Galileo the Heaven Gazer

He turned a telescope up to the night sky.

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Galileo devoted his life to scientific, especially astronomical experiments. He was the developer, though not the 'inventor', of the telescope. In 1608 a Dutch spectacle-maker, Hans Lipperhey of Middleburg, or his assistant, had discovered that a pair of lenses could make objects, such as church steeples, look closer. At least two others, including Zacharias Janssen also claimed to have been first to make the discovery. Galileo is supposed to have heard of this discovery on his arrival in Venice in July 1609. One such perspicillum had already been demonstrated in Milan. He rushed back to Padua because he had plainly understood the commercial potential of the invention and, furthermore, he had heard that no patent had been issued. (Lipperhey applied for a Dutch government patent but it was rejected). Within a fortnight he had built a perspicillum with a convex objective lens and a smaller concave eyepiece. The device achieved a magnification power of X3. This he later improved to X10, so it is not quite true, as he later boasted, that ‘The first night after my return, I solved it’. 

Galileo demonstrating his first telescope

On the right is a print by W. F. Soare, on textured board, of Galileo in 1609 with three colleagues, testing a telescope from the Tower of St. Mark's, Venice. Adapted from a popular illustration in the 'Milestones in Optical History' series, published by Bausch & Lomb in the 1930s.

Galileo returned to Venice to demonstrate the improved model before the Doge and Ruling Council, prudently deciding to offer the instrument as a gift to the Doge. This action may have contributed to the advancement in his career that followed soon afterwards. The significance of the invention for the defence of a maritime city had not been lost on the Venetians. Whilst lots of cheap imitations followed, none was as powerful as Galileo’s perspicillum.  He himself now produced a X32 device, the first model he actually called a ‘telescope’.

Galileo print
Galilaeus Galile
Lynceus, Philosop
et Mathema
, a 17th
century engraving
by N. de Larmessin

In April 1611 he demonstrated the telescope to the Papal Court in Rome, the success of which event may well have persuaded him to risk espousing Copernican theory. It was at this banquet that the name 'telescope' was originally coined by Prince Cesi, founder of the Accademia dei Lincei. This seems to have been at the suggestion of Giovanni Demisiani, a Greek mathematician who was elected to the Academy the following year.   

Galileo was the first to both apply the telescope (X32 magnification) to astronomical observations and publish the results, revealing the mountains in the Moon, numerous stars invisible to the naked eye, the nature of the Milky Way, and four of Jupiter’s satellites (named the Medicean Stars). His finds were described in Sidereus nuncius (‘The Starry Messenger’), published in Venice, 1610, in which year he went to join his patron, the Grand Duke Cosimo, in Florence.

Sidereus Nuncius

A significant copy of this may be found in the College library as part of another work, Pierre Gassendi's, Institutio Astronomica (to which is appended a copy of Galileo's 'Nuntius Sidereus' and Kepler's 'Dioptrice'), London, 1653. The inclusion of the Sidereus Nuncius in this edition of Gassendi's work was apparently the first printing in England of any of the works of Galileo.

Some readers dismissed the moons of Jupiter, concluding that they must be illusions brought about by the poor quality of the telescope lenses. With vapour from the eye early seventeenth century telescope lenses were prone to misting over and required constant wiping. Galileo's Venetian mistress Maria Gamba had married one Giovanni Bartoluzzi in 1610. From Bartoluzzi he obtained his supply of lens blanks made from Murano glass, which he then ground on his own lathes in his new house in Florence. In Rome in 1613 he published Istoria e dimostrazioni intorno alle macchie solari (History and Demonstrations Concerning Sunspots and their Phenomena) in which he explained the phases of Venus, the structure of Saturn, and the existence of Sun Spots. We know that he used a piece of paper on which to project images of sunspots so as to avoid retinal damage. he also interpreted comets as optical illusions but recurrent illness prevented him from observing any telescopically.

News of the telescope soon reached England. Sir Henry Wotton, ambassador to Venice under James I, wrote to the Earl of Salisbury, ‘I send herewith unto His Majesty the strangest piece of news (as I may justly call it) that he hath ever yet received from any part of the world; which is the annexed book of the Mathematical Professor at Padua, who by the help of an optical instrument…hath discovered four new planets rolling about the Sphere of Jupiter…so upon the whole subject he hath first overthrown all former astronomy…and next all astrology. By the next ship your Lordships shall receive from me one of the above instruments, as it is bettered by this man'.  

Detail from a painting of Galileo showing a telescope
Detail of Galileo’s telescope from
an 18th century painting in the
museum…the telescope looks
remarkably 18th century too and
is not reliable evidence for how
such instruments would have
appeared in Galileo's time.

