|Galilean Telescope Homepage||Site Map|
This page gives links to other sites dealing with Galileo and his telescopes, describes Galileo's astronomical publications and how to find them on the internet, and gives an overview of existing knowledge about the nature and optical quality of Galileo's telescopes. The page is organized as follows:
Note: all links to external sites given on this page are set to open in separate windows. To return to this page simply close the windows displaying the external sites.
Institute and Museum of the History of Science (IMSS: Florence, Italy)
This museum, located near Galileo's final home, contains the world's largest collection of scientific artifacts related to Galileo's life, including the only surviving telescopes and lenses believed to have been produced in his workshop. Their website provides images and descriptive text regarding all of these. Some links to specific images are given below. For scholarly research, their Galileo//Thek@ project, also described below, gives access to innumerable additional resources. See Accessing Galileo's Manuscripts for help on using that site.
The IMSS website includes an on-line exhibit accompanying their display "Galileo's Telescope: The Instrument that Changed the World" (March 4 - December 31, 2008). The exhibit includes discussion of the telescopes attributed to Galileo in the IMSS collection, the nature and quality of their lenses, and photographs supposedly taken through optical replicas of them prepared by the Arcetri Observatory. At least in this latter regard, the on-line exhibit is more than a bit disappointing since it shows only two photos (one of the Moon and one of Saturn), presented without explanation and too small to see any detail. Moreover, the photo of the Moon was clearly not taken through a replica of the IMSS telescopes since it shows the entire Moon in a single view. It remains unclear if photos have ever been taken through the actual telescopes. The next year, the IMSS held an even larger exhibition celebrating 400 years of the telescope: "Galileo. Images of the universe from antiquity to the telescope" (March 13 - August 30, 2009), with many web resources and a published catalog.
National Central Library of Florence (BNCF)
This library has physical custody of many manuscripts and other materials related to Galileo and his life. Most of those items are cataloged, and some of them can be viewed, through their Galileo Digitale website. Navigation is entirely in Italian; so, again, it may be helpful to consult our separate page on Accessing Galileo's Manuscripts for tips on retrieving materials.
The Galileo Project (Rice University)
The largest entry-level Internet resource centered around Galileo Galilei, his time and his discoveries. Descriptions of Galileo's astronomical discoveries will be found under "Observations, experiments, and discoveries" on the Science page. Additional information about Galileo's telescopes and descriptions of modern student-built replicas are given in the Student Projects section. There is a newer version of this website offering most of the same content in a slightly different format; while an older version of the Galileo Project can be found archived at Shippensburg University.
Photos by Shinichi Akiyama
When we began this project in the Fall of 2002, Japanese amateur astronomer (and third-generation owner of an eyeglass manufacturing company in Osaka) Shinichi Akiyama hosted a website showing photos of the Earth's Moon, Saturn and three of Jupiter's moons taken with a digital camera through an optical replica of the 14-power Galilean telescope at the IMSS. Akiyama's photographs slightly pre-date those on the present site, and show considerably more aberration. Unfortunately, his Galilean telescope website has been inaccessible since June 2004. Archived portions of it (particularly his photos of the Moon) can still be viewed by entering the URL in the Internet Archive's WayBack Machine.
Photos by Tiemen Cocquyt
Tiemen Cocquyt, a university student in the Netherlands, has posted photos taken with a 4.3 megapixel digital camera looking through an optical replica of the 20-power Galilean telescope at the IMSS. The subpages of his site show examples of terrestrial objects, the Moon and Jupiter (evidently taken under conditions of poor seeing when they were near the horizon). Cocquyt has not yet been able to fully convey in photos the crispness of the images seen through his telescope. As he says on his introductory page: "contrast and detail are much better when you have a look through the actual telescope."
Photos by Thierry Lepine (l'Ecole Supérieure d'Optique)
Lepine's new website (in French) shows a recently-constructed 70 mm aperture/ 1000 mm focal length Galilean telescope, which he implies cost only 3.6 euros to build. He shows very nice photographs of Jupiter, Saturn and the Sun taken through it, as well as providing links to some other interesting French language websites about Galileo's telescopes. Unfortunately Lepine's photos are shown only in black and white, so it is difficult to compare the amount of chromatic aberration in this much "faster" telescope with that visible in the photographs on the present website. If you do not understand French, you may find the Google language tools page helpful for translating these sites.
Replicas by Jim and Rhoda Morris
The Morrises own a company that makes replicas of historic scientific instruments, both for sale and for use as props in movies and on TV. Their most recent project has been the construction of a meticulously accurate replica of the IMSS's gilded leather 21X telescope (see description below) for display at the Griffith Observatory in Los Angeles. Their website includes many interesting details concerning its construction, as well as a description of their trip to Florence to view the original. For the telescope objective, a stock lens was re-figured to shorten its focal length. Its quality is, therefore, a bit uncertain. The Morrises have posted a "first light" photo of a window 400 feet away as viewed through their telescope. The Morris' 21X replica (as well as some lunar photos taken through our own replica telescope), appeared in an episode of the documentary series Man, Moment, Machine shown on the History Channel in January 2007. On an older webpage the Morrises explain how they created a simple, yet authentic-looking, Renaissance covering for a telescope used as a prop in another television documentary about Galileo. They are also restoring a replica of the IMSS's longer paper-covered 14X telescope for the Adler Planetarium in Chicago.
Telescopes of Joseph T. Arendt
Biomedical engineer Joseph Arendt explains in clear and simple language his experiences building two 26-power Galilean telescopes and shows a drawing of Saturn he made through it. Arendt reports some disappointment with the quality of the images, and some difficulty suppressing reflections from the interior of the PVC pipe he used for the tube. On a separate page he recounts his experiences viewing the sun by eyepiece projection and shows photos of both his setup and the results.
