Galileo’s miraculous year: 1609 and the revolutionary telescope Introduction

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Galileo’s miraculous year: 1609 and the revolutionary telescope

Galileo’s miraculous year: 1609 and the revolutionary telescope


In 1987, when I was a postdoc at Caltech, I visited the Caltech sister laboratory just up the road which was having an open day. That sister laboratory was the Jet propulsion Laboratory and a highlight of the visit for me was seeing the Galileo spacecraft being assembled in the huge clean rooms. The atoms of that spacecraft are now dispersed somewhere in the atmosphere of Jupiter following entry into the Jovian atmosphere in 2003 after a 14 year mission which included 8 years orbiting and observing Jupiter.

Naming the spacecraft after Galileo, the man, commemorates the revolutionary discoveries made by Galileo 400 years ago this year that set humanity on a new path of understanding. As a consequence of this anniversary, 2009 has been declared the International Year of Astronomy1 by the General Assembly of the United Nations.

Galileo’s discovery of the moons of Jupiter were only one part of an avalanche of remarkable discoveries made by a telescope, simple in concept but masterful in construction and application, that overturned widely held views of Earth’s place in the cosmos. In part, Galileo’s discoveries were made possible by a new way of thinking that represented a turn away from received wisdom and towards discovering and observing directly from nature. In this, Galileo stands at the boundary between the medieval world and the modern world.

This short article reviews the great discoveries made in the first few years of the astronomical telescope2 starting in the last few months of 1609. Remarkably, the notes from Galileo’s observations reveal he unwittingly observed the planet Neptune 234 years before its official discovery. As I will discuss, evidence that Galileo realised he had seen a new planet could still be hidden deep in his notebooks.

The Discoveries

Galileo Galilei was born in 1564 in Pisa. He had a position as a professor of mathematics in Pisa before moving to the University of Padua in 1592. There, in late 1608 or mid 1609, he heard3 of a “spyglass” invented in the Dutch republic. He quickly designed his own version from first principles and by November of 1609 was making observations of the night sky. After clearing up any doubt that what he was seeing was real and not artefacts in the glass of the lenses, he published his first comprehensive book of astronomical observations in March of 1610. Although Galileo is not the first to observe the sky, he is the first to publish.

So great was the interest in his work (rumours must have already been spreading widely prior to 1610) that Sir Henry Wotton, English Ambassador to the Republic of Venice obtained a copy and sent it to the King of England, on the day of publication, with a cover letter4 stating that the author “runneth a fortune to be either exceedingly famous or exceedingly ridiculous”. The book was “Sidereus Nuncius”5 which can be translated as “Starry Messenger”. It was published in the lingua franca of science in the 17th Century: Latin.

This slim tome contains an astonishing list of discoveries. The Moon was found to have dramatic surface texture including mountains and craters that cast long shadows if the phase was right. Familiar constellations were found to have vast numbers of new stars invisible to the unaided eye but clearly seen through the telescope. More than 34 stars were visible in the constellation of the Pleiades, many more than the nine or so visible to the unaided eye6. Even more astonishing was the report that Jupiter was seen to have four tiny moons in orbit around it. These four moons were discovered between January 10 and January 16 of 16107. Galileo named them the “Medicean Stars” in hope of getting financial support from Cosimo Il de’Medici, the Grand Duke of Tuscany. The discovery of moons orbiting Jupiter was very radical because this was the first time objects had been observed orbiting a planet other than Earth.

After publication of Sidereus Nuncius in March 1610 Galileo continued to make discoveries. He observed that there was something odd about Saturn. At the limit of resolution of his telescope, he saw two “lobes” sticking out each side of the planet. He established the primacy of his discovery by sending an anagram to Kepler shortly after July 25 1610 which read:


This can be rearranged to read:

“Altissimum planetam tergeminum observavi”

This translates to8:

“I have observed the highest of the planets [Saturn] three-formed”

Much later, in 1616, these lobes were observed to vanish, until the idea of thin rings around the planet explained these puzzling observations.

