Astronomia nova

Astronomia nova (English: New Astronomy, full title in original Latin: Astronomia Nova ΑΙΤΙΟΛΟΓΗΤΟΣ seu physica coelestis, tradita commentariis de motibus stellae Martis ex observationibus G.V. Tychonis Brahe) is a book, published in 1609, that contains the results of the astronomer Johannes Kepler's ten-year-long investigation of the motion of Mars.

One of the most significant books in the history of astronomy, the Astronomia nova provided strong arguments for heliocentrism and contributed valuable insight into the movement of the planets. This included the first mention of the planets' elliptical paths and the change of their movement to the movement of free floating bodies as opposed to objects on rotating spheres. It is recognized as one of the most important works of the Scientific Revolution.

Prior to Kepler, Nicolaus Copernicus proposed in 1543 that the Earth and other planets orbit the Sun. The Copernican model of the Solar System was regarded as a device to explain the observed positions of the planets rather than a physical description.

Kepler sought for and proposed physical causes for planetary motion. His work is primarily based on the research of his mentor, Tycho Brahe. The two, though close in their work, had a tumultuous relationship. Regardless, in 1601 on his deathbed, Brahe asked Kepler to make sure that he did not "die in vain," and to continue the development of his model of the Solar System. Kepler would instead write the Astronomia nova, in which he rejects the Tychonic system, as well as the Ptolemaic system and the Copernican system. Some scholars have speculated that Kepler's dislike for Brahe may have had a hand in his rejection of the Tychonic system and formation of a new one.

By 1602, Kepler set to work on determining the orbit pattern of Mars, keeping David Fabricius informed of his progress. He suggested the possibility of an oval orbit to Fabricius by early 1604, though was not believed. Later in the year, Kepler wrote back with his discovery of Mars's elliptical orbit. The manuscript for Astronomia nova was completed by September 1607, and was in print by August 1609.

In English, the full title of his work is the New Astronomy, Based upon Causes, or Celestial Physics, Treated by Means of Commentaries on the Motions of the Star Mars, from the Observations of Tycho Brahe, Gent. For over 650 pages (in the English translation), Kepler walks his readers, step by step, through his process of discovery. The work is divided into 5 parts, and contains a total of 70 chapters.

In the first part, Kepler examines the relationship between the various astronomical hypotheses that were in use at the time.

In chapters 1-3, He shows that the heliocentric, geocentric and Tychonic models are all mathematically equivalent in that they predict the same angular positions of celestial object, and the variation in distance for any planet are also the same in all three models. This is so because the epicycle in the geocentric model plays the same role as the orbit of the Earth in heliocentric model and the orbit of the sun in Tycho's model. But the motion of the planets is observed to be non-uniform, even if we ignore the effects of the epicycle or the Earth's motion. Ptolemy and Copernicus had two different explanations for this. Ptolemy used an equant and eccentric circle, whereas Copernicus realized that he could combine two epicycles to explain the same effect. Kepler showed, however, that Copernicus' explanation is simply equivalent to an equant point with a non-circular orbit. The difference in the predictions between the two explanations were minor and are for practical purposes equivalent.