Emmanuel Poulle; Deutsche Gesellschaft für Chronometrie.
|描述：||272 S. : zahlr. Ill., graph. Darst.|
|丛书名：||Deutsche Gesellschaft für Chronometrie; Jahresschrift|
|责任：||Emmanuel Poulle ...[Hrsg.:] Deutsche Gesellschaft für Chronometrie.|
Ptolemy’s Course of the Planets Displayed by Clockwork
Ptolemy’s Course of the Planets Displayed by Clockwork
A Masterwork of Astronomy and Technology of the Renaissance created by Eberhard Baldewein...
Ptolemy’s Course of the Planets Displayed by Clockwork
A Masterwork of Astronomy and Technology of the Renaissance created by Eberhard Baldewein 1563-1568
An extended Bookreview and Synopsis by Fortunat Mueller-Maerki (Sussex, NJ, USA)
Die Planetenlaufuhr –Ein Meisterwek der Astronomie und Technik der Renaissance geschaffen von Eberhard Baldewein 1563-1568. By Emanuel Poulle, Helmut Sändig, Joachim Schardin & Lothar Hasselmeyer. Published 2008 as ‘Jahresschrift 2008 – Band 47’ of the Deutsche Gesellschaft für Chronometie, Nurnberg, Germany. ISBN 978-3-89870-548-6. Hardcover, 272 pages. With a Welcome by Peter Plaßmeyer, an Explanation by Lothar Hasselmeyer, and an Introduction by Emanuel Poulle, 15 Chapters, Appendix, Bibliography, extensive Endnotes, and Image Credits. Illustrated with 11 full-page color plates, and 255 figures (mostly color photographs) in the text. Availabe for Euro 40, plus postage, from Deutsche Gesellschaft für Chronometrie, Gewerbemuseumsplatz 2, D90403 Nurnberg (Germany) (Website:www.dg-chrono.de ).
Fig. 1: The ‘Dresden Planetenlaufuhr’ by Eberhard Baldewein, made around 1565 for Duke August of Saxony. Mars/Astrolabe side, with small timedial (minute hand and day of week disc).<h1>Clocks featuring planetary displays, i.e. showing the position of the planets in real time, have always been among the most coveted timepieces, both for their use as scientific demonstration pieces, and as showpieces of ultra-complex mechanics. Even today, accurate orreries are difficult enough to design and build; but in the era before Copernicus’ heliocentric model of the solar system was accepted the difficulties were greater by orders of magnitude. The apparent retrograde movement of the planets (following the then accepted astronomic theory of Ptolemy with its epicyles) is difficult to represent in a mechanical model.</h1>
The most widely known mechanism of this kind undoubtedly is the ‘Astrarium’ which Giovanni de Dondi of Padua (Italy) built in the 1350s. Although that artifact was lost around 1530, copies of the illustrated, detailed construction notes recorded by the creator survive to the present. Several replicas were built in the second half of the 20<sup>th</sup> century and now serve as focal points of the early horology sections of a handful of prominent museums, including the newest design by Hank Gipmanns, to be unveiled in August 2009 at the Carillionmuseum in Asten, the Netherlands. Dondi’s notes were transcribed, translated and published in several different languages. The 1974 edition by Baillie, Lloyd and Ward, published by the Antiquarian Horological Society as Monograph No. 9 is probably the most widely known English language publication discussing such geo-centric planetary clocks.
While Dondi’s ‘Astrarium’ was probably the first clockwork-driven mechanical model of the motion of the planets according to the theory of Ptolemy, it certainly was not the only one, and it was designed to show the principle, rather than to model the epicyclical motion on a ongoing basis in real time. In other words: it was not much of a timekeeper. In the 1400s and throughout the 1500s certainly a number of them were built; there exists reliable documentary evidence on about 10 of them. It appears that only five planetary clocks in the Polemic tradition have survived to the present. (The sixth one, the public planetary clock by Giovan-Paolo Rainieri in the Piazza San Marco in Venice, has been so much modified that it has virtually lost its original planetary functions). Two of the five (the Philipp Immser (Strasburg) clock of 1557, now at the Technical Museum in Vienna, and the Georg Kostenbader (Strasburg), clock of 1588, now at the Gaasbeek Castle near Brussels, the latter now missing part of its internal gearing) hide the nature of the epicyclical motion behind the dial. The three others are based on the tradition of the unmechanized planetary position calculators based on the ‘epicycle on deferent’ theory of Campanus of Novarra, as published in 1260.