In September of 1610 was the discovery of the astonishing phases of Venus9. Although Venus just looks like a very bright star to the unaided eye, telescopic observations showed it to exhibit phases like the Moon. Galileo again established the primacy of his discovery by sending an anagram (in Latin) to Kepler10:

“Haec immatura a me jam frustra leguntur oy”11

This may be rearranged to read:

“Cynthiae figuras aemulatur mater amorum”

Which can be translated to:

"The mother of love imitates the shape of Cynthia"

Sadly, we no longer report important scientific discovers in such poetical terms. The variations of the visible phase of Venus and the correlations with the observed size of the planet in the eyepiece of Galileo’s telescope could only be explained if Venus orbited the Sun and not Earth.

Later, he discovered sunspots12 (as had many other people by then) and began demonstrating them to other people in 1611.

The Telescope

All this was made possible by a telescope of unprecedented power and quality, fabricated by Galileo’s own hand. Indeed Galileo’s telescope was of such high quality that for more than 20 years after 1609 he had a monopoly on the supply of high quality astronomical telescopes. His access to the products of the great glass industries of Venice, his knowledge of optics and his high manual skills were the key factors in his success. The key innovation in his astronomical telescope was a high-precision plano-convex objective lens which was combined with a plano-concave objective lens in the now immortal “Galilean telescope configuration”. Convex lenses were already in mass production to combat presbyopia13 (from the Greek for “old person”). Likewise concave lenses to combat myopia14 were also in use. It was Galileo’s genius that saw the potential of combining these lenses, with carefully selected focal lengths, to make a workable astronomical telescope.

The actual objective used in some of the important discoveries is on display15 at the Museum of Science in Florence, which I had the pleasure of visiting in December of 2008. In fact I was obliged to purchase a set of +1.5 Dioptre spectacles from a supermarket in Florence to allow me to read the fine print in the museum guidebook.

When looking at Galileo’s original objective lens in the museum display cabinet, now unfortunately broken in several pieces, I was amazed that so slight a piece of glass could have been responsible for so many revolutionary discoveries. Galileo’s telescope incorporated many innovations, not just the lenses, as revealed by people today seeking to make replicas as close as possible to the original16.

Then, as now, Galileo had to keep a close eye on the financial support for his research. So in a letter to the Duke of Tuscany, a potential funding agency, he wrote17:

“Most Serene Prince. Galileo Galilei most humbly prostrates himself before Your Highness, watching carefully, and with all spirit of willingness, not only to satisfy what concerns the reading of mathematics in the study of Padua, but to write of having decided to present to Your Highness a telescope ("Occhiale") that will be a great help in maritime and land enterprises. I assure you I shall keep this new invention a great secret and show it only to Your Highness. The telescope was made for the most accurate study of distances. This telescope has the advantage of discovering the ships of the enemy two hours before they can be seen with the natural vision and to distinguish the number and quality of the ships and to judge their strength and be ready to chase them, to fight them, or to flee from them; or, in the open country to see all details and to distinguish every movement and preparation.”

He tried a number of different combinations of lenses, aiming for the highest possible magnification with the sharpest view. The most successful combination appeared to consist of a hand-ground plano-convex objective with a focal length of 980 mm and a diameter of 37 mm teamed with a plano-convex eyepiece with a focal length of -47.5 mm and a diameter of 22 mm. In use, the objective was stopped down to 12 – 25 mm to limit aberrations and the combination gives a magnification of about 20. The magnification is handy, but the benefits of collecting light through a 25 mm diameter lens and focussing it through the 6 mm diameter of a human iris should not be overlooked! This telescope allowed Galileo to see objects with a brightness down to about 9th magnitude compared to the unaided eye which can see down to about 6th magnitude. For comparison, the moons of Jupiter have a brightness between 5th and 6th magnitude.

