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14.2. Johannes Kepler's astronomical instruments
14.2.
Johannes Kepler's Astronomical Instruments V. L. CHENAKAL
University of Leningrad Summary The life and activity of Kepler coincided with an important period in the history of astronomy, when every decade new instruments were being invented, bringing about many great discoveries. At this time many scientists and artisans participated in the construction of astronomical instruments. The contribution made by them is fairly well known, but Kepler's work in this field has been little investigated. However, in this domain he has done no less than others and in some respects surpassed them and even forestalled future developments. In Kepler's time great advances were made in the construction of astronomical instruments for measurements of angles. A large triquetrum proved to be well adapted for Kepler's purpose, though not exceeding in precision Tycho Brahe's instruments. A device constructed by Kepler for the observation of the Moon deserves attention. An invaluable contribution to observational astronomy was made by Kepler when he designed his telescope. The scheme proposed by him, consisting of a biconvex object lens and a biconvex e3;epiece, determined for centuries the progress of instruments for observations and angular measurements. Establishing, on the basis of Copernicus' work, the true structure of the solar system, Kepler opened a new page in the field of practical modelling of armillary spheres, planetaria and telluria. Thanks to his initiative many such instruments were made. Kepler himself proposed in 1596 and 1624-5 two original models of planetaria, and his investigations were also used in the design of celestial globes.
T h e life and creative activity of Johannes Kepler (1571-1630) coincided with an important period in the history of astronomy, when with each passing decade that
819
V. L. Chenakal branch of science was enriched with new instruments for studying the heavens, enabling many significant discoveries to be made in a short time. Unlike the case in our day, when astronomical instruments are built primarily by engineers who specialize in that field of design, in Kepler's time instruments were constructed by the astronomers themselves. We need only recall the efforts of such illustrious astronomers of that period as Tycho Brahe (1546-1601), Galileo Galilei (1564-1642), Christoph Scheiner (1573-1650), Jan Hevelius (1611-87), and others to see that this is true. Of course one should not forget the talented scientific instrument makers who by that time were living in various Central European countries. But their contribution to the design of new astronomical instruments was considerably smaller than the contribution of the astronomers themselves. In the papers that have thus far been published on Kepler's scientific work, his contribution toward the development of astronomy has usually been described in terms of his three laws, which for all time were to govern further progress in celestial mechanics; the Tabulae Rudolphinae; his research in the field of geometrical optics; and certain comparatively minor investigations, such as his observations of sunspots and his discovery of the nova of 1604. But his creativity as a designer of new astronomical instruments as yet remains inadequately explored, despite the fact that his contribution in this area was no less significant than that of other scientists among his contemporaries, while in some respects he was far ahead of his time. The heliocentric system devised by Nicolas Copernicus (1473-1543) to explain the structure of the solar system, although adopted by leading scholars of the late sixteenth and early seventeenth centuries, was in need of further development, and for this purpose instruments were required that were capable of measuring angles more accurately than ever before. Along with other astronomers, Kepler participated in designing such instruments. To be sure, he did not achieve such outstanding success as Tycho Brahe, 1 but the angle-measuring instruments he developed were of considerable interest at the time. Despite their very high precision for that period, Tycho Brahe's angle-measuring instruments were quite cumbersome, and hence ill suited for use under field conditions. Intending to carry out geodetic work in Lower Austria, Kepler proposed a design for an instrument capable of measuring the altitude of the Pole Star conveniently in the field, as is necessary in order to determine latitudes. Two brief 1 The most comprehensive description of Tycho Brahe's astronomical instruments, including those for measuring angles, appears in his collected works: Tychonis Brahe Dani Opera Omnia, Copenhagen (1913-29). Further descriptions are given in the following: J. L. E. Dreyer, Tycho Brahe, Karlsruhe, 1894; L. Weinek, Die Tychonischen Instrumente auf der Prager Sternwarte, Prague, 1901; J. A. Repsold, Zur Geschichte der Astronomischen Messwerkzeuge yon Purbach bis Reichenbach, 1450 bis 1830, pp. 21-30, Leipzig, 1908; A. Rohde, Die Geschichte der Wissenschaftlichen Instrumente vom Beginn der Renaissance b.is zum Ausgang des 18. Jahrhunderts, Leipzig, 1923.
