Galileo and the Telescope

‘Now it is our advantage, that by the help of Galileus’ glass, we are advanced nearer unto them, and the heavens are made more present to us than they were before’.

John Wilkins, Discovery of a World in the Moone (London, 1638).


Galileo Galilei, Opere (Florence, 1718), vol 1. frontispiece portrait.

2009 as the International Year of Astronomy marks the four hundredth anniversary of Galileo’s use of the telescope. That he himself did not create it he makes clear in his Siderius nuncius (Venice, 1610):

‘A few days later the report [that a certain Fleming had constructed a spyglass] was confirmed to me in a letter from a noble Frenchman at Paris, Jacques Badovere, which caused me to apply myself wholeheartedly to inquire into the means by which I might arrive at the invention of a similar instrument. This I did shortly afterwards, my basis being the theory of refraction.’ *

In fact the telescope originated in 1608 in the Netherlands and there was some competition over who actually invented it: James Metius (also known as Jacob Adriaanzoon) argued that he had discovered the secret but Pierre Borel, in his work on the invention of the telescope (collected by Worth), awarded the discovery to two lensmakers, Hans Lipperhey and Zacharias Jansen who are pictured here:

Galileo2 Galileo3
Pierre Borel, 
De Vero Telescopii Inventore (The Hague, 1655), 
portrait of Lipperhey.
Pierre Borel, 
De Vero Telescopii Inventore (The Hague, 1655), 
portrait of Jansen.

Of the two men it is likely that the German-Dutch Lipperhey had the greater claim to the actual discovery but his failure to secure a patent for the new invention ensured that when its fame spread to France and Italy, astronomers soon started experimenting with the new device.

Galileo did not claim to have invented the telescope but he was keen to show that he had improved on Lipperhey’s original which was only three-powered. As Van Helden argues (1974), his ‘reasoning’ outlined in the Siderius Nuncius behind his own improved telescope of the summer of 1609 and his thirty-powered telescope of January of the following year, left much to be understood:

‘My reasoning was this. The device needs either a single glass or more than one. It cannot consist of one alone, because the shape of that one would have to be convex… or concave…, or contained between parallel surfaces. But the last named does not alter visible objects in any way, either by enlarging or reducing them; the concave diminishes them; and the convex, while it does indeed increase them, shows them very indistinctly and confusedly. Therefore a single glass is not sufficient to produce the effect. Passing next to two, and knowing as before that a glass with parallel faces alters nothing, I concluded that the effect would still not be achieved by combining such a one with either of the other two. Hence I was restricted to trying to discover what would be done by a combination of the convex and the concave, and you see how this gave me what I sought.’*

In fact it was not until Kepler’s Dioptrice in 1611 (another work purchased by Worth), that the optical theory of the new instrument was fully worked out. Kepler’s telescope was far more suited to astronomical investigation but its advocacy of the inverted image made it slow to gain adherents – it was not until the middle of the seventeenth century that it came into widespread use.

Siderius Nuncius

Galileo was not the discover of the telescope and nor was he the only astronomer putting it to good use in the summer of 1609 but crucially he was the first to publish his finding in a short tract called Siderius Nuncius, usually translated as the Sidereal Messenger or Starry Messenger (Venice, 1610). And the messages it brought were startling – so startling that Galileo was sure to put his four major discoveries on the title page. We see here Worth’s London 1683 copy of Pierre Gassendi’s edition of Siderius Nuncius:

Galileo Galilei, Siderius Nuncius (London, 1683) ed, by Pierre Gassendi, title page.

Undoubtedly the highlights were his discovery of the four moons or satellites of the planet Jupiter which are explored elsewhere in this website and which in a diplomatic and ultimately successful bid for patronage he named the ‘Medicean Moons’; the increased extent of the ‘fixed stars’, the difference between their appearance and that of the planets and, finally, the mountains of the Moon.


Galileo Galilei, Siderius Nuncius (London, 1683) ed, by Pierre Gassendi, p. 15.

Galileo describes his view of the Moon in the Sidereus Nuncius as follows:

‘I would by no means be silent about something deserving notice, observed by me while the Moon was rushing toward first quadrature, the appearance of which is also shown in the above figure. For toward the lower horn a vast gulf projected into the brighter part. As I observed this for a long time, I saw it very dark. Finally, after about 2 hours, a bit below the middle of this cavity a certain bright peak began to rise and, gradually growing, it assumed a triangular shape, still entirely separated from the bright face. Presently three other small points began to shine around it until, as the Moon was about to set, this enlarged triangular shape, now made larger, joined together with the rest of the bright part, and like a huge promontory, surrounded by the three bright peaks already mentioned, it broke out into the dark gulf. Also, in the tips of both the upper and the lower horns, some bright points emerged, entirely separated from the rest of the light, as shown in the same figure. And there was a great abundance of dark spots in both horns, especially in the lower one.’