Our knowledge of Galileo's astronomical technique and observations derives from his published books, and what survives of his letters and manuscripts, many of which are accessible via the internet, although usually only in their original languages (Latin or Italian). Both Google and AltaVista (they give identical results) provide free on-line tools with a limited ability to translate these materials from Italian to other languages. The Freelang Dictionary tool with its numerous downloadable word lists may also be useful for decoding texts in both Latin and Italian to English. Additional resources can be found at yourDictionary.com William Whitaker's WORDS program, and its cut-and-paste capable interface for Windows computers by In Rebus, is even more helpful for trying to understand passages is Latin. Although its vocabulary is smaller, Adam MacLean's Latin Parser and Translator is also quite useful, especially for its numerous examples of Latin usage and the nomenclature of Latin grammar. For additional resources on understanding Latin, Kevin Cawley's webpage at the University of Notre Dame is a good starting point.
The story of the fate of Galileo's papers and the attempts to assemble them into a collected edition of his works is told in some detail in the bibliographical note at the end of Fahie (1903). According to that account, by far the largest collection of Galileiana was collected by Galileo's final student, Vincenzo Viviani; whose plans for a grand edition never came to fruition. Much of what Viviani had collected is said to have been dissipated by his heirs, the remaining papers allegedly being sold for scrap by about 1750. It was apparently not until some time later that the Tuscan government made a concerted effort to re-assemble the then scattered manuscripts. The result was more than 300 volumes of manuscript papers, of which 86 are by Galileo and the remainder related to him, deposited at the National Library in Florence.
The definitive collection of Galileo's writings is the monumental 20-volume Italian Edizione Nazionale (National Edition) of his works [Le Opere di Galileo Galilei] edited by Antonio Favaro, published between 1890 and 1909, whose predecessors are described in some detail on the IMSS website. This contains not only almost everything written by Galileo, including most of what survives of his unpublished journals and notebooks, but also many things written about him or referring to him by contemporaneous writers. Favaro is said to have located more than a thousand manuscripts and documents related to Galileo in public and private collections outside Florence. Favaro's Edizione Nazionale supersedes an earlier Le Opere compiled by Eugenio Alberi in 1842-1856, which itself replaced earlier Collected Works, one published as early as 1656. The story of the compilation of Favaro's National Edition of Le Opere is told by Castagnetti and Camerota. A revised and corrected version of Favaro's National Edition was published in 1929-39 under the direction of Giorgio Abetti, in consultation with such luminaries of 20th century Italian science as Enrico Fermi; and it was reissued again in 1964-1966 (perhaps, in part, to remove the imprint of fascist dictator Benito Mussolini on the title pages of the 1929-39 version). The corrections and additions of the later editors were added at the ends of the original volumes, so as not to upset Favaro's volume and page numbering. On this website, we use the term National Edition, rather than Le Opere, to distinguish Favaro's Le Opere from the earlier ones.
The Galileo//Thek@ project, hosted by the Institute and Museum of the History of Science (IMSS), provides free searchable electronic access to the complete Italian National Edition as well as to photographic images of additional documents, links to items in their virtual museum, and an extensive bibliography of books and articles about Galileo. The originals of many of the Galileo-related manuscripts are held in the National Library in Florence (BNCF) and images of many of them may also be displayed via the Galileo Digitale project on their website Since navigation of both of these sites is difficult, especially for non-Italian speakers, those interested may wish to check our tips on Accessing Galileo's Manuscripts.
The complete National Edition of Galileo's works is also freely available on-line (but less readily searchable) through the National Library of France's (BNF's) Gallica service. In addition, Gallica permits free access to the published works of numerous other early astronomers and mathematicians, as well as to scanned copies of many historic European journals. Most of the National Edition is also viewable in the University of Michigan's Digital General Collection.
Within the National Edition, Favaro confined his comments about the material he was editing to a brief explanatory "Avvertimento" (literally, "Warning") at the beginning of most sections. He did, however, write copiously about the results of his years of Galilean research and issued those thoughts as separate publications, now mostly hard to find. In 1992, the University of Padua issued three volumes under the editorship of Lucia Rossetti and Maria Laura Soppelsa. We have not seen these, but they appear to bring together some of Favaro's scattered writings, and perhaps some of the material he chose to omit from the National Edition. Favaro, like Galileo, was a mathematician by training; and like Galileo (and the rest of us) he was not infallible in his interpretation of facts and evidence. For example, in his Avvertimento to the reprinting of Sidereus Nuncius he states (at the start of the second paragraph) that the observations communicated to the world in that book were made between January 7 and March 2, 1610. These are, of course, the dates of the observations of Jupiter's moons reported in Galileo's book. Although Galileo says in the book (published in March) that he is reviewing observations made during the previous two months, Favaro might have pointed out there was reason to suspect the lunar observations were made earlier. For Galileo's famous letter of January 7, 1610 announcing his first observation of three little stars next to Jupiter describes nearly all the other observations reported in Sidereus Nuncius. So it seems likely that in the lunar section Galileo is, for the most part, describing observations made before January 7, 1610 (it is, of course, possible that Favaro did not yet know about this letter, printed in Volume 10, when Volume 3 of the National Edition was issued). Favaro also omitted many manuscripts that seemed of little interest to him (see, for example, the section below on Galileo's artistic skills), and cleaned up others by deleting items that seemed distracting or irrelevant (such as the horoscopes on the sheets of watercolor drawings of the Moon).
(Note: clicking the following numbered links will open copies of Galileo's works using the IMSS's Galileo//Thek@ web service. To understand Galileo//Thek@, please see our separate page on Accessing Manuscripts.)