820
Johannes Kepler's Astronomical Instruments descriptions of it have come down to us, one 2 dating from 1595 and the other 3 from 1598. They show that the instrument consisted of a rather large-sized wooden right triangle (according to the first description it was 10 feet long and 5 feet high; according to the second, its sides measured 10, 8 and 6 feet), suspended at the right-angle point, and equipped with a plumb-bob attached at the same corner; a scale of degrees was drawn on the rod forming the hypotenuse, and a sight was mounted on the rod forming the longer leg of the triangle. Clearly there is no need to explain the operation of this instrument (a "triquetrum"). It is of greater interest to observe that other astronomers also later used it for the same purpose, determining the altitudes of celestial bodies. For example, from 1627 to 1635 a triangle of this type was often used by the Tiibingen mathematics professor and scientific-instrument maker Wilhelm Schickard (15921635). In particular, with its aid Schickard twice accurately determined the latitude of Tiibingen. 4 The instrument Kepler proposed for measuring the solar disc and the disc and features on the moon is of exceptional interest. As described in the Astronomiae Pars Optica, this instrument consisted of a wooden pole 12 feet long that could be pointed to any part of the sky; a fixed metal plate with a pea-sized aperture was attached at one end of the pole, and at the other there was a small plank, movable along the pole and carrying two rulers, mounted perpendicular to each other, which could be turned around the centre of the plank and moved parallel to its axis. The rotation angle of the rulers and the amount of their linear displacement were measured with special scales. When the pole was pointed at the Sun or Moon, the aperture in its upper plate projected the image of the solar or lunar disc, or of the lunar crescent, onto the plank, where the rulers and scales provided a measurement of features in the image that were of interest to the observer. ~ Kepler's greatest contribution to the development of astronomical instruments is indisputably his effort to build an astronomical telescope. It is known to all what a revolution took place in the study of the heavens when in 1609 Galileo used a telescope to examine celestial bodies, although the prior history of the invention of the telescope unfortunately is still not entirely clear. And it is an equally familiar fact that the tube Galileo built for this purpose, later called a Dutch or Galilean telescope, was equipped with a convex lens as objective and a concave lens as eyepiece; it was incapable of providing either high magnification or a large field of view. This circumstance naturally limited its possibilities as a universal instrument for studying celestial bodies. Even so, a good many discoveries, which were 2Joannis Kepleri Astronomi Opera Omnia, ed. Ch. Frisch, Frankfurt and Erlangen (1858-71; hereafter cited as Opera Omnia), 1, 19-20. 3 Opera Omnia, 1, 67-68. 4 E. Zinner, Deutsche und Niederldndische Astronomische Instrumente des 11.-18. Jahrhunderts, p. 211, Munich, 1956. 5 Opera Omnia, 2, 340-397.