A significant difficulty with the Galilean telescope for astronomy is that the field of view is very narrow. For example the 20 magnification telescope has a field of view of only about 0.13o which is about one quarter of the full Moon. This would have made it difficult to get an overview of the Moon, to say nothing of the need to constantly tweak the telescope to compensate for the rotation of the Earth!

The Moons of Jupiter and the heliocentric model

Following his discovery of the four Galilean moons of Jupiter from January 7 – 10 of 1610, Galileo spent many years tracking their orbits. Page after page of his notebooks records his meticulous observations18. Reproductions of most of these notebooks are now available online at the Institute and Museum of the History of Science in Italy19,20.

By 1612 he was using a fully quantitative and precise technique, now lost, to measure the distance of each of the four moons from the centre of Jupiter in units of the diameter of Jupiter. He also took great care to record “fixed stars” that drifted through the field of view during his observations as he tracked Jupiter’s movement across the sky.

The discovery of objects orbiting a planet other than Earth created severe difficulties for one of the primary justifications of the geocentric model of the solar system that required everything to orbit the Earth. The observed variations in the phases of Venus could really only be explained by the heliocentric model. But the most salient feature of the heliocentric model was that the Earth moved relative to the Sun AND the stars. Therefore stars, if assumed to be at varying distances from the Earth, should exhibit parallax as the Earth changes position in its orbit over the course of a year. Galileo was well aware of this feature of the heliocentric model and made several attempts to detect this parallax.

He first had to know the distances to the stars to calculate the magnitude of the expected parallax. He made the very reasonable assumption that stars were suns like our own and therefore that the size and brightness of the star disk observed in his telescope could be used to estimate their distance. In the case of the double star Mizar he estimated that the two stars were 300 and 450 Astronomical Units (A.U.) distant from the Earth, with the consequence that the annual parallax should be significantly larger than their angular separation21,22 that he observed in January 1617. Unfortunately his assumptions about the distances to the stars were significantly underestimated. Wave optics was unknown to Galileo and so he did not recognise that the star diameters he measured were due to diffraction through the collimators of his lenses (and other effects), not the true magnified star diameters.

Over the years, the two stars of Mizar did not show any parallax that Galileo could observe. Of course no such parallax of Mizar, true distance around 5 million A.U., or any other closely spaced pair of stars, could be observed with Galileo’s telescope or any other technology before the 18th century. Indeed it was the observation of stellar aberration23 by James Bradley in 1725 that first detected Earth’s orbit around the Sun. This is an effect that arises from the annual change in direction of Earth’s velocity vector that produces a shift in the positions of the stars an order of magnitude larger (and 3 months out of phase) compared to stellar parallax.

Galileo’s failure to observe stellar parallax must have puzzled him. Especially as critics of the heliocentric model demanded this hard evidence24 before abandoning the geocentric model. Perhaps he realised there was something wrong with his assumptions? In any case his observations of Mizar and other closely spaced stars do not appear to have made it out of his notebooks and into any of his publications.

Galileo’s Observations of Neptune

Just about all of the “fixed stars” Galileo records in his notebooks while observing Jupiter appear in modern star catalogues. However one of those “fixed stars”, seen in December 1612 and January 1613 does not appear in any star catalogue. This particular “fixed star” turns out to be something entirely different: Galileo was actually observing the planet Neptune. These observations were made 234 years earlier than the official discovery of Neptune in 184625. It is remarkable that Neptune has yet (in 2009) to complete one orbit around the Sun since its official discovery, because its orbital period is 165 years. The first orbit will therefore be completed in 2011.

The story of Galileo’s observations of Neptune is remarkable, and is a striking example of his skill and care at making quantitative observations with very simple apparatus that has stood the test of time. Galileo’s observations of Neptune were discovered by Kowal and reported in very interesting articles in the journals Nature26 and Scientific American27 in 1980. Kowal also provides a commentary on the circumstances of the discovery and the aftermath in a short essay28 posted on the web site of DIO: The International Journal of Scientific History in 2008.