Of these three the French example by Oronce Fine, Paris, 1533 (possibly based on the work of Regomontanus in Nurnberg, today in the St.Geneviève Library in Paris) is the oldest one. The two others were designed by Eberhard Baldewein, the genius mechanic at the court of Wilhelm IV in Kassel (Germany) in the mid 16<sup>th</sup>-century, and built with the help of clockmaker Hans Bucher, while the cases are mainly the creations of master goldsmith Hermann Diepel. The earlier (and smaller) one - known as the ‘Willhelmsuhr’ - was conceived by Wilhelm IV of Kassel himself in the 1550s, while his astronomer Andreas Schöner provided the astronomical calculations. It is still in Kassel, still at the historic observatory at the Orangerie.
The second Baldewein astronomical clock – the subject of the book under review (see Figure 1) – was also built by the same craftsmen in Kassel and Marburg, but it was created for the court in Dresden, specifically for Count Elector August I of Saxonia. That clock, significantly larger, much more opulent, and mechanically more refined, is known as the ‘Dresden Planetenlaufuhr’ [Dresden Planetary Clock] and it also remains to this day in the building it has been in since the 16<sup>th</sup> century, the “Mathematisch-Physikalischer Salon” in the Zwinger in Dresden. (If you want to see it you have to wait till 2010 as the Museum is currently closed for renovations). That clock is unquestionably the pinnacle of planetary clocks based on the theory of Ptolemy.
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The literature –in any language- on any of these clocks modeling planetary motion based on Ptolemy is extremely sparse. There are, of course, the various transcripts/translations/facsimiles of the Dondi records, including the 1985 book produced by DGC of Henri Bach’s text, and the best of them, the magnificent French facsimile edition of 1998 (<a href="http://www.hsn161.com/BHM/bhm4-superusers/bhmfind2.php?sqlType=detail&BookID=11555">Horologium Amicorum - Emmanuel Poulle: L'Astrarium de Giovanni Dondi</a> -Facsimile du manuscrit "Tractatus Astrarium" de Padoue, Bibliothèque capitulaire, ms D.39 – ISBN 2 900791 16 2 –Published: 1998, Paris, <a href="http://www.hsn161.com/BHM/bhm4-superusers/bhmfind2.php?sqlType=title&PublisherID=3248">École Nationale des Chartes</a>), but they cover a clock no longer in existence. There are tangential references to the subject in several broader themed books, and there are a few scholarly papers and articles. Some of the clocks are decribed relatively concisely in various museum catalogs, such as the relatively short description of the ‘Willhelmsuhr’ of Kassel in their 1991 publication (Von Mackensen: Naturwissenschaftlich Technische Sammlung in Kassel - Geschichte, Bedeutung und Ausstellung in der Kasseler Orangerie; ISBN 3 87013 025 3; Published 1991, Kassel: Georg Wenderoth Verlag), and a more extensive entry in their excellent 2007 catalog (Gaulke: Ptolomaeus von Kassel - Landgraf Wilhelm IV. von Hessen-Kassel und die Astronomie - Kataloge der Museumslandschaft Hessen-Kassel Bd. 38 – ISBN 3 91787 43 1 – Published Kassel, 2007: <a href="http://www.hsn161.com/BHM/bhm1-allusers/bhmfind2.php?sqlType=title&PublisherID=3408">Museumslandschaft Hessen Kassel</a>). I know of only two small publications on the Oronce-Fine clock (Poulle & Hillard: <a href="http://www.hsn161.com/BHM/bhm1-allusers/bhmfind2.php?sqlType=detail&BookID=4716">Oronce Fine et l'Horloge Planetaire de la Bibliotheque Sainte-Genevieve</a>; in Bibliotheque Humanism et Renaissance, Travaux et Documents, Tomme XXXIII, 1971; and: Poulle & Hillard: <a href="http://www.hsn161.com/BHM/bhm1-allusers/bhmfind2.php?sqlType=detail&BookID=4717">Science et Astrologie au XVIe Siècle - Oronce Fine et son Horloge Planetaire</a>; 22 Nov - 22 Dec 1971 Bibilothèque Sainte-Geneviève Paris [Catalogue]). And then there is of course Emanuel Poulle’s unequalled two volume 1980 study of planetary motion devices in the 13<sup>th</sup> to 16<sup>th</sup> centuries (Poulle: <a href="http://www.hsn161.com/BHM/bhm1-allusers/bhmfind2.php?sqlType=detail&BookID=11750">Equatories et Horlogerie Planetaire du XIIIe au XIVe Siecle -Tomme I [Texte] - Les Instruments de la Theorie des Planetes selon Ptolomee</a>; published Paris, 1980, <a href="http://www.hsn161.com/BHM/bhm1-allusers/bhmfind2.php?sqlType=title&PublisherID=3309">Centre des Recherces d'Histoire et de Philologie</a>) which devotes 34 pages to the two Baldewein clocks, and which mentions – in 1980, in a footnote on page 595 – the book now under review as being published “dans un avenir proche [in the near future]”.
