Editing Johannes Kepler (section)

=== ''Epitome of Copernican Astronomy'' ===
{{further|Epitome Astronomiae Copernicanae}}

Since completing the ''Astronomia Nova'', Kepler had intended to compose an astronomy textbook that would cover all the fundamentals of [[Heliocentrism|heliocentric astronomy]].<ref>Caspar, ''Kepler'', pp.&nbsp;239–240, 293–300</ref> Kepler spent the next several years working on what would become ''Epitome Astronomiae Copernicanae'' (''Epitome of Copernican Astronomy''). Despite its title, which merely hints at heliocentrism, the ''Epitome'' is less about Copernicus's work and more about Kepler's own astronomical system. The ''Epitome'' contained all three laws of planetary motion and attempted to explain heavenly motions through physical causes.<ref name="Gingerich pp 302">Gingerich, "Kepler, Johannes" from ''Dictionary of Scientific Biography'', pp.&nbsp;302–304</ref> Although it explicitly extended the first two laws of planetary motion (applied to Mars in ''Astronomia nova'') to all the planets as well as the Moon and the [[Galilean moons|Medicean satellites of Jupiter]],{{NoteTag|By 1621 or earlier, Kepler recognized that Jupiter's moons obey his third law. Kepler contended that rotating massive bodies communicate their rotation to their satellites, so that the satellites are swept around the central body; thus the rotation of the Sun drives the revolutions of the planets and the rotation of the Earth drives the revolution of the Moon. In Kepler's era, no one had any evidence of Jupiter's rotation. However, Kepler argued that the force by which a central body causes its satellites to revolve around it, weakens with distance; consequently, satellites that are farther from the central body revolve slower. Kepler noted that Jupiter's moons obeyed this pattern and he inferred that a similar force was responsible. He also noted that the orbital periods and semi-major axes of Jupiter's satellites were roughly related by a 3/2 power law, as are the orbits of the six (then known) planets. However, this relation was approximate: the periods of Jupiter's moons were known within a few percent of their modern values, but the moons' semi-major axes were determined less accurately. Kepler discussed Jupiter's moons in his ''[[Epitome Astronomiae Copernicanae|Summary of Copernican Astronomy]]'':<ref>Linz ("Lentiis ad Danubium"), (Austria): Johann Planck, 1622, book 4, part 2, [https://books.google.com/books?id=wa2SE_6ZL7YC&pg=PA554 p. 554]</ref><ref>Christian Frisch, ed., ''Joannis Kepleri Astronomi Opera Omnia'', vol. 6 (Frankfurt-am-Main, (Germany): Heyder & Zimmer, 1866), [https://books.google.com/books?id=xjMAAAAAQAAJ&pg=PA361 p. 361].)</ref><!--Original : ''4) Confirmatur vero fides hujus rei comparatione quatuor Jovialium et Jovis cum sex planetis et Sole. Etsi enim de corpore Jovis, an et ipsum circa suum axem convertatur, non-ea documenta habemus, quae nobis suppetunt in corporibus Terrae et praecipue Solis, quippe a sensu ipso: at illud sensus testatur, plane ut est cum sex planetis circa Solem, sic etiam se rem habere cum quatuor Jovialibus, ut circa corpus Jovis quilibet, quo longius ab illo potest excurrere, hoc tardius redeat, et id quidem proportione non eadem, sed majore, hoc est sescupla proportionis intervallorum cujusque a Jove: quae plane ipsissima est, qua utebantur supra sex planetae. Intervalla enim quatuor Jovialium a Jove prodit Marius in suo Mundo Joviali ista: 3, 5, 8, 13 (vel 14 Galilaeo)&nbsp;... Periodica vero tempora prodit idem Marius ista: dies 1. h. 18 1/2, dies 3 h. 13 1/3, dies 7 h. 3, dies 16 h. 18: ubique proportio est major quam dupla, major igitur quam intervallorum 3, 5, 8, 13 vel 14, minor tamen quam quadratorum, qui duplicant proportiones intervallorum, sc. 9, 25, 64, 169 vel 196, sicut etiam sescupla sunt majora simplis, minora vero duplis.''-->{{blockquote|(4) However, the credibility of this [argument] is proved by the comparison of the four [moons] of Jupiter and Jupiter with the six planets and the Sun. Because, regarding the body of Jupiter, whether it turns around its axis, we don't have proofs for what suffices for us [regarding the rotation of ] the body of the Earth and especially of the Sun, certainly [as reason proves to us]: but reason attests that, just as it is clearly [true] among the six planets around the Sun, so also it is among the four [moons] of Jupiter, because around the body of Jupiter any [satellite] that can go farther from it orbits slower, and even that [orbit's period] is not in the same proportion, but greater [than the distance from Jupiter]; that is, 3/2 (''sescupla'' ) of the proportion of each of the distances from Jupiter, which is clearly the very [proportion] as [is used for] the six planets above. In his [book] ''The World of Jupiter'' [''Mundus Jovialis'', 1614], [[Simon Marius|[Simon] Mayr]] [1573–1624] presents these distances, from Jupiter, of the four [moons] of Jupiter: 3, 5, 8, 13 (or 14 [according to] Galileo)&nbsp;... Mayr presents their time periods: 1 day 18 1/2 hours, 3 days 13 1/3 hours, 7 days 3 hours, 16 days 18 hours: for all [of these data] the proportion is greater than double, thus greater than [the proportion] of the distances 3, 5, 8, 13 or 14, although less than [the proportion] of the squares, which double the proportions of the distances, namely 9, 25, 64, 169 or 196, just as [a power of] 3/2 is also greater than 1 but less than 2.}}}} it did not explain how elliptical orbits could be derived from observational data.<ref>Wolf, ''A History of Science, Technology and Philosophy'', pp.&nbsp;140–141; Pannekoek, ''A History of Astronomy'', p. 252</ref>

Originally intended as an introduction for the uninitiated, Kepler sought to model his ''Epitome'' after that of his master [[Michael Maestlin]], who published a well-regarded book explaining the basics of [[Geocentric model|geocentric astronomy]] to non-experts.<ref name=":1">{{Cite journal|last=Rothman|first=A.|date=2021|title=Kepler's Epitome of Copernican Astronomy in context|url=https://onlinelibrary.wiley.com/doi/epdf/10.1111/1600-0498.12356|journal=Centaurus|volume=63|pages=171–191|doi=10.1111/1600-0498.12356|issn=0008-8994|s2cid=230613099}}</ref> Kepler completed the first of three volumes, consisting of Books I–III, by 1615 in the same question-answer format of Maestlin's and have it printed in 1617.<ref>{{Cite journal|last=Gingerich|first=Owen|date=1990|title=Five Centuries of Astronomical Textbooks and Their Role in Teaching|url=http://adsabs.harvard.edu/abs/1990teas.conf..189G|journal=The Teaching of Astronomy, Proceedings of IAU Colloq. 105, Held in Williamstown, MA, 27–30 July 1988|pages=189|bibcode=1990teas.conf..189G}}</ref> However, the [[Index Librorum Prohibitorum|banning of Copernican books]] by the Catholic Church, as well as the start of the [[Thirty Years' War]], meant that publication of the next two volumes would be delayed. In the interim, and to avoid being subject to the ban, Kepler switched the audience of the ''Epitome'' from beginners to that of expert astronomers and mathematicians, as the arguments became more and more sophisticated and required advanced mathematics to be understood.<ref name=":1" /> The second volume, consisting of Book IV, was published in 1620, followed by the third volume, consisting of Books V–VII, in 1621.