Further refinements followed; for example Galileo devised a method for checking the lens curvature, thus aiding astronomic observation. All early telescopes were of the refracting type. A good example is seen in our painting of Galileo though the artist has depicted an instrument of early eighteenth century style!

The Galilean Telescope is still a recognised concept today and forms the basis of the simple spyglass. With a positive lens as the eyepiece it becomes an astronomical telescope - a somewhat different function.

Galileo constantly stressed the need for quantitative experiments and sound hypothetical reasoning, and these were matters which he discussed in Il saggiatore (Rome, 1623). His belief in the Copernican system and views about astronomy are apparent from his Dialogo sopra i due massimi sistemi del mondo (Florence, 1632), a highly technical account presented as a dialogue between a supporter of the Aristotelian-Ptolemaic tradition and an advocate of the new astronomy. Even though Galileo let the old world view win out, the new arguments were so strong that the Holy Office summoned Galileo to Rome, forced him to abjure his Copernican convictions (which it regarded as heretical), and sentenced him to confinement in his home at Arcetri and to constant supervision by the Inquisition.

In 1992, after three and a half centuries, the Vatican admitted making ‘errors’ over its handling of Galileo.

Galileo engaged with almost every branch of physics. His final, and most important work, the Discorsi e dimostrazioni matematiche intorno a due nuove scienze (Leiden, 1638) contained, among other things, the proof of the laws of the fall in vacuum, the principle of the independence of forces, and a complete theory of parabolic ballistics. This work became a cornerstone on which scientists of the next generation, including Isaac Newton, built up classical mechanics.

Galileo’s influence lasted throughout the century. His observation of Jupiter’s moons moving in and out of ‘eclipse’ was used by the Danish astronomer Ole Roemer in 1675 to estimate the speed of light (the first such attempt). This side of Galileo’s work ensured his lasting fame with a wider, educated public, far more than his important work on mechanical forces.

'The Noblest Eye is Darkened' - Galileo's blindness

Galileo's eye from a painting in the BOA Museum

Galileo’s sight began to deteriorate in the middle of 1636 when he was 68 years old, and by the end of June 1637 he had lost the use of his right eye (pictured in a detail from the museum's painting of the great man) while his left eye was affected by a constant discharge. He described seeing a 'luminous halo' around candle flames. To date there has been little speculation by modern optometrists or physicians about the possible causes of Galileo’s blindness.

In July 1636 he wrote to his friend, an Italian lawyer living in France, Elia Diodati:

I have been in bed for five weeks oppressed with weakness and other infirmities. Added to the (proh dolor!) the sight of my right eye - that eye whose labours (I have no hesitation in saying) have had such glorious results, is lost forever. That of the left, which was and is imperfect, is rendered null by a continual running.

He became totally blind early in December 1637, a few months after using the telescope to discover that the moon wobbles on its axis ('lunar libration') which was quite a remarkable observation to make with only one useful eye. At this point he wrote to Father Castelli noting that:

Galileo's eyes from a sculpture in the BOA Museum

The noblest eye is darkened which nature ever made, an eye so privileged and so gifted with rare qualities that it may with truth be said to have seen more than the eyes of all those who are gone, and to have opened the eyes of all those who are to come.


  • Galileo's medical friends diagnosed cataracts but no attempt to restore the transparency of the crystalline lens succeeded.
  • Therefore it has been suggested that Galileo suffered from corneal disease, for which there was little treatment in the seventeenth century.
  • Professor Brodetsky suggested eyestrain due to the continual use of primitive telescopes prone to spherical and chromatic aberration. Modern optometrists reject the thinking behind this explanation.
  • Similarly it is doubted that he could ever have caught an infection from sharing telescope eyepieces.
  • Glaucoma is an improbable if possible cause, a theory supported by correspondence from his nephew that appears to suggest he recovered some vision by late 1639 and only went permanently blind in 1641.

The Pope seems to have mellowed on hearing of Galileo's severe visual impairment (and accompanying deafness) and allowed him to 'see' his friends once more, including the poet John Milton (who would later go blind himself due to glaucoma). Galileo told Diodati that, without sight, the universe had shrunk to the meagre confines of his body. From October 1638 he had a live-in companion, the 16-year old Vincenzio Viviani (1622-1703) who acted as his eyes and later wrote a gushing biography (1654). Nevertheless, despite his medical training at the University of Pisa in the 1580s neither Galileo nor his friends left behind sufficient symptomatic evidence to draw a firm diagnosis.


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