Galileo’s notes show he made several observations of the planet Neptune in December of 1612 and January of 1613. His uses the label “fixa” where he plotted the position of Neptune in his notebook, indicating, at least initially, that he believed he was observing a fixed star and not a planet.

Even more remarkable is that Galileo’s notes of January 28 1613 suggest he saw Neptune move when it passed in close conjunction to an actual star. Yet it appears he did not follow up this observation, and no further entries in his notebooks have been identified that suggest Galileo was aware of the possibility of a new planet. If Galileo had used his observations to propose the discovery of a new planet, it would have been the first time a planet had been discovered by humanity since deep antiquity, and would be without precedent in recorded history.


The first two of three observations of Neptune identified by Kowal were both made on December 28 1612. The third observation was on January 28 1613. On January 4 1613, in between these two dates, Neptune was actually occulted by Jupiter.

The reason why Neptune was visible in close proximity to Jupiter over a time span of a month was because Jupiter and Neptune executed a retrograde loop (direction reversal) on January 13 1613 as the Earth overtook them in orbit. This maximises the amount of time they are visible in close proximity in the sky.

frame2A further article by Standish and Nobili29 report a possible additional observation of Neptune that is represented by an unlabelled mark in Galileo’s notes from January 6 1613. This mark, which is not reproduced in images of Galileo’s notebooks posted on the web, is clearly seen to be a deliberate record of an observation because of the physical evidence of the page itself. This reveals, as reported by Standish and Nobili, a dimple in the page made by the deliberate press of the nib of an ink pen into the paper.

As pointed out by Kowal and Drake, the observations made by Galileo on January 28 are quite remarkable. First, Neptune appeared in close proximity to an actual star. Second, although the actual star could be plotted in Galileo’s notebook on the same page as Jupiter and its satellites, Neptune could not because it lay further from Jupiter beyond the position of the actual star. So Galileo included an inset drawing of the actual star with Neptune included as well.

Galileo’s notes on his observations, as pointed out by Kowal and Drake, indicate that Galileo recalled seeing both Neptune and the actual star the previous night, but he did not record them in his notebook. However he notes on his January 28 observations that “After the fixed star a, another was following in the same line in the same way as b did, which was also observed on the night before; but they seemed to be further away relative to each other”30 (here a is an actual star and b is Neptune). Kowal and Drake point out that from January 27 to January 28 Neptune would have moved 2.5 Jovian radii closer to the actual star. Clearly Galileo had seen this motion.


The absence of follow-up observations is puzzling. Kowal and Drake speculate that bad weather or difficulties relocating Neptune once it moved out of the field of view when the telescope was trained on Jupiter prevented further observations. There is no evidence yet found that Galileo formed the hypothesis that he had seen a new planet on the nights of January 27 and 28.

However, I suggest the unlabelled mark on January 6 might be a retrospective record made by Galileo after he made the remarkable observation on January 28. Given he depended only on his memory of the observations made, but not recorded, on January 27 to identify the motion of Neptune (b) relative to the fixed star (a), then it is possible the mark on January 6 was made from memory AFTER he made the observations on January 28. If so, this would suggest he did indeed form the hypothesis that he had seen a new planet which had moved right across the field of view during his observations of Jupiter over the month of January 1613.

Therefore it would be very interesting to see if trace element analysis of the unlabelled spot from January 6 could identify the date on which it was recorded. January 6 or January 28? If the latter, I would propose this could construe evidence that Galileo was thinking about the possibility he had discovered a new planet.