Johannes Hevelius, Selenographia (Danzig, 1647), opposite p. 298.

In this illustration of the Moon, engraved by Hevelius in Danzig in 1644, the jagged edge of the terminator which Galileo had seen through his telescope in 1610 is clearly visible, as is the uneven surface of the Moon. The concept of an uneven lunar surface, along with a jagged terminator, was perhaps the hardest discovery for Galileo’s aristotelian colleagues to accept – and this is precisely why Christoph Clavius held out on this issue for so long when Galileo was being lauded at Rome by the Collegio Romano. Clavius, the editor par excellence of the Sphaera of Sacrobosco, one of the core astronomical textbooks of the Middle Ages, was not initially open to any of Galileo’s findings but following instructions from Galileo on the use of the new instrument he began to change his mind. By the time Galileo arrived in Rome in March 1611 the Jesuits, including Clavius, had confirmed his major discoveries – Jupiter’s satellites, the strange shape of Saturn and the phases of Venus. But the uneven lunar surface and the jagged terminator remained the sticking point with Clavius for unlike the rest of the discoveries which might, with some persuasion, be made to fit into an aristotelian cosmos, the terminator specifically contradicted Aristotle’s theory that the Moon as a celestial body was a perfect sphere. Lattis (1994) suggests that Clavius may also have rejected the jagged surface of the Moon for the simple reason that he linked the sphericity of the body with its circular motion.

Initial Reaction

The reaction of the foremost Jesuit astronomers in Rome to Galileo’s findings is perhaps best summed up by Clavius’s comment on the discovery of the telescope and Galileo’s findings in his 1611 edition of Sacrobosco’s Sphaera (given by Lattis (1994)

‘I do not want to hide from the reader that not long ago a certain instrument was brought from Belgium. It has the form of a long tube in the bases of which are set two glasses, or rather lenses, by which objects far away from us appear very much closer, and indeed considerably larger, than the things themselves are. This instrument shows many more stars in the firmament than can be seen in any way without it, especially in the Pleiades, around the nebulas of Cancer and Orion, in the Milky way, and other places… and when the moon is a crescent or half full, it appears so remarkably fractured and rough that I cannot marvel enough that there is such unevenness in the lunar body. Consult the reliable little book by Galileo Galilei, printed at Venice in 1610 and called Sidereal Messenger, which describes various observations of the stars first made by him.

Far from the least important of the things seen with this instrument is that Venus receives its light from the sun as does the moon, so that sometimes it appears to be more like a crescent, sometimes less, according to its distance from the sun. At Rome I have observed this in the presence of others more than once. Saturn has joined to it two smaller stars, one on the east, the other on the west. Finally, Jupiter has four roving stars, which vary their places in a remarkable way both among themselves and with respect to Jupiter – as Galileo Galilei carefully and accurately describes.

Since things are thus, astronomers ought to consider how the celestial orbs may be arranged in order to save these phenomena.’

Clavius refers here not only to the terminator and the satellites of Jupiter but also to Galileo’s subsequent discoveries in 1610: the phases of Venus, which, as Clavius rightly pointed out, was a discovery with important implications since it was totally incompatible with the Ptolemaic system. The following illustration is of the phases of Venus sighted by Hevelius in Danzig in 1644:

Johannes Hevelius, Selenographia (Danzig, 1647), Fig. K.

A fairly crude depiction of the strange formation of the planet Saturn followed – to Galileo Saturn looked as if it had three components- the planet itself and two moons. He, like many astronomers after him, realised that the shape of the planet varied but it was not until Huygens that a true appreciation of Saturn’s Rings came into being. Galileo’s depictions might seem crude but it important to remember that this was the first time that many had been depicted in a naturalistic fashion (Winkler and Van Helden, 1992). Here we see illustrations of the Pleiades in the 1663 edition of the Siderius Nuncius and from a later edition of Galileo’s complete works in the Worth Library, the celebrated three volume Florentine edition, Opere di Galileo nobile Fiorentino primario filosofo, e mattematico del serenissimo Gran Duca di Toscana. 3 vols. (Florence, 1718).

Galileo8 Galileo9
Galileo Galilei, 
Siderius Nuncius, plate of Pleiades.
Galileo Galilei, 
Opere vol 1, p. 21.

At the request of Cardinal Robert Bellarmine the Jesuit astronomers had investigated Galileo’s claims: apart from the caveats mentioned above, the response was an enthusiastic acceptance of Galileo’s findings and an invitation to a banquet in his honour, held in the Collegio Romano itself. It was at a dinner during his Roman sojourn that Galileo received the added honour of being made a member of the Accademia dei Lincei by Prince Federigo Cesi – at which the new instrument was formally named ‘telescope’. Galileo’s star was in the ascendant.