Given that it has taken 30 years for this book to get published, and that I am not aware on anything written in English on the subject, I plan to be a bit more exhaustive in this book review than is customary, as I know that only few readers of ‘Antiquarian Horology’ will acquire this German language book, and because I am convinced that the ‘Dresden Planetenlaufuhr’ deserves to be documented somewhere in the English language.
But before I describe and review the book in more detail, the history on how this unique publication came about deserves to be told as well: The initiative came from an engineer named Helmut Sändig from eastern Germany, who -back around 1970- became enamored with the mechanics of the clock. While prominently exhibited, its inner workings were at that time completely undocumented. In the course of several years Sändig recorded the mechanism in great technical detail. But lacking the historical and astronomical context he was unable to really understand what the various dials showed. By the mid 1970s the French scholar Emanuel Poulle showed up in Dresden to analyze the clock for his big book on planetary mechanisms. Although neither understood the other’s language, and in spite of the ‘iron curtain’ restricting communications significantly, these two ‘fans’ of the clock developed a rapport and a friendship, cumulating in a collaborative first manuscript in French, written by Poulle, but based on the mechanical analysis of Sändig, which described and analyzed all the dials and indications of the clock in some detail. Given that the clock was German made and located in Germany the plan always envisioned a publication in German. The then curator of the clock in Dresden, Joachim Scharadin, researched and documented the elaborate icononography, while the conservator at the Dresden museum, Lothar Hasselmeyer, documented the time and strike trains with the help of Sändig. This unique collaboration between an astronomer, an engineer, an art historian and a curator resulted in an unusually rich and deep manuscript. In the meantime, the former regime in the German Democratic Republic had collapsed, and publication budgets for the museum in Dresden had evaporated. Given the rather specialized nature of the subject matter no commercial publisher was willing to pick up the project. After many years of futile searches for a publisher, the Deutsche Gesellschaft fur Chronometrie, the German sister organization to the AHS, rose to the challenge. They generously decided to forgo their normal annual scholarly publication, the ‘Jahrbuch’ (which usually contains some 20+ illustrated, scholarly horological articles) and use the funds to create a ‘single subject Jahrbuch’ based on the 20 years old, four author manuscript. The staff photographers of the Museum in Dresden (Hans-Peter Klut and Elke Estel) took many hundred pictures, and the yearbook editorial team under Dieter Tondock went to work. By the time the book was finally published in late 2008 two of the four authors (Sändig and Scharadin) had passed away, and Poulle -who had drafted the first manuscript when he was in his fifties- had recently celebrated his 80<sup>th</sup> birthday.
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While it certainly took a long to time to produce the book, this reviewer feels it was worth the wait. The final publication is unquestionably the most ambitious publication ever attempted to describe a single timepiece. Given that many readers of this review may choose not to acquire the book because it is written in German, the following synopsis of the book’s content will be more detailed than most reviews.
After an 8-page introductory text by Poulle, the first two chapters set the stage: Chapter 1 (15 pages) provides an introduction to the Ptolemic theory of planetary movement. At the time this clock was created it was believed that heavenly bodies moved at regular speed in circular motion. Because the observed motions did not correspond to the theory, a way had to be found to approximate the observed motion through compounding various circular motions, leading to epicyclical motions. While somewhat challenging for the astronomically illiterate, the explanation of this theory in Chapter 1, characterized by the phrase “mechanically saving the phenomena” is vital both to appreciating the mechanical genius of the creators of this clock, and to understanding its displays. Here –as elsewhere in the book- the numerous and very clear graphic diagrams help the reader significantly in understanding the subject matter.