Galileo's principal astronomical publications were The Starry Messenger [Sidereus Nuncius, 1610; III-17 (manuscript) and III-53 (text)] (a large part of which is a series of nightly drawings of Jupiter and its moons), The Sunspot Letters to Marc Welser [Istoria e Dimostrazioni intorno alle Macchie Solari, 1613; V-72] (containing 38 daily sunspot drawings plus additional observations of Jupiter's moons and comments on the other planets), The Assayer [Il Saggiatore, 1623; VI-199] (in which Galileo primarily argued that comets were a phenomenon caused by the reflection of sunlight off emanations from the Earth's atmosphere, provides a diagram illustrating his observations of Venus, Jupiter and Saturn), and The Dialog Concerning the Two Chief World Systems [Dialogo Sopra i due massimi sistemi del mondo, 1632; VII-23] (full of Galileo's theoretical interpretations of his observations, this is the book that precipitated his final condemnation by the Catholic church). These are all reprinted, often in facsimile form, in the National Edition of Le Opere (the exact Volume-Page numbers are indicated above). In addition, Sidereus Nuncius, Il Saggiatore, and Dialogo are available in electronic book form, complete with all the original figures, at the LiberLiber project in Italy. Also included is the text from several other volumes of Favaro's National Edition, a separate book of 25 letters written by Galileo (including the text of the original sunspot letters), and a number of Galileo's non-astronomical publications. Newer facsimile copies of early print editions of Galileo's works (plus those of many of his contemporaries) as well as selections from earlier editions of Le Opere are viewable via the IMSS's digital library. Access to epresentative digital copies of Galileo's prinicipal works are also neatly organized in the Galileo Portal's Reading Room. High resolution photographic images of selected items from Galileo's texts are also displayed on a web exhibit at the University of Oklahoma, which includes a complete facsimile copy of an early presentation copy of Sidereus Nuncius. Galileo's Moon drawings, as they appear in that volume, are shown in their Lunar Topography exhibit. University of Oklahoma librarian Kerry Magruder has also prepared an excellent and very thorough Timeline and Guided Tour of Galileo's life and published works, summarizing the contents of each work and pointing out a standard English translation. Besides the Galileo's official publications, pirated copies of Sidereus Nuncius with inferior illustrations were printed in other parts of Europe, as may be seen in the on-line Face of the Moon exhibit hosted by the Linda Hall Library of Science in Kansas City. Over the years these pirated editions, the illustrations of which are still occasionally reproduced, have caused considerable confusion in the minds of many readers who imagined these crude drawings were accurate representations of the original engravings.
Apparently almost paranoid about establishing his priority in the field of celestial discovery, Galileo often slipped announcements about his observations into seemingly unrelated publications. For example, in addition to the examples cited above, the opening paragraphs of his Discourse on Bodies Floating in Water [Discorso intorno alle cose che stanno in sull'acqua, 1612; IV-61] mention his discovery of the phases of Venus, the three-bodied appearance of Saturn, and sunspots as well as giving the orbital periods for Jupiter's moons. Galileo's Letter to the Grand Duchess Christina is also often numbered among his astronomical works, although it is primarily an expression of his fervent belief that his observations, although unexpected, were not contradictory to the spirit of the Bible.
The titles of the standard English language translations of Galileo's books are only occasionally self-explanatory, a tradition perhaps started by Thomas Salusbury, whose 1661 translation of Galileo's Dialogo appears in the first volume of a work uninformatively entitled Mathematical collections and translations. The tradition is particularly strong in the hands of the prolific American Galileo scholar Stillman Drake, who has been credited with bringing more of Galileo's works within reach of more English-speaking readers than any other single man. Drake's most complete translation of Galileo's Sidereus Nuncius, for example, is in the oddly titled 1983 Telescopes, Tides and Tactics, where fragments of translation are interspersed with commentary in a Galilean dialog-type fashion. Drake used a similar format in his 1981 Cause, Experiment, and Science, which is primarily a translation of Galileo's Discorso on floating bodies. Drake's Discoveries and Opinions of Galileo contains of his translations of portions of Sidereus Nuncius, Istoria e Dimostrazioni intorno alle Macchie Solari, and Il Saggiatore, as well as the Letter to the Grand Duchess Christina. Il Saggiatore is also translated in his The Controversy on the Comets of 1618. His translation of Galileo's Dialogo appears under the uncharacteristic clear title of Dialogue Concerning the Two Chief World Systems. The later should not, incidentally, be confused with Galileo's final publication, his Discorsi e Dimostrazioni Matematiche, intorno � due nuove scienze, which summarizes his thoughts and discoveries in physics, and was rendered into English as Dialogues Concerning Two New Sciences in the 1914 translation by Henry Crew and Alfonso de Salvio.
To the best of our knowledge, Favaro's National Edition has never been translated into English. Although many of Galileo's individual books have been translated and are readily available in print, internet links to translations of Galileo's astronomical works are rare and may be ephemeral. The University of Oklahoma has a nice PDF version of Edward Stafford Carlos translation of Sidereus Nuncius paginated to match the 1610 publication. Carlos' book can be read on-line in its original format at the Linda Hall Library (not that you Bard College also has a PDF copy. WebExhibits.org has a condensed version of Stillman Drake's translation of Galileo's Dialogo as well as selections from Copernicus' De Revolutionibus (1543). An identical version of Galileo's Dialogo is also available on Douglas O. Linder's Famous Trials website at the University of Missouri. Readers may also be interested in the English language translation of Volume 17 of Giambattista della Porta's Natural Magick, a pre-telescopic work giving some idea of the limited understanding of optical science in the 1590's. Della Porta, a fellow member with Galileo of the elite Florentine Academy of the Lynxes, claimed priority in the invention of the telescope (see Van Helden, 1977), saying it had been described in his books, and accused Galileo of stealing his idea, although Galileo claimed only to have improved on an earlier invention.
Samples from a growing number of the standard English translations of Galileo's works, as well as a number of the other books mentioned on this website, may be viewed via Google Books, or other such services; although viewing most of them is frustrating because of the omitted pages and intentionally awkward navigation. John Ockerbloom's Online Books Page is a good resource for checking for the existence of full free online versions of books in English unencumbered by copyright or viewing restrictions. Ockerbloom currently give links to English language translations of three of Galileo's books: Sidereus Nuncius, Discorso intorno alle cose che stanno in sull'acqua, and Dialogo Sopra i due massimi sistemi del mondo (see his Online Books Page -- he actually give two links for translations of the latter, in addition to the one mentioned in the previous paragraph).