That Clavius makes no mention of another of Galileo’s discoveries, sunspots, is probably because he was writing prior to Galileo’s demonstration of them at Rome in April 1611. Though Galileo first sighted them in 1610 he was not the first to use a telescope to view sunspots – the Englishman Thomas Harriot just pipped him to that particular post – but he worked on them avidly once he arrived in Rome. As Stephenson (1990) relates, sunspots had been viewed prior to Galileo but previous interpretations, including one by Kepler, had assumed that the dark spots on the surface of the Sun were the planets Mercury and Venus passing in transit. While at Rome he was given a newly published tract on sunspots by the Jesuit professor of mathematics at Ingolstadt University, Christoph Scheiner, who argued that sunspots were small satellites of the Sun. Galileo set out to disprove this theory by demonstrating that the spots were in fact phenomena of the Sun itself. In the course of a number of letters and finally in 1613 his work on sunspots came to light: History and Demonstration concerning Sunspots and their Properties.



Galileo Galilei, Opere (Florence, 1718), vol 2, pp 138-9.

As Winkler and Van Heldin (1992) make plain, Galileo’s illustrations of sunspots played a crucial role for their acceptance by the astronomical community. By displaying their shape over a series of days in which all the spots were oriented the same way, the changes in their shape became clear – showing the impossibility of Scheiner’s satellite theory.

That sunspots continued to be a source of fascination in the following decades is shown by the deliberations of the Dublin Philosophical Society of which Worth’s father, John Worth, Dean of St Patrick’s cathedral, Dublin, had been a founding member. On 26 May 1684 William Molyneux, whose Sciothericum Telescopicum (Dublin, 1686) Edward Worth would later purchase, read a paper on the subject which had been communicated to him by no less a figure that the Astronomer Royal, John Flamsteed, and in August 1707, the English astronomer William Derham wrote to his Dublin based colleague Samuel Molyneux of his own observation of sunspots in June 1706, outlining his method:

‘No longer since than June 29 last a facula was on the sun, in which (July 1) a faint spot arose and made his transit almost over the disc. The places and stages of most of the spots I carefully measured viz. their declinations from the poles of the sun with a micrometer and their distances from the limb with the pulses of a pendulum clock. And whilst they were a novelty, I took their figures as well as I could. As to which, I observed them to be very variable, so as sometimes I have seen them change their shapes while I have been viewing them through my 16 foot telescope. Yea, I have seen their figure change even with the help of only a 6 foot telescope when I have transmitted the sun’s rays through it into a darkened room and have received the spacies of the sun on a shield of white paper, which I found to be the most commodious way of viewing the solar spots, as to look through the tube is the surest way of measuring them.’

This is taken from K. T. Hoppen (ed.) Papers of the Dublin Philosophical Society 1683- 1709 (Dublin, 2008), 2 vols. Derham’s method is illustrated by Hevelius in the Sun web page of this website.

The 1616 Decree against Copernicanism and the 1633 Trial of Galileo

Galileo’s adherence to Copernicanism predates by ‘some years’ his letter to Kepler thanking the latter for his gift of the Mysterium cosmographicum in 1597 but he was, as Van Heldin relates (2003), a cautious Copernican – at least at the start of his career. It was really only in his 1613 work on sunspots that Galileo related his new discoveries specifically to the issue of the Copernican system and emerged as an advocate of the reality of the heliocentric system.

Storm clouds gathered: in 1614 a Dominican priest preached a sermon criticising Galileo and by 1615 he was being accused of heresy. When Bellarmine was appealed to adjudicate the matter he fell back on the argument that Copernicanism could only be accepted as a mathematical construct since it clearly contradicted a number of biblical texts. Galileo’s response was his celebrated letter to the Grand Duchess Christina of Tuscany wherein he argued that Science and Scripture should be treated separately. By December of 1615 Galileo decided to return to Rome in a bid to argue the Copernican case but his reception was radically different from his previous experience.

Pope Paul V called on his theologians to make a judgment on Copernicus and the theologians unsurprisingly called for De revolutionibus orbium to be banned until it was corrected. Despite the fact that Galileo was not specifically mentioned it was clear to all that the decree of 1616 was a blow to his Copernican campaign – and the seriousness of the situation was made clear to him in a private audience with Cardinal Bellarmine.

On the death of Paul V and the succession of Maffeo Barberini as Pope Urban VIII in 1624 Galileo, who had judiciously remained silent on the subject of Copernicanism in the intervening years, began to test the waters. Bellarmine had made it clear that Copernicanism could only be commented on as a mathematical hypothesis so Galileo decided to frame his emerging work as a discussion, a dialogue between adherents of two rival cosmological systems: Simplicio, the spokesman for the Aristotelian system; Salviati, the proponent of Copernicanism; and Sagredo, and intelligent bystander. This was undoubtedly keeping to the letter of the law, but the resulting Dialogi that emerged in 1632 was seen by all to be a very public condemnation of the Aristotelian cosmos, as defended by Simplicio.