Chapter 2 (7 pages) traces the history of the clock, its construction, its makers and its original owner, and compares the clock to other pre-Copernican planetary mechanisms. It is worth noting that Baldewein was originally a tailor by trade, then became the court architect, but by the mid 1560s was also the astronomical aide to Count Willhelm IV of Hessen in Kassel, as well as his instrument maker and mechanic. In 1562 he had just finished making the ‘Willhelms-Uhr’ mentioned above for his master; when Duke August of Saxony asked Duke Willhelm of Hessen if Baldewein could also make one for him. When referring to Baldewein as the ‘maker’ we must note that while the clock is certainly his creation - as he conceived it and directed the works - he was more a ‘general contractor’ who subcontracted much of the work with countless tradesmen like clockmakers (Bucher) and goldsmiths (Diepel) to build the many components of the physical artifact. However, some of the most creative gearing, such as e.g. the cutting of wheels with uneven spaced teeth, is probably by Baldewein’s own hand.
Chapter 3 (19 pages) describes the case and its iconography. This artifact is a massive piece (see Fig. 2); its basic shape is a square pillar with sides of approximately 63cm, and today about 120 cm high including the mechanized celestial globe rotating in the top surface. (The top finial of 15cm got lost during World War 2). Each of the four sides has two large dials of approximately 30 cm diameter. The resulting eight primary dials are dedicated to what were at the time thought to be the seven planets (Moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn). Six of the planets have their own dedicated large dials, with the seventh dial being the calendar, and the eighth dial being an astrolabe. The firmament of stars makes the ninth large display of this clock, shown on the celestial globe on the top. The position of the sun, the seventh Ptolemic planet, is indicated three times: 1. on the moon dial, 2. on the astrolabe dial and 3. on the celestial globe.
The corners are decorated with pairs of pillars and the bottom, center and top friezes are richly ornamented in silver or gilt brass. Every square centimeter of dials and spandrels is also decorated. Of particular interest are the large engravings in the centers of the large dials. These show the antique deities associated with the respective planets. The subjects shown all relate to the theme of the dial or to the original owner of the clock. The symbolism of all these engravings and images, and how they relate to a specific planet or the original owner, are explained in considerable detail.
Chapters 4 through 6 (together 25 pages) describe the time train, the quarter striking train and the hour striking train respectively. These three trains occupy essentially all of the space in the center of the clock case. Structurally, this is a four corner-pillar movement (non-plated) in the gothic tradition (see Figure 2), with horizontal frames on
Fig. 2: Frontal view of the time movement
top and bottom, which carry additional verticals into which the three trains are planted. All three trains are spring driven. The time train occupies half of the central space (behind the Mars and astrolabe dials). The clock has a fussee of 12 turns connected by 329 cm of gut line to a 9 cm wide spring barrel. It runs 55 hours on one winding. As the clock predates the invention of the pendulum it originally had a verge escapement with either a foliot or a ring balance. This was changed in 1828 by Friedrich Gutkaes to a short pendulum (286mm) anchor escapement, which was ‘enhanced’ in 1901 by Ernst Schmidt with a fine adjustment mechanism of the pendulum suspension point. Extra holes in the frame allow a conclusive determination of the geometry of the original verge escapement as fully described in the book. All three trains are described in detail both verbally and in clear diagrams (see Fig. 3) which include detailed teeth counts.
Fig. 3: Schematics of the time movement
The time train directly drives the small time dial (day disc and minute hand) and the astrolabe dial. All other indications are driven indirectly from the 24 hour wheel behind the astrolabe dial. The top of that wheel also drives a pinion that goes up to the celestial globe and the bottom of the wheel drives the ‘power plant’ in the bottom of the clock which in turn powers the other 7 major dial systems. See Fig. 4 for the power distribution diagram.
Fig. 4: The power distribution plant
The two strike trains are located behind the calendar/moon side of the clock. The quarter train is tripped by the time train, and the hour train is started by the quarter train. Both striking trains have sophisticated count wheel systems, whose operations are described in detail both verbally and graphically in the book. The hour strike train was originally switchable from 2 x 12 hour striking to 4 x 6 hour striking. The strike style selector is still in the clock but the count wheel for 4 x 6 is missing today. The book documents in detail how the dual strike mode had worked, as well as the location and dimensions of its original components.