Scholarship regarding Galileo and the early history of the telescope is much too vast to be covered in any detail on this website.
Nonetheless, we must note that innumerable other copies of Galileo's telescopes have been built over the years, often as student projects at colleges and universities. The articles by Edison Pettit, Michael Gainer, William Peters, Reinhart Claus, and Stephen Ringwood, listed below, describe visual observations through replica telescopes, as do the student sites listed above. The article by Elizabeth Cavicchi is an attempt to reproduce Galileo's watercolor paintings of the Moon using a modern 20 power spotting scope.
Despite all this documentation, Galileo is rarely explicit about the instrumentation he used. One of the few exceptions is a January 7, 1610 letter translated by Stillman Drake, which reads like a first draft of Sidereus Nuncius. In it, Galileo says he is describing observations made with a 20 power telescope and "about to finish" one of 30 power. At the beginning of the published version of that book says he will describe the beauty of Moon observed at slightly less than 30 power, but a few sentences later says he constructed an "excellent instrument" that magnified areas "nearly 1000 times" and diameters "more than 30", which has led many to assume he made his first discoveries with a telescope of 32 power. He also says that 20 power is the minimum that would be required to confirm his observations. Many prominent scholars claim that although mentioned in Sidereus Nuncius, Galileo was never able to perfect his higher power telescopes, and that 20 power was his preferred magnification, and perhaps the only one he ever used, for astronomical work. This idea was first put forward by Drake (1976), who says he found references to telescopes of 18, 19 and 20-power in Galileo's notes on the moons of Jupiter from mid-1612 (unfortunately he does not indicate the pages). In his 1990 book, Drake says that although Galileo not only mentions 30-power in Sidereus Nuncius, and demonstrated such an instrument to his friend Sarpi a few days before the book was published, "I have found no evidence that Galileo used it in his astronomical observations." This statement is, it appears, belied by an undated scrap of text, recently noticed by Harald Siebert, on page 878 of Volume 3 of the National Edition, in which Galileo estimates the size of the star Sirius by starting from the observation that its apparent diameter as seen through a 32 power telescope is 1 part in 1200. After correcting for the magnification, Galileo concludes this means that its diameter, without the telescope, would subtend an angle of 1 part in 38,400 or a little over 5 seconds of arc (what a modern astronomer would refer to as the resolution limit of his telescope). Although Galileo's idea that the apparent sizes of stars as seen through a telescope has any relation to their true diameters was erroneous, this calculation seems to indicate not only that he had a working 32 power telescope, but also that it produced quite decent images (see our Galilean Optics Page: 5.4 arc seconds is the smallest dot of light achievable with a one inch aperture). It should be emphasized that even with the 32X magnification, the image of Sirius that Galileo is reporting he saw in the telescope was only about 3 arc minutes in apparent diameter, that is, even for this bright and dazzling star he saw a very small point of light close to the limit of what the eye can resolve, and less bright stars undoubtedly looked even smaller and sharper. In the following (also undated) entry on the same page, Galileo gives the "true" diameter of Sirius as 1 part in 56,000 (about 3.7 arc seconds). This was presumably based on an observation with a telescope of even higher resolution. Also, a part of Galileo's notebook entry from 1617, regarding his observation of the Trapezium stars in Orion, recently noticed by Czech amateur Leos Ondra, suggests he was using 27 power at that time. Although our website telescope was built to 20 power, there does appear to be substantial evidence that Galileo himself occasionally, if not routinely, used higher powers.
As to the light grasp of his telescopes, Galileo claims in Sidereus Nuncius that fifth or sixth magnitude stars, when viewed through his telescope, appear as bright as first magnitude stars do to the unaided eye, perhaps even as bright as Sirius (magnitude -1.5 in modern notation). If this were true, and if one could, with the naked eye, routinely see stars of sixth magnitude or so in the dark skies over 17th century Italy, then through his telescopes Galileo should have been able to see stars as faint as magnitude 11 or 12. But modern scholars do not believe he ever recorded stars much fainter than 8th or 9th magnitude (e.g., Neptune). For example, he appears to have completely missed Saturn's large moon Titan (magnitude 8.5), despite claiming to have looked at that planet "thousands" of times. So Galileo's claim about the light grasp of his telescopes may be slightly exaggerated. Increasing stars from sixth to first magnitude requires an aperture of well over an inch. For further information on the limiting magnitude of Galileo's star observations, see our Photo-Drawing Comparison page.
The exact number of telescopes built by Galileo is not known. Galileo evidently had a workshop in his home for the production and sale of his military compass (a kind of drafting/surveying instrument he had developed in the early 1600's), and when he became involved with telescopes, the workshop was presumably adapted for their production. Most scholars seem to believe he had the equipment necessary to grind his own lenses, or at least to test and modify ones purchased from spectacle makers, although documentary evidence describing the workshop appears to be virually non-existent. A single letter from 1610 mentions a delay in moving to Florence caused by the difficulty of moving some kind of unspecified apparatus connected with building telescopes that was attached to a wall of his Padua home. A number of scholars have concluded this could refer to nothing other than a lens grinding machine, but the basis of that conclusion is unclear. The April 23, 1616 letter from Sagredo, mentioned later, indicating that only 3 out of 300 lenses in a recently purchased lot would be suitable for making telescopes, argues, to the contrary, that the lenses were being purchased at that time. According to Greco et al. (1993), Galileo's letters also indicate that by the time Sidereus Nuncius was published in March, 1610, he had already constructed more than 60 telescopes, most, apparently, intended as gifts.