821
V. L. Chenakal so unexpected as to astonish the world, were made by Galileo himself and by other astronomers who availed themselves of his instrument soon afterward: Thomas Harriot (1560-1621), Christopher Clavius (1537-1612), Simon Marius (1573-1624), Johannes Fabricius (1587-ca. 1615), Scheiner, and Johann Baptist Cysat (1586-1657). In his own research in the field of optics, set forth in 1604 in the Astronomiae Pars Optica and in 1611 in the Dioptrice, Kepler not only became the first in the history of science to give a carefully developed theory for the refraction of light in optical glasses of different shapes, but he also proposed several new optical instruments, including a telescope that was free from the shortcomings of the Dutch design. As described in the Dioptrice, Kepler's telescope, unlike the Dutch instrument, consisted of a convex objective and also a convex eyepiece. 6 It had the advantages of providing a larger field of view, as well as a higher magnification, and it also permitted a thread, or cross hairs, to be placed in the back focal plane of the objective, where the real image of the observed objects is formed. With the thread or the centre of the cross hairs aligned on the optic axis of the tube, the instrument could be set very accurately on an observed object or an individual part of the object; thus in turn a telescope of this type could be used as a viewfinder in astronomical angle-measuring instruments, instead of viewfinders with sights, leading to a substantial rise in accuracy. Compared to the Galilean telescope, the Keplerian design did have the disadvantage that it gave an inverted image of the object being viewed. However, this property was of no consequence for observations of celestial bodies. Kepler himself did not have occasion to construct such a telescope. This was done several years later by Scheiner, after the Dioptrice had been published. The advantages of the Keplerian or, as it later came to be called, the astronomical telescope over the Dutch design for studying the sky were apparent from the very first years that it was used for this purpose, and were so striking that soon afterward the Dutch telescope was no longer employed in astronomy at all. Even before publication of Kepler's Dioptrice, the Heidelberg professor Jakob Christmann (1554-1613) mounted a telescope on the alidade of his sextant, thereby turning it into a viewfinder for that instrument. 7 But this was a Galilean telescope, and it had to be set on a celestial body by looking through a narrow slit, so that the opportunity of using the whole field of view of the telescope to search for an object was lost, and the observer was greatly inconvenienced. Soon afterward the Keplerian telescope appeared; it enabled this shortcoming to be avoided and in a short while it became an inseparable part of every angle-measuring instrument. A further possibility for wide application of the Keplerian telescope in astrometric work opened up during the period 1640-72 with the development of micrometers by William Gascoigne (1612-44), Adrian Auzout (1622-91), and Ole Romer (1644-1710).
e Opera Omnia, 2, 549-550. 7H. Ludendorff, "f2ber die erste Verbindung des Fernrohres mit astronomischenMessinstrumenten", Astron. Nachr. 213, 385-390 (1921). 822
Johannes Kepler's Astronomical Instruments The facts we have given--and their number could be multiplied further--show what an important role the Keplerian telescope played in the continuing improvement of astronomical instruments, and thereby in the development of observational astronomy generally. Kepler also contributed in a major way to the design of mechanical models of the solar system--that is to say, armillary spheres, planetaria and telluria. Armillary spheres, which appeared long before Kepler's time, were intended to demonstrate the structure of the solar system and the motions of the celestial bodies relative to one another; but their construction had rested on geocentric principles. Models of the six planets of the solar system known at that time, as well as a model of the Sun itself, were placed in rotation about a model of the Earth, by means of either manual drives or clock mechanisms. In working out the Copernican doctrine regarding the heliocentric structure of the solar system, Kepler had established the true scale of the system and the type of motion followed by its components; he then proceeded to supervise designers and makers of scientific instruments in the construction of armillary spheres that not only were based on heliocentric rather than geocentric ideas, but also obeyed accurate scale relationships, such as in the relative orbital periods of the model planets around the model Sun. There is no need to enumerate here the names of all the scientific-instrument makers of the early seventeenth century who built armillary spheres based on Kepler's investigations. Without anticipating when his researches might be used by others to build new models of the solar system, Kepler as early as 1596 developed a plan for a model solar system, consisting of a spherical bowl with images of the planets and stars drawn on its inner surface. This model was actually constructed by a certain Stuttgart goldsmith, but it did not prove too successful and did not come into widespread use. But this did not stop Kepler. In 1624-5 he worked out a more complete model for the solar system, a planetarium, and offered the drawing to Schickard, whom we have mentioned above. It comprised a glass hemisphere mounted on the cover of a metal box and containing a mechanism inside. Vertical dowels extended up from the mechanism to the surface of the cover where they passed through special slits, carrying model planets in rotation about a model Sun placed at the centre, while on the inner surface of the glass hemisphere were outlined the principal constellations. 8 It is not without interest to observe--although there would appear to be no reason to regard it as borrowing--that nearly all the scholars and scientific-instrument makers of the seventeenth and early eighteenth centuries who built planetaria, namely Andreas Busch, Ole Romer, Christian Huygens (1629-95), Johann van Keulen, Thomas Tompion (1639-1713), John Rowley, and others, actually made them according to the Keplerian design. The talented craftsman Schickard had contacts with Kepler even earlier. By 1618, for example, he had built a celestial globe on which the positions of the stars were s Zinner, op. cit., p. 43.
