Astronomia nova
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↑  Greek, αἰτιολογητός can be translated as "explained, justified" (from αἰτιολογῶ "I explain, I justify"), but it also combines two roots αιτία "cause" and λόγος "reason". Kepler's concern with causes, as clearly shown in the book, indicates that he intended something more specific in the title than a generic 'justified' or 'explained', thus the title Astronomia Nova ΑΙΤΙΟΛΟΓΗΤΟΣ can be understood as "New astronomy based on causes" or "reasoned from causes".
↑  Here G.V. is a siglum for "Generositas Vestra", see Winiarczyk, Marek (1995). Sigla Latina in libris impressis occurrentia: cum siglorum graecorum appendice (ấn bản thứ 2). OCLC 168613439. Bản gốc lưu trữ ngày 7 tháng 2 năm 2019. Truy cập ngày 27 tháng 12 năm 2018.
↑  Voelkel, James R. (2001). The composition of Kepler's Astronomia nova. Princeton: Princeton University Press. tr. 1. ISBN 0-691-00738-1.
↑  Koestler, Arthur (1990). The Sleepwalkers: A history of man’s changing vision of the universe. London: Penguin Books. tr. 1. ISBN 0-14-019246-8.
↑  Kepler, Johannes; William H. Donahue (2004). Selections from Kepler’s Astronomia Nova. Santa Fe: Green Lion Press. tr. 1. ISBN 1-888009-28-4.
1 2  Koestler, Arthur (1990). The Sleepwalkers: A history of man’s changing vision of the universe. London: Penguin Books. tr. 325. ISBN 0-14-019246-8.
↑  Koestler, Arthur (1990). The Sleepwalkers: A history of man’s changing vision of the universe. London: Penguin Books. tr. 338. ISBN 0-14-019246-8.
↑  In his Astronomia nova, Kepler presented only a proof that Mars' orbit is elliptical. Evidence that the other known planets' orbits are elliptical was presented later.
See: Johannes Kepler, Astronomia nova … (1609), p. 285. After having rejected circular and oval orbits, Kepler concluded that Mars' orbit must be elliptical. From the top of page 285: "Ergo ellipsis est Planetæ iter; … " (Thus, an ellipse is the planet's [i.e., Mars'] path; …) Later on the same page: " … ut sequenti capite patescet: ubi simul etiam demonstrabitur, nullam Planetæ relinqui figuram Orbitæ, præterquam perfecte ellipticam; … " (… as will be revealed in the next chapter: where it will also then be proved that any figure of the planet's orbit must be relinquished, except a perfect ellipse; …) And then: "Caput LIX. Demonstratio, quod orbita Martis, …, fiat perfecta ellipsis: … " (Chapter 59. Proof that Mars' orbit, …, be a perfect ellipse: …) The geometric proof that Mars' orbit is an ellipse appears as Protheorema XI on pages 289-290.

Kepler stated that all planets travel in elliptical orbits having the Sun at one focus in: Johannes Kepler, Epitome Astronomiae Copernicanae [Summary of Copernican Astronomy] (Linz ("Lentiis ad Danubium"), (Austria): Johann Planck, 1622), book 5, part 1, III. De Figura Orbitæ (III. On the figure [i.e., shape] of orbits), pages 658-665. From p. 658: "Ellipsin fieri orbitam planetæ … " (Of an ellipse is made a planet's orbit …). From p. 659: " … Sole (Foco altero huius ellipsis) … " (… the Sun (the other focus of this ellipse) …).
↑ In his Astronomia nova … (1609), Kepler did not present his second law in its modern form. He did that only in his Epitome of 1621. Furthermore, in 1609, he presented his second law in two different forms, which scholars call the "distance law" and the "area law". In 1621, Kepler restated his second law for any planet: Johannes Kepler, Epitome Astronomiae Copernicanae [Summary of Copernican Astronomy] (Linz ("Lentiis ad Danubium"), (Austria): Johann Planck, 1622), book 5, page 668. From page 668: "Dictum quidem est in superioribus, divisa orbita in particulas minutissimas æquales: accrescete iis moras planetæ per eas, in proportione intervallorum inter eas & Solem." (It has been said above that, if the orbit of the planet is divided into the smallest equal parts, the times of the planet in them increase in the ratio of the distances between them and the sun.) That is, a planet's speed along its orbit is inversely proportional to its distance from the Sun. (The remainder of the paragraph makes clear that Kepler was referring to what is now called angular velocity.)
His "distance law" is presented in: "Caput XXXII. Virtutem quam Planetam movet in circulum attenuari cum discessu a fonte." (Chapter 32. The force that moves a planet circularly weakens with distance from the source.) See: Johannes Kepler, Astronomia nova … (1609), pp. 165-167. On page 167, Kepler states: " …, quanto longior est αδ quam αε, tanto diutius moratur Planeta in certo aliquo arcui excentrici apud δ, quam in æquali arcu excentrici apud ε." (…, as αδ is longer than αε, so much longer will a planet remain on a certain arc of the eccentric near δ than on an equal arc of the eccentric near ε.) That is, the farther a planet is from the Sun (at the point α), the slower it moves along its orbit, so a radius from the Sun to a planet passes through equal areas in equal times. However, as Kepler presented it, his argument is accurate only for circles, not ellipses.
His "area law" is presented in: "Caput LIX. Demonstratio, quod orbita Martis, …, fiat perfecta ellipsis: … " (Chapter 59. Proof that Mars' orbit, …, is a perfect ellipse: …), Protheorema XIV and XV, pp. 291-295. On the top p. 294, it reads:  "Arcum ellipseos, cujus moras metitur area AKN, debere terminari in LK, ut sit AM." (The arc of the ellipse, of which the duration is delimited [i.e., measured] by the area AKM, should be terminated in LK, so that it [i.e., the arc] is AM.) In other words, the time that Mars requires to move along an arc AM of its elliptical orbit is measured by the area of the segment AMN of the ellipse (where N is the position of the Sun), which in turn is proportional to the section AKN of the circle that encircles the ellipse and that is tangent to it. Therefore, the area AMN that is swept out by a radius from the Sun to Mars as Mars moves along an arc AM of its elliptical orbit is proportional to the time that Mars requires to move along that arc. Thus, a radius from the Sun to Mars sweeps out equal areas in equal times.
↑  Johannes Kepler, Harmonices Mundi [The Harmony of the World] (Linz, (Austria): Johann Planck, 1619), p. 189. From the bottom of p. 189: "Sed res est certissima exactissimaque quod proportio qua est inter binorum quorumcunque Planetarum tempora periodica, sit præcise sesquialtera proportionis mediarum distantiarum, … " (But it is absolutely certain and exact that the proportion between the periodic times of any two planets is precisely the sesquialternate proportion [i.e., the ratio of 3:2] of their mean distances, … ")

An English translation of Kepler's Harmonices Mundi is available as: Johannes Kepler with E.J. Aiton, A.M. Duncan, and J.V. Field, trans., The Harmony of the World (Philadelphia, Pennsylvania: American Philosophical Society, 1997); see especially p. 411.
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