According to Greco et al.'s interpretation of those letters, Galileo normally supplied only the lenses and a piece of string indicating how far apart they should be mounted. The tube was built locally (by the recipient) to Galileo's specifications (although no document containing Galileo's instructions for building a telescope seems to exist). Of these many telescopes only two survive, a long paper-covered one of 14 power and a shorter leather-covered one of 21 power. These two telescopes, the latter of which is known to have an old but non-original biconvex eyepiece, are lovingly preserved in the Galileo Room at the Museum of the History of Science in Florence (click on "Room IV" and "Room V") together with a broken, yet cherished, 58 mm diameter plano-convex lens (stopped to 38 mm diameter) which, according to legend, was presented to the Duke of Tuscany as the objective (front lens) of the telescope Galileo had used to discover the moons of Jupiter. The museum website lists a number of conflicting values for the diameter of Galileo's large objective. The dimensions given here are those given by Ronchi (1923) and by Greco et al. (1992, 1993).
The evidence that the broken lens once belonged to Galileo seems strong. The other two telescopes may well have been Galileo's, but the tradition that they are seems to rely more on the long repetition of perhaps once dubious claims, rather than on any hard historical documentation. Nonetheless, as pointed out by replica maker Jim Morris these telescopes have a great amount of symbolic and cultural value, not only from their intrinsic beauty, but also from their long association with Galileo. The question of their attribution to Galileo seems first to have been seriously investigated by Antonio Favaro, editor of the National Edition of Galileo's works. His results, which we have not seen, appeared in the Proceedings of the Royal Institute of Venice in 1901. Writing in 1903, J. J. Fahie, a great admirer of Favaro, says that the telescope with which Galileo discovered the moons of Jupiter (which he later called the "old discover") had long been promised to Galileo's former student Grand Duke Cosimo II; but Galileo was unwilling to actually part with it during his lifetime. After Galileo's death, it was turned over to Prince Leopoldo, a brother of the then-current Duke of Tuscany; but, as Fahie says, "of its subsequent history little for certain is known." The instrument was apparently broken at some time, possibly during Galileo's life, for a 1675 inventory of Leopoldo's effects includes a "broken object-glass with which Galileo discovered the four new planets;" and there is a 1677 commission for the ivory frame in which that broken lens is currently displayed. As to the two telescopes displayed with it, Fahie says they are "said to have been made by Galileo and certainly of his time."
Writing in 1990, Stillman Drake, a usually reliable scholar, says that Galileo gave the name "discoverer" to the 20X telescope with which he discovered the moons of Jupiter when he later presented it to the Grand Duke and that he "also asked that it be preserved as it was, though of cardboard and not covered with decorated leather as was a telescope already left with the duke on a visit to Florence." Since no references are given, we do not know if this statement is the result of later research or a somewhat fanciful attempt to account for the paper and leather-covered telescopes in the museum (the broken lens is of much too long a focal length to have ever been used in the surviving cardboard tube).
More recently, the attribution of the two IMSS telescopes to Galileo, including Favaro's findings, has been reviewed in some depth by Carlo Triarico. In addition to his conclusions, Triarico's article contains a bibliography of 85 other references published since 1900 that deal with Galileo's telescopes. As best we can understand, the traditional attribution seems to have begun with a terse entry referring to a "telescope of Galileo" in a 1704 catalog of the items in the Uffizi Gallery. Whether that anonymous clerk meant to indicate a telescope personally belonging to Galileo, or merely one the Galilean type or of his period, and what his identification was based on, we cannot say. Triarico concludes his review by claiming that the connection between the two telescopes and the broken lens was reinforced by the 1992 findings of the INOA research team (see description below), who, he says, by using "advanced technical analysis" found a clear link between the optical properties of the lenses. We are unable to find any such assertion in any publication by the INOA researchers, nor any reason to suspect that such a link should exist, since Galileo appears to have obtained his lenses from a variety of suppliers over an extended period of time. On the contrary, each lens appears to have unique properties. Indeed, the INOA researchers state that they themselves understand the authenticity of the telescope tubes is more certain than the authenticity of their lenses; but they are simply repeating what they have read elsewhere. The stops (covers) over the objective lenses of the IMSS telescopes seem inconsistent with Galileo's description of them as being oval in shape; as do the stops over the eyepieces, which restrict the field of view, and, to the best of our knowledge, are not mentioned in any of Galileo's writings. As explained at the end of the previous section, the present magnifying power of these telescopes is also significantly lower than the 27-32 power known to have been used by Galileo for at least some of his later astronomical observations. As currently configured, the smaller one is consistent with the 18-20 power mentioned by Drake , but since the eyepiece has been replaced, it is very difficult to know what its original power may have been.
As pointed out by historian Silvio Bedini, what is probably the objective end of one presumably authentic Galilean telescope is shown in a portrait of Galileo, believed to have been painted during his lifetime by the Medici court painter Justus Sustermans, and probably later copied by others. The version in the Palatina Gallery of the Palazzo Pitti may be the original. An archival photograph of this painting, including its frame and nameplate, is available in the Iconography database at Galileo//Thek@, where the artist's name is listed as Giusto Suttermans and the date is given as 1640, two years before Galileo's death. A larger version, including Galileo's hands, but showing no more of the body of the telescope, is in the collection of the National Maritime Museum in London. Bedini says that details of the optical stop, the opening of which appears to be surrounded by a decorative twelve-point star, can be seen in the Palatina portrait (Bedini seems to misinterpret this decorative pattern as being the actual shape of the opening, which seems highly unlikely to us). Because Galileo made many telescopes, it is not surprising that although very similar in appearance to the IMSS telescopes, the telescope shown in Galileo's portrait does not seem to match either of those at the IMSS. The outer part of the lens holder, visible even in the reproductions on the internet, shows a series of decorative (or possibly actual) ribs or grooves that only vaguely corresponds to the pattern of stamping on the smaller IMSS telescope. In addition, the lens holder (a pale khaki tan) is a distinctly different color from the body of the telescope (a rich reddish brown, visible most clearly in the Maritime Museum portrait and in the Galileo//Thek@ version of the Palatina portrait). The paper covering of the 14X IMSS telescope (whose power was too low for astronomical observation) is too deteriorated to evaluate its original color, while the 21X version was clearly of a single color.