823
V. L. Chenakal
drawn in accordance with Kepler's instructions. ~ There is no question but that these contacts were furthered to a large extent by Schickard's increasing skill. Kepler's work also influenced the development of a third variety of model for the motion of celestial bodies: the "telluria", which demonstrate the revolution around the Sun of just one planet, the Earth, and also the revolution around the Earth of its satellite, the Moon. The earliest designers of telluria, Adrian Antonius (d. 1620), Willem Janszoon Blaeu (1571-1638), and again Schickard, performed calculations for their instruments on the basis of Keplerian data for the distances between the three bodies and the character of their motions. Kepler also did much to assist in the perfection of astronomical instruments by his personal contacts with the craftsmen who built such instruments. We have mentioned above his contacts with Schickard, and have indicated the beneficial effect they had upon Schickard's workmanship. Unquestionably Kepler had just as favourable an influence on the output of the Prague astronomical-instrument maker Erasmus Gabermel. The most fruitful years of his creative activity as a builder of excellent astrolabes, sundials, sextants, and other astronomical instruments were the years of his close contact with Kepler after the latter had travelled to Prague in 1600. There was an even closer relationship between Kepler and the Kassel watchmaker and builder of scientific instruments, Jost Btirgi (1552-1632). Their mutual friendship which began during the time when the watchmaker built for the astronomer his "hydraulic machine", lasted for many years, 1° and without doubt Biirgi owed much to Kepler for the way that his craftsmanship improved with each passing year. Among the artisans who profited from Kepler's influence, his son-in-law and assistant Jakob Bartsch (1600-33) must surely be mentioned. From an acquaintance with his numerous instruments, as described by Bartsch himself, we can easily recognize the influence of Kepler's work upon their design. This influence is most striking in the design of his celestial globes and planispheres. We have every reason to conclude, then, that Kepler's role in the area of planning and building astronomical instruments was indeed a very great one.
9 Opera Omnia, 5, 51-53. lo Opera Omnia, 1, 219; 2, 80, 754, 769-770; 5, 104, 639. 824
Johannes Kepler's Astronomical Instruments V. L. CHENAKAL
University of Leningrad Summary The life and activity of Kepler coincided with an important period in the history of astronomy, when every decade new instruments were being invented, bringing about many great discoveries. At this time many scientists and artisans participated in the construction of astronomical instruments. The contribution made by them is fairly well known, but Kepler's work in this field has been little investigated. However, in this domain he has done no less than others and in some respects surpassed them and even forestalled future developments. In Kepler's time great advances were made in the construction of astronomical instruments for measurements of angles. A large triquetrum proved to be well adapted for Kepler's purpose, though not exceeding in precision Tycho Brahe's instruments. A device constructed by Kepler for the observation of the Moon deserves attention. An invaluable contribution to observational astronomy was made by Kepler when he designed his telescope. The scheme proposed by him, consisting of a biconvex object lens and a biconvex e3;epiece, determined for centuries the progress of instruments for observations and angular measurements. Establishing, on the basis of Copernicus' work, the true structure of the solar system, Kepler opened a new page in the field of practical modelling of armillary spheres, planetaria and telluria. Thanks to his initiative many such instruments were made. Kepler himself proposed in 1596 and 1624-5 two original models of planetaria, and his investigations were also used in the design of celestial globes.