Trace Element Analysis of Galileo’s Inks

This proposal arises from prior use of trace element analysis of Galileo’s inks to establish the dates of some of his undated writings. The National Institute of Nuclear Physics at the University of Florence has had considerable experience in the analysis of Galileo’s inks using the technique of Proton Induced X-ray Emission (PIXE). Manuscripts from the years 1600, 1605-09, 1617 and 1636 have been analysed31. The analysis covered both the background parchment and the inks employed in the writing. On the PIXE evidence, the relative concentration of K, Ca, Fe, Cu, Zn and Pb allow, in some cases, identification of the date at which a document was written to a precision of three months (see for example a case study in the development of Galileo’s theories of mechanics32).

The challenge of using trace elements to identify the date of the unlabelled spot on the page for January 6, 1613, is much greater than using trace elements to date a manuscript. This is because when two manuscripts are compared, the entire surface area of each manuscript can be used for comparison. Here we seek only to compare the composition with a single spot to the composition of the writing ink in the remainder of the manuscript. However, the Florence group have identified large variations in the composition of the ink in a single manuscript33. These variations which are larger than the accuracy of the PIXE measurements themselves, suggest real variations in the ink composition are responsible.

It would be very interesting to see if the ink composition could be employed to link the unlabelled dot of January 6, 1613, with the ink used on January 28, 1613. If such a link could be established, this could be interpreted that Galileo understood he was seeing something unusual that was, perhaps, a new planet.

But I would suggest another intriguing possibility presents itself. Galileo’s habit of sending cryptic anagrams to his correspondents to establish the primacy of his discoveries has already been explained here. It is therefore possible that there remains, undiscovered in the Galileo literature, an anagram put there by Galileo to establish the date of his discovery of Neptune. However, as yet no such anagram has been uncovered. Perhaps there is such an anagram hidden in his notebooks or in his voluminous correspondence revealing that he considered the possibility he had discovered a new planet. This would indeed be an even more remarkable addition the already impressive list of discoveries that make the 400th anniversary of Galileo’s telescope worth celebrating!

Obviam Valens Tamen Pavor34


2 See F. Watson, Stargazer: the life and times of the telescope for an entertaining history of the instrument

3 A. Carugo, Galileo in Venice, Microchemical Journal 79 (2005) 7-14.

4 M.B. Hall, The Scientific Renaissance 1450-1630, Dover 1994 p 320.







11 Translation: "This was already tried by me in vain too early" from










21 C.M. Graney, Letter to the editor of Sky and Telescope concerning Galileo’s observations of Mizar, May 2006.

22 C.M. Graney, On the accuracy of Galileo’s Observations, Baltic Astronomy, 16 (2007) 443-449.


24 Cardinal Bellarmine:


26 C.T. Kowal and S. Drake, Galileo’s observations of Neptune, Nature 287 25 Sept 1980 pp 311-313

27 S. Drake and C. T. Kowal, Galileo’s Sighting of Neptune, Scientific American Dec 1980 pp 52-59.

28 C.T. Kowal’s history of his discovery of Galileo’s observations of Neptune: (Dio: The International Journal of Scientific History, Vol 15, December 2008)

29 E. M. Standish and A. M. Nobili, Galileo’s Observations of Neptune, Baltic Astronomy 6 1977 pp 97-104

30 Translation by Sonya Wurster, University of Melbourne 2009

31 L. Giuntini, F.Lucarelli, P.A. Mando, W. Hooper, P.H. Barker, Galileo’s writings: chronology by PIXE, Nuclear Instruments and Methods in Physics B 95 (1995) 389-391.

32 F. Lucarelli, P.A. Mando, Recent applications to the study of ancient inks with the Florence external-PIXE facility, Nuclear Instruments and Methods in Physics B 109/110 (1996) 644-652.

33 P. Del Carmine, L. Giutini, W. Hooper, F. Lucarelli, P.A. Mando, Further results from PIXE analysis of inks in Galileo’s notes on motion, Nuclear Instruments and Methods in Physics B 113 (1996) 354-358.

34 Anagram by David Jamieson 2009, translation of the anagram by Dr Alberto Cimmino: “I Boldly Follow The Path Even Though Scared”.

D.N. Jamieson – 13 May 2009

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