Bedini notes that many of the earliest artistic depictions of telescopes, including the one held by the right-hand cherub in the frame surrounding Galileo's portrait of himself in the frontispiece to the Academy of the Lynx's 1613 printing of his Sunspot Letters, show the instruments to be distinctly flared or trumpet shaped. While the cherub's telescope has quite likely been artistically altered to resemble Gabriel's trumpet, Galileo's own drawing of his telescope in Sidereus Nuncius indicates a tube clearly tapering from a small eyepiece end to a large objective. Pettit's assembly drawing of a replica of the 14X telescope at the IMSS shows a somewhat similar, but less extreme, tapering. A distinctly tapered tube is also very clearly depicted in a July 1612 letter from the artist Lodovico Cardi (known as Cigoli), illustrating the method he used for projecting images of the Sun.
Aside from the evidence from Galileo's drawings and verbal descriptions of astronomical objects that we present on our Photo-Drawing Comparison page, there are several hints in the literature as to the optical performance of his telescopes. There are a couple of reports, by witnesses to the event, of objects around Venice that could be resolved with the small and low powered telescopes that Galileo demonstrated in the summer of 1609. More relevant is the letter dated March 23, 1612 to Galileo from his artist friend Cigoli mentioning that he (Cigoli) had recently constructed a telescope through which he could see the hands of the clock on St. Peter's Basilica from Santa Maria Maggiore; but that the numbers marking the hours were not nearly as distinct as through the telescope of Galileo's that he had tried (Cigoli was working, at this time, on his famous frescoes in the latter church, which include his large image of the Virgin Mary as the Woman of the Apocalypse standing atop a cratered crescent Moon). Cigoli included with this letter a set of 26 daily sunspot drawings for February and March. These drawings do, indeed, despite Cigoli's great artistic talents, appear poorer than Galileo's; although they precede by many months the highly detailed engravings shown in Galileo's Sunspot Letters. They probably suffer not only from a poorer telescope, but also from the fact that they were made by looking at the Sun through a thick green glass, a much more difficult technique than tracing the outlines of the spots by eyepiece projection, which was the method used for making the engravings shown in the Sunspot Letters. Cigoli's comment about the clock not only indicates that the idea of comparing telescopes with terrestrial test targets was already evident to these early users, but it also provides some quantitative information as to the resolution of those telescopes, if anyone is able to determine the size of the hour markings Cigoli is referring to (it seems unlikely that the clock of 1612 would have survived without change, since the modern idea of regulating clocks by synchronization to a pendulum was a later idea of Galileo himself). Perhaps an interested reader would like to repeat Cigoli's experiment with a modern telescope.
In his letter, Cigoli also describes the smallest features he could distinguish on buildings at Fraschati and Tigoli, whose distances he gives as 10 to 18 miles. Of course looking at terrestrial targets over long distances is highly subject to variations in the state of lighting and atmospheric turbulence; and there is no way of knowing if the lack of distinctness Cigoli noted in his new telescope might have been due to looking at a less favorable time of day. Other reports of objects that could be distinguished through the early telescopes are scattered among the letters in the National Edition; but, most of the targets referred are probably more difficult to identify than the clock on St. Peters.
There is a common misconception that the optical quality of Galileo's telescopes must have been quite low. In fact, optical experts have twice concluded from careful examination of the surviving instruments that, although Galileo's glass suffered from many cosmetic defects, over the small and carefully-selected apertures used, the lenses preserved in Florence should have performed essentially as well as any modern lens of similar design. The first examination was conducted in 1923 by famed solar astronomer and longtime director of the Arcetri Observatory, Giorgio Abetti . Abetti tested the surviving telescopes visually, examining the Sun, the Moon, Jupiter, Saturn and the star Mizar. George Ellery Hale, Director of the Mount Wilson Observatory, who had apparently suggested the idea, was present and participated in part of the visual testing. Abetti concluded that the 14X telescope offered a resolution of about 20 arc-seconds, while the 20X telescope (which actually has a smaller aperture), as well as the large broken lens, gave 10 arc-second resolution. His results were published in L'Universo -- the journal of the Geographic and Military Institute of Florence. Note that after magnification by 20X a 10 arc-second feature becomes 3 arc-minutes - not far above the limit of resolution of the human eye. In other words, Abetti and Hale saw very sharp images through Galileo's telescopes. The Arcetri Observatory, where the tests were performed, is located a short distance from Galileo's final home, and their website has an on-line presentation (in Italian) about the long scientific collaboration between Abetti and Hale. Hale was apparently more impressed than Abetti by the 14X telescope and had a very accurate replica of it made for himself during his 1923 visit. It is described in great detail in the article by Pettit , which is an excellent resource for anyone wanting to build a historically authentic replica of their own. As noted above, Galileo constructed many telescopes during his lifetime, most intended as gifts for presentation to dignitaries. Not having yet had a chance to read the commentaries by Favaro  and Triarico, our understanding is that the surviving 14X and 20X telescopes are thought to be characteristic of the telescopes made by Galileo, but we are unaware of any historical evidence indicating that these telescopes were among the best he made nor that either of these was actually used by Galileo for any of his astronomical observations. The evidence in Galileo's own hand would seem to suggest that he routinely achieved a resolution somewhat exceeding that reported by Abetti and Hale (see our Photo-Drawing Comparison page).