T h e life and creative activity of Johannes Kepler (1571-1630) coincided with an important period in the history of astronomy, when with each passing decade that
819
V. L. Chenakal branch of science was enriched with new instruments for studying the heavens, enabling many significant discoveries to be made in a short time. Unlike the case in our day, when astronomical instruments are built primarily by engineers who specialize in that field of design, in Kepler's time instruments were constructed by the astronomers themselves. We need only recall the efforts of such illustrious astronomers of that period as Tycho Brahe (1546-1601), Galileo Galilei (1564-1642), Christoph Scheiner (1573-1650), Jan Hevelius (1611-87), and others to see that this is true. Of course one should not forget the talented scientific instrument makers who by that time were living in various Central European countries. But their contribution to the design of new astronomical instruments was considerably smaller than the contribution of the astronomers themselves. In the papers that have thus far been published on Kepler's scientific work, his contribution toward the development of astronomy has usually been described in terms of his three laws, which for all time were to govern further progress in celestial mechanics; the Tabulae Rudolphinae; his research in the field of geometrical optics; and certain comparatively minor investigations, such as his observations of sunspots and his discovery of the nova of 1604. But his creativity as a designer of new astronomical instruments as yet remains inadequately explored, despite the fact that his contribution in this area was no less significant than that of other scientists among his contemporaries, while in some respects he was far ahead of his time. The heliocentric system devised by Nicolas Copernicus (1473-1543) to explain the structure of the solar system, although adopted by leading scholars of the late sixteenth and early seventeenth centuries, was in need of further development, and for this purpose instruments were required that were capable of measuring angles more accurately than ever before. Along with other astronomers, Kepler participated in designing such instruments. To be sure, he did not achieve such outstanding success as Tycho Brahe, 1 but the angle-measuring instruments he developed were of considerable interest at the time. Despite their very high precision for that period, Tycho Brahe's angle-measuring instruments were quite cumbersome, and hence ill suited for use under field conditions. Intending to carry out geodetic work in Lower Austria, Kepler proposed a design for an instrument capable of measuring the altitude of the Pole Star conveniently in the field, as is necessary in order to determine latitudes. Two brief 1 The most comprehensive description of Tycho Brahe's astronomical instruments, including those for measuring angles, appears in his collected works: Tychonis Brahe Dani Opera Omnia, Copenhagen (1913-29). Further descriptions are given in the following: J. L. E. Dreyer, Tycho Brahe, Karlsruhe, 1894; L. Weinek, Die Tychonischen Instrumente auf der Prager Sternwarte, Prague, 1901; J. A. Repsold, Zur Geschichte der Astronomischen Messwerkzeuge yon Purbach bis Reichenbach, 1450 bis 1830, pp. 21-30, Leipzig, 1908; A. Rohde, Die Geschichte der Wissenschaftlichen Instrumente vom Beginn der Renaissance b.is zum Ausgang des 18. Jahrhunderts, Leipzig, 1923.
820
Johannes Kepler's Astronomical Instruments descriptions of it have come down to us, one 2 dating from 1595 and the other 3 from 1598. They show that the instrument consisted of a rather large-sized wooden right triangle (according to the first description it was 10 feet long and 5 feet high; according to the second, its sides measured 10, 8 and 6 feet), suspended at the right-angle point, and equipped with a plumb-bob attached at the same corner; a scale of degrees was drawn on the rod forming the hypotenuse, and a sight was mounted on the rod forming the longer leg of the triangle. Clearly there is no need to explain the operation of this instrument (a "triquetrum"). It is of greater interest to observe that other astronomers also later used it for the same purpose, determining the altitudes of celestial bodies. For example, from 1627 to 1635 a triangle of this type was often used by the Tiibingen mathematics professor and scientific-instrument maker Wilhelm Schickard (15921635). In particular, with its aid Schickard twice accurately determined the latitude of Tiibingen. 4 The instrument Kepler proposed for measuring the solar disc and the disc and features on the moon is of exceptional interest. As described in the Astronomiae Pars Optica, this instrument consisted of a wooden pole 12 feet long that could be pointed to any part of the sky; a fixed metal plate with a pea-sized aperture was attached at one end of the pole, and at the other there was a small plank, movable along the pole and carrying two rulers, mounted perpendicular to each other, which could be turned around the centre of the plank and moved parallel to its axis. The rotation angle of the rulers and the amount of their linear displacement were measured with special scales. When the pole was pointed at the Sun or Moon, the aperture in its upper plate projected the image of the solar or lunar disc, or of the lunar crescent, onto the plank, where the rulers and scales provided a measurement of features in the image that were of interest to the observer. ~ Kepler's greatest contribution to the development of astronomical instruments is indisputably his effort to build an astronomical telescope. It is known to all what a revolution took place in the study of the heavens when in 1609 Galileo used a telescope to examine celestial bodies, although the prior history of the invention of the telescope unfortunately is still not entirely clear. And it is an equally familiar fact that the tube Galileo built for this purpose, later called a Dutch or Galilean telescope, was equipped with a convex lens as objective and a concave lens as eyepiece; it was incapable of providing either high magnification or a large field of view. This circumstance naturally limited its possibilities as a universal instrument for studying celestial bodies. Even so, a good many discoveries, which were 2Joannis Kepleri Astronomi Opera Omnia, ed. Ch. Frisch, Frankfurt and Erlangen (1858-71; hereafter cited as Opera Omnia), 1, 19-20. 3 Opera Omnia, 1, 67-68. 4 E. Zinner, Deutsche und Niederldndische Astronomische Instrumente des 11.-18. Jahrhunderts, p. 211, Munich, 1956. 5 Opera Omnia, 2, 340-397.
