Astronomia Nova (full Latin title: Astronomia Nova ΑΙΤΙΟΛΟΓΗΤΟΣ seu physica coelestis, tradita commentariis de motibus stellae Martis ex observationibus Tychonis Brahe, meaning “New Astronomy, Based on Causes, or Celestial Physics, Treated by Means of Commentaries on the Motions of the Star Mars from the Observations of Tycho Brahe”) is a work in astronomy and natural philosophy published by Johannes Kepler in 1609. The book represents a turning point in the history of astronomy: it introduced what are now called Kepler's First and Second Laws of planetary motion (the Third Law would follow in Harmonices Mundi in 1619), establishing that planets travel in elliptical orbits with the Sun at one focus, and that a line connecting a planet to the Sun sweeps equal areas in equal times. Unlike earlier astronomical works that sought only to predict planetary positions mathematically, Astronomia Nova argued explicitly for a physical cause of planetary motion - a move that fused astronomy with natural philosophy and anticipated the later development of classical mechanics.

Kepler received access to the extraordinarily precise observational records of Tycho Brahe, with whom he worked briefly before Brahe's death in 1601. The bulk of the work concerns Kepler's prolonged and painstaking analysis of the orbit of Mars, whose pronounced eccentricity made it uniquely suited - and uniquely demanding - for testing orbital models. Kepler described his failed attempts to fit the Martian data to circular orbits, including the vicarious hypothesis and various equant-based models, before arriving at the ellipse as the correct geometric solution. The methodological transparency of Astronomia Nova - including the documentation of wrong turns - was unusual for the period and has been noted by historians of science as significant in its own right.

The work also advanced a proto-physical account of the force driving planetary motion, drawing on William Gilbert's work on magnetism. Kepler proposed that the Sun emits an animating power (anima motrix) that sweeps the planets along their orbits, with its influence weakening with distance - a precursor to, though distinct from, the inverse-square gravitational theory later formulated by Isaac Newton.

Astronomia Nova was published in Prague in 1609, the same year Galileo Galilei began his telescopic observations. Initial reception among astronomers was mixed. The departure from circular orbits - a constraint inherited from Aristotelian cosmology and retained even by Copernicus - was a significant conceptual barrier for contemporaries. Adoption of Kepler's elliptical model was gradual; it became widely accepted only after Newton demonstrated in the Principia (1687) that elliptical orbits follow necessarily from an inverse-square law of gravitation.

Questions surrounding the provenance of Brahe's data - including whether Kepler's use of that data was fully authorized under the terms governing his access to the Brahe estate's records - have been a subject of historical discussion.

There is broad consensus in the history of science that Astronomia Nova marks a foundational moment in the Scientific Revolution and that Kepler's First and Second Laws, as presented in the work, correctly describe planetary orbital geometry within the Newtonian framework. See Astronomy Consensus and History of Science Consensus.

Interpretations of Astronomia Nova vary across several dimensions:

• Methodological significance - Some historians of science regard Kepler's self-documented trial-and-error as an early exemplar of empirical method; others question whether his account accurately reflects his actual reasoning process. See Kepler's Method - Viewpoint.
• The role of Neoplatonism and theology - Kepler's natural philosophy was deeply shaped by his theological commitments and a Neoplatonic conception of cosmic harmony. The degree to which these shaped, rather than merely accompanied, his scientific conclusions is debated. See Kepler's Neoplatonism - Viewpoint.
• Relationship to the Scientific Revolution - Whether Astronomia Nova represents a clean break from Aristotelian cosmology or a hybrid transitional work is a point of ongoing historiographical discussion. See Scientific Revolution - Debate.
• The Brahe data question - The circumstances under which Kepler obtained and used Tycho Brahe's observations have raised questions about intellectual and legal propriety by some scholars. See Brahe Data - Viewpoint.

• Astronomia Nova - History
• Astronomia Nova - Debate
• Astronomia Nova - Astronomy Consensus
• Johannes Kepler
• Tycho Brahe
• Scientific Revolution
• Kepler's Laws of Planetary Motion
• Harmonices Mundi

1. Kepler, Johannes. Astronomia Nova. Prague, 1609. Standard modern edition: Caspar, Max, ed. Gesammelte Werke, vol. 3. Munich: C. H. Beck, 1937.
2. Caspar, Max. Kepler. Translated by C. Doris Hellman. London: Abelard-Schuman, 1959. The standard biographical reference.
3. Donahue, William H., trans. New Astronomy. Cambridge: Cambridge University Press, 1992. The standard English translation with commentary.
4. Gingerich, Owen. “The Astronomy and Cosmology of Copernicus.” Highlights of Astronomy 3 (1974). Background on the pre-Keplerian tradition.
5. Voelkel, James R. The Composition of Kepler's Astronomia Nova. Princeton: Princeton University Press, 2001. Detailed study of the work's construction and the Brahe data question.
6. Newton, Isaac. Philosophiæ Naturalis Principia Mathematica. London, 1687. Demonstrated the gravitational basis of Kepler's laws.