Also in 1923, as part of the evaluation, the two telescopes and the broken lens were carefully measured and optically tested by the famed optical engineer Vasco Ronchi (click on "The Founder") using his newly invented Ronchi test, as well as with the more traditional Foucault test (Ronchi, 1923). He also published photographs of an artificial star (i.e., a point source of light) imaged by the three objectives at a variety of focal positions. Ronchi concluded that all three objective lenses were of excellent quality, with the large broken one being the best of the lot. The surfaces of the eyepiece lenses are not as good, but, like eyeglasses, they are used over only a very small portion of their diameter. Ronchi, who was founding director of the Italian National Institute of Optics in Florence (known, since 2000, as the National Institute of Applied Optics or INOA), developed a lifelong interest in the history of telescopes and over the course of his long career eventually wrote three books and approximately 35 articles related to Galileo and his telescopic discoveries.
In 1992 the two telescopes and broken lens were tested with a state-of-the-art laser interferometer by Vincenzo Greco, Giuseppe Molesini and Franco Quercioli (Greco et al. 1992, 1993), also of the National Institute of Applied Optics in Florence. For those with an understanding of modern optical testing, we give some examples of their results on a separate page. The INOA team concluded that the "instruments allow for nearly diffraction-limited imagery at a single wavelength," and that the real limit on the resolution was the chromatic (color) aberration inherent to singlet lenses. The first article placed this limit at 10-20 arc seconds ; while the second concluded Galileo could not see better than about 20 arc seconds. Giuseppe Molesini also published a more popular article describing the results of the INOA's studies of these and other early Italian telescopes. An English language translation of that article appears in the June 2004 issue of Optics & Photonics News (the monthly newsletter of the Optical Society of America -- copies can be obtained at colleges with OSA accounts). Among the many new insights provided by this very interesting article is mention of an April 23, 1616 letter from GiovanFrancesco Sagredo to Galileo stating that out of a lot of 300 lenses purchased for Galileo from Venetian glass maker Maestro Antonio only three proved suitable for use in his telescopes (for those able to read Italian, this is Letter 236 starting on p. 257 of Vol. 12 of the National Edition; Vol. 12 is also among the texts available in machine-readable form from LiberLiber, where this letter is listed as #1198).
Molesini's recent article seems to repeat the assertion of the 1993 article that chromatic aberration degraded the resolution of Galileo's objectives to "values of the order of 20 arc sec, three times better than that which is conventionally attributed to the naked eye," even though (as to the large objective) "there is no doubt that the quality of the wave front is of the order of 1/8-th wave for a diameter of 38 mm" and "his observational abilities ... in practice were limited only by diffraction." The conclusion regarding a 20 arc sec visual resolution limit, initially based on a somewhat arbitrary theoretical calculation, is said to be supported by a supposed absence of any observations made by Galileo showing finer detail. We, on the contrary, believe there is abundant evidence indicating Galileo saw finer detail, and are not surprised by this since our experience with the replica telescope indicates the effects of chromatic aberration are hardly noticeable to the eye.
A telescope, used visually, is only as good as the observer and his ability to understand the images he sees through it. Galileo obviously excelled at interpreting the images he saw. Somewhat surprisingly, however, given the detailed drawings he made based on the very small images seen in a 20-30X telescope, his eyesight, even in his youth, was apparently less than perfect. Galileo is said to have suffered from bouts of severe ophthalmia (inflammation of the eyes) even as a child of 12 or 13 years age (Fahie, p. 5). In Il Saggiatore (The Assayer) he mentions a malady that caused bright lights to appear surrounded by colored halos and rays so intense as to prevent him from seeing objects behind them. He says his eyes never totally recovered. And in his Dialogo (as well as in Il Saggiatore) he explains that when one suffers from such a problem, the effect of the haloes can be alleviated by looking through the tiny aperture formed by the fingers of a clenched fist. There is little further mention of Galileo's eye problems until about 1637, when symptoms associated with the onset of blindness become a frequent topic of his letters. In a letter to his friend Elia Diodati, that he dictated to his assistant Viviani on July 4, 1637 (at age 73), Galileo complained that he had recently become completely blind in the good right eye that he had used for all his famous discoveries, and that although some vision remained in the other, it had never been of much use. By the next year he had become completely blind in the left eye as well. According to Fahie (1903) "it is certain that Galileo's blindness was due to glaucoma," but we do not know the basis of that statement.
Dava Sobel, on page 85 of her popular book Galileo's Daughter, advances the idea that the episode of eye trouble recounted in The Assayer may have resulted from an infection Galileo contracted through sharing the eyepiece of his telescope with others. However, Galileo scholar Stillman Drake (the translator of the passage Sobel quotes) places the episode much earlier. Drake believes Galileo is describing an intermittent condition he had suffered from youth. In fact Drake believes it is Galileo's prior experience with visual haloes and how to alleviate them that led him, when he began his telescopic observations at age 45, to hit upon the idea of placing stops over the objective lenses (see, for example, Drake, 1978, page 148). As it turns out, the lenses available to Galileo were not well figured around the edges, and stopping them down may have materially improved the sharpness and contrast of the images he saw. Drake believes Galileo would have been unable to discover the moons of Jupiter without them.
It is sometimes claimed that Galileo's blindness resulted from, or was in some way connected with, observing the Sun through his telescope. Few modern scholars believe this. Not only did Galileo's blindness begin long after his sunspot observations, but nearly all of those observations were made by looking at an image of the Sun projected by the telescope onto a piece of paper; a completely safe technique discovered by his student Benedetto Castelli, and probably, independently, by others. Even when looking directly at the Sun through a Galilean telescope (a technique we do not recommend, except through a thick haze as at sunrise or sunset), the light is not concentrated into nearly as intense a patch as is the case with a modern refractor using a positive (Keplerian) eyepiece. For more about the cause of Galileo's blindness and its relation (if any) to his sunspot observations, see the webpage by Andrew Young.
As pointed out by Sobel (page 355) and others, even as he was being overtaken by blindness, Galileo seems to have remained an active and interested observer, and as late as the fall of 1637 reported his discovery of the librations of the Moon, an apparent rocking motion in which the Moon's visible features move slowly from side to side and up and down. In truth, however, this is an observation that more than anything else requires accurate records showing how the Moon looks on different days, and a firm belief in the accuracy of those records. The magnitude of the rocking is actually large enough that it should have been noticeable to a keen-sighted observer even in pre-telescopic days. To detect the librations through a telescope does not require sharp eyesight.