821
V. L. Chenakal so unexpected as to astonish the world, were made by Galileo himself and by other astronomers who availed themselves of his instrument soon afterward: Thomas Harriot (1560-1621), Christopher Clavius (1537-1612), Simon Marius (1573-1624), Johannes Fabricius (1587-ca. 1615), Scheiner, and Johann Baptist Cysat (1586-1657). In his own research in the field of optics, set forth in 1604 in the Astronomiae Pars Optica and in 1611 in the Dioptrice, Kepler not only became the first in the history of science to give a carefully developed theory for the refraction of light in optical glasses of different shapes, but he also proposed several new optical instruments, including a telescope that was free from the shortcomings of the Dutch design. As described in the Dioptrice, Kepler's telescope, unlike the Dutch instrument, consisted of a convex objective and also a convex eyepiece. 6 It had the advantages of providing a larger field of view, as well as a higher magnification, and it also permitted a thread, or cross hairs, to be placed in the back focal plane of the objective, where the real image of the observed objects is formed. With the thread or the centre of the cross hairs aligned on the optic axis of the tube, the instrument could be set very accurately on an observed object or an individual part of the object; thus in turn a telescope of this type could be used as a viewfinder in astronomical angle-measuring instruments, instead of viewfinders with sights, leading to a substantial rise in accuracy. Compared to the Galilean telescope, the Keplerian design did have the disadvantage that it gave an inverted image of the object being viewed. However, this property was of no consequence for observations of celestial bodies. Kepler himself did not have occasion to construct such a telescope. This was done several years later by Scheiner, after the Dioptrice had been published. The advantages of the Keplerian or, as it later came to be called, the astronomical telescope over the Dutch design for studying the sky were apparent from the very first years that it was used for this purpose, and were so striking that soon afterward the Dutch telescope was no longer employed in astronomy at all. Even before publication of Kepler's Dioptrice, the Heidelberg professor Jakob Christmann (1554-1613) mounted a telescope on the alidade of his sextant, thereby turning it into a viewfinder for that instrument. 7 But this was a Galilean telescope, and it had to be set on a celestial body by looking through a narrow slit, so that the opportunity of using the whole field of view of the telescope to search for an object was lost, and the observer was greatly inconvenienced. Soon afterward the Keplerian telescope appeared; it enabled this shortcoming to be avoided and in a short while it became an inseparable part of every angle-measuring instrument. A further possibility for wide application of the Keplerian telescope in astrometric work opened up during the period 1640-72 with the development of micrometers by William Gascoigne (1612-44), Adrian Auzout (1622-91), and Ole Romer (1644-1710).