The next eight chapters deal with the mechanisms driving the eight major dials (Chapter 7: Saturn (26 pages), chapter 8: Jupiter (8 pages), chapter 9: Venus (10 pages), chapter 10: Mars (22 pages), chapter 11: Mercury (22 pages), chapter 12: Moon (32 pages), chapter 13: Calendar with sunrise/sunset indications(13 pages), and chapter 14: Astrolabe(10 pages). The varying lengths of these chapters is due to the greatly varying complexity of the geometric/mathematical models constructed in Ptolemy’s system to simulate the apparent motions of these heavenly bodies as seen from earth. The exception is that chapter 7 which is so long because it explains in detail both the geometrical and mechanical concepts underlying the epicyclical gearing that is central to understanding all of Baldewein’s planetary motion works.
Seven of these eight display mechanisms (all except the astrolabe) are rather complex gear trains in their own right, and they are ‘semi-autonomous’ from the drive train, as they can each, be decoupled individually from the drive train if setting or maintenance is required. Each is located behind its respective dial. The dials are all hinged to the case, so that they can be swung out of the case individually including their respective display trains. Swinging out the dial disengages the trains from their respective drive shafts that reach up to them from the central ‘power distribution plant’ located in the bottom of the case. In most cases, some of the down gearing to the rotational speed needed at a specific dial already happens in the bottom of the case, at the ‘power plant’ level, before the long vertical shafts reaching up to the display gearing behind the dials. (Fig.5)
Fig. 5 The clock is 18 cm high, with a square footprint. The eight main dials swing out including the gearing for their displays for setting or maintenance. The time and strike movements are in the center of the case
Each of these eight chapters begins with full page images of the dial and the underdial display gearing. They also contain a detailed description of the mathematical-geometric model needed to achieve the variable angular speed to show the correct apparent position of the planet as seen from earth, as well as detailed schematics with teeth counts, and numerous images of special parts and subsidiary dials as present on or behind that dial. An analysis of how close the clock comes to the ‘theoretically correct’ solution is also part of each chapter.
The Saturn and Jupiter mechanisms are relatively straightforward examples of epicyclical gearing, although Saturn needed wheels with unevenly spaced teeth do display the desired values. The Venus mechanism is logically similar to Saturn, except that the satellite gear needed to be executed in two stages (including a wheel with internal teeth) to avoid an overly large wheel. The mechanism for the Mars dial features the additional complication of a circular, but eccentric wheel, which is driven by an ‘endless screw’ in a cleverly designed ‘sliding cage’. The driveshaft supplying power to the moving cage varies its direction by 15º in the course of a rotation of the eccentric wheel. That eccentric 360 tooth wheel also features the most uneven arrangement of teeth in the mechanism. In the theory of Ptolemy, Mercury follows a different geometric model than the other planets, leading to a different and more complex gear train. As representative samples of this section of the book’s content a schematic and one of the dial detail pictures from the Mercury dial are shown in this review (Fig 6 and 7).
Fig. 6: Schematics for displaying the position of Mercury with epicyclic gearing and Fig. 7: Detail of the display on the Mercury dial
The astrolabe, calendar dial and moon dial are a bit different from the planet dials. The astrolabe, as one of the dials showing the position of the sun, is driven directly from the going train (before the power flows to the ‘power plant’). The rete of the astrolabe, rotating once per sideral day, has long and slender points marking the location of 39 different stars, 29 of which are labeled at the base of their pointers. Just above the astrolabe dial is the small minute hand and the day of the week disc.
Fig. 8: The Calendar dial, with holidays including Easter, time of sunset and sunrise, and bohemian hours.
The calendar dial (see Fig, 8 ) is one of the few without epicyclical gears. It has a centered hour hand (on a 2x12 hour display) as its prominent feature. The silvered shutters in the center of the dial show daylight hours, while blued shutters are showing nighttime hours, with the shutter edges indicating the time of sunrise and sunset. These shutters, of course, move as the seasons change; that mechanism is not built for any specific latitude, but can be adjusted through a setting dial (inside the mechanism) for latitudes from 0º to 64º. Tied to the sunset mechanism is a special drivesystem incorporating a clever ‘step-wheel’ with a mechanical crank for the dial ring indicating bohemian hours (i.e. hours since nightfall, or in other words, seasonally equal hours counted from a seasonably variable starting point). The change in nightfall over the seasons is a nonlinear function that exceeded the limits of using unevenly spaced teeth, and forced Baldewein to invent yet another custom solution. The outer rings are dedicated to calendar data, with the longer hand giving the day of the year (no automatic leap year feature) and related data such as 57 holidays with fixed dates (including saint days and holy days with fixed dates) which are explicitly labeled on the calendar dial. An additional short pointer allows reading the year, the date of Easter and the Sunday letter(s) for any year between 1532 and 1619. These values are then used to manually set the outermost ring of the dial to indicate all holy days dependent on the date of Easter.