The final element involved in visual astronomy is the ability to put one's observations on paper. Galileo prided himself on his artistic abilities, although the results have been both ridiculed and praised by modern scholars. Much of the criticism has probably been generated by the crude woodcut illustrations that appeared in pirated editions of Sidereus Nuncius (see the Linda Hall Library's web exhibit).
According to Galileo's assistant and first biographer, Vincenzio Viviani, Galileo, whose father was a prominent musician, wished to become a painter in his youth, but was overruled. He was in fact elected to the fledgling Florentine Accademia del Disegno (the society of professional artists in whose building the original of Michelangelo's David is currently housed) in October 1613; albeit probably more for his astronomical fame and contributions to the science of mathematical perspective than for his output as a scientific illustrator.
Horst Bredekamp has published images of a number of pages from the many manuscripts at the National Library in Florence that have not yet been, but hopefully eventually will be, placed on-line as part of their Galileo Digitale project. Bredekamp shows examples from folios Gal. 46, 48, 50, and 57. These include pages of calculations on which Galileo doodled sketches of designs, landscapes and human and animal figures, many executed with a very considerable artistic flair.
Note: there seems to be a problem with Bredekamp's citation for his Figure 10, a Rembrandt-like ink sketch by Galileo of what is perhaps the river Arno with the rooftops of Florence emerging from a sea of fog in the distance. Bredekamp says it appears on the reverse side of page 54 in Gal. 48. But Gal. 48 is the only folio he mentions that is currently on-line, and page 54 is a blank page at the end of an Italian translation of Sidereus Nuncius by Viviani.
For those interested in building replica telescopes of their own, the articles by Greco et al. (1992 and 1993) and Ronchi give detailed specifications of all the lenses used in Galileo's telescopes; while that by Pettit gives historically accurate mechanical details of the 14X scope. We give a parts list for the website telescope and suggest commercially-available singlet lenses that can be used for constructing similar replica telescopes under How to Build a Galilean Telescope.
We are confident that a telescope constructed from modern singlet lenses gives a reasonable impression of the images that must have been seen by Galileo through the instruments used for his epoch-making astronomical discoveries. Our photographs of terrestrial objects taken with the 23 mm aperture reveal, after some enhancement, detail very close to the Rayleigh limit of 6 arc-seconds -- somewhat better than the visual estimates on astronomical subjects of Abetti and Hale. It should be noted, however, that at 20X the images presented to the eye appear much smaller than do most of the photographs as shown on this website. That Galileo was able so accurately to interpret what he was seeing in those tiny images with no foreknowledge of the objects he was examining is truly remarkable.
The technology of Galileo's day appears to have been incapable of producing diffraction limited lens surfaces over diameters of more than a little over an inch, and his ability to further magnify his tiny astronomical images was limited by a fundamental problem with his negative eyepiece design: the angular diameter visible on the astronomical target is roughly the same as the apparent diameter of the observer's eye as seen from the objective (see the field of view diagram on the separate Galilean Optics Page). Looking through our 20 power model one sees an excellent magnified image, but the visual sensation is much like peeping through a long hollow tube: the illuminated patch of light representing about 14 arc-minutes on the source is only 20x14 arc-min = 4.7 degrees in apparent diameter. If we doubled the focal length of the objective (and telescope tube), we would see roughly the same 4.7 degree patch of light, but it would now represent only 7 arc-minutes on the target at 40 power. With higher power and a smaller field of view, pointing would be very difficult. Doubling the power of the eyepiece (by replacing the 50 mm negative lens with a 25 mm focal length one) would double the apparent size of the images without reducing the amount of the target visible at any one time, but it would probably necessitate using a smaller diameter eyepiece (since the curve would be very steep), and this would restrict the observer's ability to see different fields by moving his eye around the lens. Again, pointing would become very difficult. The two classic articles by Albert Van Helden (1974) and 1977)) are excellent introductions to the technological challenges facing the makers of telescopes of Galileo's era, with abundant references to original sources; as is, of course, the earlier well-known book by Henry King. Van Helden's 1977 article is particularly valuable for reprinting both the original text and a careful translation of many early documents related to its interesting topic. Van Helden's later thoughts on Galileo's early telescopes and his observations with them may be found in the introduction and notes to his translation of Sidereus Nuncius (Van Helden, 1989).
Despite these limitations, the Galilean telescope continues in use to the present day, in laser beam expanders (especially for high-wattage lasers where it is undesirable to allow the beam to come to a sharp focus), in inexpensive "opera glasses," in low-vision magnifiers and in the similar magnifiers sometimes worn by dentists and surgeons (since it remains the simplest and most compact way to obtain an erect magnified image of a distant object). In some of these applications, however, particularly in so-called Galilean binoculars, a modern achromatic objective may replace the simple plano-convex lens of the original Galilean design.
The following is a list of references cited on this webpage.
Additional references will be found on other pages of this website. As might be expected, even when those are included, the references given here are only a tiny sample of the countless articles written about Galileo's telescopes. The IMSS maintains a world-class bibliography of print references related to all aspects of Galileo's life and work on their Galileo//Thek@ website, although not all articles dealing with Galileo and his telescopes can be found listed even there.
Cambridge University's Galileo Pages, the PBS/ NOVA special Galileo's Battle for the Heavens, and the Profile of Galileo in the High Altitude Observatory's History of Solar Physics. All of these sites give additional print references. The Space Telescope Institute hosts an interesting elementary level history of telescopes. A special section is devoted to the theory and operation of the Galilean telescope.
Finally, with Google one can easily find further examples of replicas of Galileo's historic telescopes and many additional references to his astronomical observations.
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Last modified: December 8, 2010