e Opera Omnia, 2, 549-550. 7H. Ludendorff, "f2ber die erste Verbindung des Fernrohres mit astronomischenMessinstrumenten", Astron. Nachr. 213, 385-390 (1921). 822
Johannes Kepler's Astronomical Instruments The facts we have given--and their number could be multiplied further--show what an important role the Keplerian telescope played in the continuing improvement of astronomical instruments, and thereby in the development of observational astronomy generally. Kepler also contributed in a major way to the design of mechanical models of the solar system--that is to say, armillary spheres, planetaria and telluria. Armillary spheres, which appeared long before Kepler's time, were intended to demonstrate the structure of the solar system and the motions of the celestial bodies relative to one another; but their construction had rested on geocentric principles. Models of the six planets of the solar system known at that time, as well as a model of the Sun itself, were placed in rotation about a model of the Earth, by means of either manual drives or clock mechanisms. In working out the Copernican doctrine regarding the heliocentric structure of the solar system, Kepler had established the true scale of the system and the type of motion followed by its components; he then proceeded to supervise designers and makers of scientific instruments in the construction of armillary spheres that not only were based on heliocentric rather than geocentric ideas, but also obeyed accurate scale relationships, such as in the relative orbital periods of the model planets around the model Sun. There is no need to enumerate here the names of all the scientific-instrument makers of the early seventeenth century who built armillary spheres based on Kepler's investigations. Without anticipating when his researches might be used by others to build new models of the solar system, Kepler as early as 1596 developed a plan for a model solar system, consisting of a spherical bowl with images of the planets and stars drawn on its inner surface. This model was actually constructed by a certain Stuttgart goldsmith, but it did not prove too successful and did not come into widespread use. But this did not stop Kepler. In 1624-5 he worked out a more complete model for the solar system, a planetarium, and offered the drawing to Schickard, whom we have mentioned above. It comprised a glass hemisphere mounted on the cover of a metal box and containing a mechanism inside. Vertical dowels extended up from the mechanism to the surface of the cover where they passed through special slits, carrying model planets in rotation about a model Sun placed at the centre, while on the inner surface of the glass hemisphere were outlined the principal constellations. 8 It is not without interest to observe--although there would appear to be no reason to regard it as borrowing--that nearly all the scholars and scientific-instrument makers of the seventeenth and early eighteenth centuries who built planetaria, namely Andreas Busch, Ole Romer, Christian Huygens (1629-95), Johann van Keulen, Thomas Tompion (1639-1713), John Rowley, and others, actually made them according to the Keplerian design. The talented craftsman Schickard had contacts with Kepler even earlier. By 1618, for example, he had built a celestial globe on which the positions of the stars were s Zinner, op. cit., p. 43.
823
V. L. Chenakal
drawn in accordance with Kepler's instructions. ~ There is no question but that these contacts were furthered to a large extent by Schickard's increasing skill. Kepler's work also influenced the development of a third variety of model for the motion of celestial bodies: the "telluria", which demonstrate the revolution around the Sun of just one planet, the Earth, and also the revolution around the Earth of its satellite, the Moon. The earliest designers of telluria, Adrian Antonius (d. 1620), Willem Janszoon Blaeu (1571-1638), and again Schickard, performed calculations for their instruments on the basis of Keplerian data for the distances between the three bodies and the character of their motions. Kepler also did much to assist in the perfection of astronomical instruments by his personal contacts with the craftsmen who built such instruments. We have mentioned above his contacts with Schickard, and have indicated the beneficial effect they had upon Schickard's workmanship. Unquestionably Kepler had just as favourable an influence on the output of the Prague astronomical-instrument maker Erasmus Gabermel. The most fruitful years of his creative activity as a builder of excellent astrolabes, sundials, sextants, and other astronomical instruments were the years of his close contact with Kepler after the latter had travelled to Prague in 1600. There was an even closer relationship between Kepler and the Kassel watchmaker and builder of scientific instruments, Jost Btirgi (1552-1632). Their mutual friendship which began during the time when the watchmaker built for the astronomer his "hydraulic machine", lasted for many years, 1° and without doubt Biirgi owed much to Kepler for the way that his craftsmanship improved with each passing year. Among the artisans who profited from Kepler's influence, his son-in-law and assistant Jakob Bartsch (1600-33) must surely be mentioned. From an acquaintance with his numerous instruments, as described by Bartsch himself, we can easily recognize the influence of Kepler's work upon their design. This influence is most striking in the design of his celestial globes and planispheres. We have every reason to conclude, then, that Kepler's role in the area of planning and building astronomical instruments was indeed a very great one.
9 Opera Omnia, 5, 51-53. lo Opera Omnia, 1, 219; 2, 80, 754, 769-770; 5, 104, 639. 824