Fig 9: Schematics of the Lunar Display, with epicyclic gears and with the green drum rotating excenticaly inside the dial
The moon dial has the most complex display mechanisms of this clock, even if it shows no visible epicyclical gearing (see Fig. 9). Inside the stationary rim showing the enameled symbols of the ecliptic (It is the only one of the six ecliptic dial rings that is not manually adjustable by a few degrees for the ever so slow precession of the equinoxes over the centuries, but it does have a fixed scale of 1.2º covering at least a few centuries) the moon dial is made up of four moving concentric rings, each with a different rotational speed. From the outside in they are 1<sup>st</sup> an otherwise unmarked ring showing where on the ecliptic the sun is currently located, indicating conjunction and opposition of moon and sun (uneven rotation due to uneven teeth spacing), and 2<sup>nd</sup> is the dragon ring (with pointers for head and tail of the dragon, and scales for lunar latitude, and sectors in which partial eclipses are possible). The 3<sup>rd</sup> ring serves as a carrier for the pin moving the central hand (which cannot be driven through the central axis because the innermost pinion is needed to drive the 4<sup>th</sup> ring). The innermost 4<sup>th</sup> ring rotates around the deferent, i.e. eccentrically to the dial as a whole; it carries windows for the age (numerically) and the phases (graphically) of the moon, as well as the mechanism driving this moon phase disc. Furthermore, the mechanism driving the epicyclical elements needed to display the lunar position in the ecliptic is also mounted on the drum carrying ring 4. That latter mechanism is made up of two separate components, the first determining the basic period for rotating the epicycle (anomalistic month), and the second – the correcting mechanism - which compensates for the unwanted excentricity element in the angular speed of the first component. Describing the complexity of the resulting arrangement in more detail exceeds the bounds of this review; the book takes more than 10 densely packed pages explaining the resulting geometry and gearing in detail.
The 15<sup>th</sup> and final chapter (12 pages) of the book describes the functions of the driven celestial globe that sits on top of the whole machine. That globe basically provides the same functionality as the other famous mechanized celestial globes of the Renaissance, such as the five Buergi globes (several of which were also made in Kassel, a few decades after the Baldewein clock), the Roll/Reinhold globe of 1586 (now in Dresden) or the Emmoser globe of 1579 (now at the Metropolitan Museum in New York) and others. However the one topping the Baldewein clock, with a diameter of 30 cm, is significantly larger. Unlike most other mechanized star globes, in this one only half the celestial sphere is visible at any one time, as the horizon ring is flush with the roof of the clock case. The globe is the only part of the mechanism which has been described in detail in the literature before (in Leopold: Astronomen, Sterne, Geräte [Luzern 1986], Poulle  and in King: Geared to the Stars). It is made from silver and is engraved with numerous individual stars (based on the second Kassel Catalog of Stars), only three of which are individually labeled. Images of 52 constellations (following the images of Mercator) are engraved on the globe, 51 of which are also labeled.
The mobile quadrant to measure the latitude of a given star has been missing since the 1940s. The position of the sun within the zodiac is marked by a small gilt sun symbol (see Fig. 10) circling the ecliptic in real time (i.e. in one mean solar day) in
Fig. 10: The solar indicator on the celestial globe
the slit between the two halves of the globe when the clock is operating. The equation of time is accounted for through a ‘reverse display’, i.e. the sun moves evenly and the celestial globe moves unevenly. The globe rotates once around its axis in one mean sideral day. The tilt of the rotation axis of the globe was originally adjustable in a range between 39.5º and 70,northern latitude, depending on where on the northern hemisphere the clock is operating (but since converting to a pendulum time train adjustment is limited to 43º to 46º). The mechanical drive system for all these functions is clearly outlined in image (Fig. 11 )schematics (see Fig. 12) and in the text.
