already by November 1608.

In November, Galileo had at his disposal a telescope with twenty magnifications, that is, more powerful by far that all the others circulating through Europe at the time, which utilised ordinary lenses made for spectacles, of low quality and with unsuitable focal lengths.

The instruments developed by Galileo were highly superior in performance, for example, to the telescope with six magnifications with which the Englishman Thomas Harriot (1560-1621) had conducted observations and made drawings of the lunar surface in July 1609.

Thanks to the power of his instrument, Galileo achieved exceptional results in his observations of the moon, demonstrating, in fact, that its surface is not perfectly spherical nor immaculate and even managing to calculate the height of the lunar mountains.

Subsequently, Galileo was to make the exceptional series of astronomical discoveries, described in the Sidereus Nuncius [The Starry Messenger] published in March 1610, and destined to revolutionize forever the traditional view of the cosmos.

He was to discover, first of all, the existence of a myriad of new stars, showing that the Milky Way is "no other than a mass of innumerable stars scattered in clusters".

And again, he was to observe the strange appearance of Saturn, whose true cause, the presence of a ring around the planet, was to be found by Christiaan Huygens (1629-1695) nearly half a century later.

He was the first to observe the phases of Venus, which conclusively demonstrated that the planet moved, orbiting around the Sun.

But the discovery that brought him immortal fame, in January 1610, was that of the four satellites of Jupiter, which Galileo, in homage to the dynasty that ruled Tuscany, named Astri Medicei, or Medicean Planets.

Galileo (1564-1642) was to remain always faithful to the optical combination that bears his name.

However, in the 1630s, the Keplerian telescope began to be widely used, mainly due to the work of the Neapolitan optician Francesco Fontana (c. 1580-1656), to the point of entirely superceding the Galileian one toward the middle of the century.