Galileo’s identification with Copernicanism can perhaps best be seen in a simple illustration: the frontispiece of his Dialogi sopra I due massimi sistemi del mondo (1632), of which Worth has a Latin edition published at London in 1663:


Galileo Galilei, Dialogus de systemate mundi (London, 1663), frontispiece.

In the 1632 edition we see the same three figures but unlike Worth’s copy which has the representation of Copernican modelled on portraits of the Polish astronomer, the 1632 frontispiece had an odd quirk: Copernicus, far from looking like known portraits, was remarkably like Galileo himself. The identification was further strengthened iconographically by the contrast between the upstanding figure of Copernicus/Galileo and the blind Ptolemy and decrepit Aristotle.

What the frontispiece hinted at the text made plain. The contrast on the title page was mirrored in the ‘dialogue’ between Simplicio (a simplification and caricature of the Aristotelian system) and the far more learned and compelling arguments of Salviati, the advocate of the Copernican system. Galileo must have known that the text would create a sensation and it certainly did. To make matters worse, Simplicio, presented to the reader as a reactionary dunce, was made to echo the opinions of Pope Urban VIII.

Urban VIII was not long in setting up an Inquisitorial court, over which he presided himself, to call Galileo to Rome for a trial. Despite attempts to delay the process Galileo was eventually forced to go, arriving once more in Rome in February 1633. Initially residing in the apartments of the Tuscan ambassador, by June Galileo was imprisoned by the Inquisition and was formally interrogated. The result was a climb-down: Galileo publicly abjured, stating that he had perhaps made the figure of Salviati too strong a proponent of Copernicanism. He was sentenced to house arrest in Siena where he remained until December of 1633 when he returned to his villa at Arcetri, where he died on 8 January 1642, having completed and published his Discourse on the Two New Sciences (1638).

Selected Reading

*All translations of Siderius Nuncius are from Stillman Drake’s translation in his Discoveries and Opinions of Galileo (Garden City: Doubleday, 1957) apart from the quotation on the mountains of the Moon which is from Albert Van Helden’s translation Siderius nuncius, or the Sidereal Messenger Chicago: University of Chicago Press.
The Galileo Project: Available at Galileo
Ariew, R. (2001). ‘The Initial Response to Galileo’s Lunar Observations’, Studies in History and Philosophy of Science, 32. No. 3., 571-581.
King, H.C. (1955). The History of the Telescope. Wycombe: Charles Griffin and Co.
Lattis, J. M. (1994) Between Copernicus and Galileo. Christoph Clavius and the Collapse of Ptolemaic Cosmology. Chicago: University of Chicago Press.
Machamer, P. (ed.) (1998). The Cambridge Companion to Galileo. Cambridge: Cambridge University Press.
Pantin, I. (1999). ‘New Philosophy and Old Prejudices: Aspects of the Reception of Copernicanism in a Divided Europe’, Studies in History and Philosophy of Science 30, no. 2., 237-262.
Remmert, V. R. (2006). ‘‘Docet parva pictura, quod multae scripturae non dicunt’. Frontispieces, their Functions, and their Audiences in Seventeenth-Century Mathematical Science’, in Sachiko Kusukawa and Ian Maclean (eds). Transmitting Knowledge. Words, Inages, and Instruments in Early Modern Europe. Oxford: Oxford University Press, pp. 239-270.
Stephenson, F.R. (1990). ‘Historical evidence concerning the Sun: Interpretation of Sunspot Records during the Telescopic and Pre-Telescopic Eras’, Philosophical Transactions of the Royal Society of London. Series A. Mathematical and Physical Sciences, 330 no. 1615, 499-512.
Van Helden, A. (1974). ‘The Telescope in the Seventeenth Century’, Isis 65, no. 1, 38-58.Van Helden, A. (2003). ‘Galileo, Telescopic Astronomy, and the Copernican System’, in R. Taton and C. Wilson (eds.) Planetary astronomy from the Renaissance to the rise of astrophysics. Part A: Tycho Brahe to Newton. Cambridge: Cambridge University Press. Series: The General History of Astronomy, pp. 81-105.
Wilson, F. (2001). ‘Galileo’s Lunar Observations: Do they imply the rejection of traditional lunar theory?’, Studies in History and Philosophy of Science, 32 no. 3, 557-570.
Winkler, M. G. and Van Helden, A. (1992). ‘Representing the Heavens: Galileo and Visual Astronomy’, Isis 83, no. 2, 195-217.
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