Fig. 11: The mechanism of the celestial globe Fig. 12: Schematics with teeth-counts for the celestial globe
The end-matter of the book includes several appendices of use mainly to those astronomically more savvy than I. First is a short expose on the characteristic parameters of the planet motions. This is followed by a table of planetary parameters, followed by a comparative table of cycle times of the heavenly bodies according to the Alphonseinic tables and according to Baldewein, including the differences between the two expressed in both as a percentage and as absolute length of time. It is worth noting that Baldewein’s construction differed from the theoretically correct values of his era by values between as much as 0.01595% (Venus was the worst) to as little as 0.00036% (Saturn was best). A table giving the actual location of the planets in the ecliptic for the year 1568 (they move over the centuries) concludes this section.
A four page glossary of (German language) terminology of medieval astronomy follows. There is also a bibliography of 97 titles on six pages, followed by six pages of endnotes, and a list of picture credits. A four page photo essay documenting the project of photographing the Baldewein clock concludes the book.
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There is no question in my mind that the artifact described in this book is one of the most important timepieces in the history of horology, if not in the history of mechanics. There is little doubt that – at the time of its construction - this machine was the most complex geared apparatus ever constructed in the history of mankind. To have a ‘superstar’ as the subject of a book alone does not automatically assure that the book under review is a great or even an important book. But this reviewer believes that this book is a highly important book because the skills and qualifications of its creators, and the extreme care, the unending work and thoughtfulness that went into producing the book, are commensurate with the quality of the artifact. The fact that four extremely skilled people from very different disciplines (a mechanical engineer, an astronomer, an art historian and a museum curator) converged in one time period with the burning desire to fully document the same object is a rare coincidence. And this chance constellation produced an extraordinary book. A publication of this complexity is bound to include some glitches, but I have found only one minor error (the legend of the schematic of the quarter train misidentifies the count wheels in Bild 6-1).
It would be unfair to hold it against the title that this book has appeared ‘only’ in a German language edition – and the likelihood of it ever being fully translated into an other language unfortunately is miniscule. Undoubtedly, an English language book would have been easier to understand for more readers globally; but the Baldewein clock, while being of wide interest as a part of human culture, is primarily a core element of Germany’s cultural heritage, and thus deserves to be documented primarily in German. The clock can also bee seen as a symbol of the cultural and political upheavals of its time, of reformation vs. counter-reformation, astronomy vs. astrology, Catholicism vs. Protestantism, the ‘saving the phenomena’ science vs. observational science, etc. In spite of all these complexities the publishing team at DGC deserves credit for creating a book that is relatively easy to follow – and useful – even for readers whose primary language is not German. Admittedly, the casual foreign reader will not understand much, but the most likely foreign reader, either somebody deeply either interested in Ptolemy’s planetary model, or someone already quite familiar with complicated renaissance timekeepers, is likely to not only understand quite a bit of the content, but also to enjoy the reading. Having more than 250 superb photographic images and very clear schematics spread throughout the text is an immense help for the foreign reader. Even if you cannot read a word of German the pictures alone make the book worth buying if you have any interest in the subject matter.
If you do read German the text of the book is delightfully easy to read considering the sometimes highly technical and extremely complex subject matter. It provides a glimpse into the time it was created, an appreciation for the geniuses who have thought it up and built it. Reading the book evokes gratitude for the person who commissioned the clock, and the many custodians who have helped it survive to the present. I am deeply grateful to the authors for having created the book, and in awe of the countless people who - over the decades - have not given up in spite of overwhelming odds against them and have persevered in getting the story of the Baldewein clock into print more than 400 years after it was created. Thank you to the authors, to the Mathematisch Physikalischer Salon at the Dresden Museum, to the DGC and all others involved.
Acknowledgements: Given his lack of astronomical knowledge the reviewer is much indebted to Dr. Eberhard Zelinsky for helping me better understanding Ptolemy’s theory and the historic/political context of the clock, and to Dieter Tondock of DGC and Lothar Hasselmeyer of the Mathematisch-Physikalischer Salon for securing the reproduction rights of the images accompanying this review. All the photographs are © 2008 Mathematisch-Physikalischer Salon, Staatliche Kunstsammlungen Dresden (Foto: Klut/Estel), and the schematics are © 2008 Lothar Hasselmeyer. They may not be further reproduced, and the copyright for all pictures remains with the museum in Dresden.
Fortunat Mueller-Maerki, Sussex NJ