Why didn't Johannes Kepler use data about more planets? In Astronomia Nova (1609) Johannes Kepler used observations of Mars in order to refute circular orbits. Throughout Astronomia Nova he hardly even mentions other planets than Mars, Earth and the Sun (and the Moon in a different context). Were there any astronomical reasons for him not to continue, or even beginning, his investigation by using data about other planets?

Tycho Brahe had collected corresponding data about the other planets too, right? 20 years of data is enough for Saturn to make 2/3 of its period. Jupiter and Saturn have almost twice as frequent oppositions as does Mars. And Mercury has twice the eccentricity and its much shorter period should help getting data about its apsides when it is best seen.


Would it had helped him to use several planets during the process as he describes it in Astronomia Nova?
Was it simply the calculation capacity which limited him from dragging several planets into it?
Or could he somehow conclude that it is enough to solve for Mars, to know that his physical conclusions of interests are valid for all planets?
I know that quoting Wikipedia is frowned upon here, but as there has been no other answer posted, this is what the Wikipedia article on Kepler has to say about the matter:


  He then set about calculating the entire orbit of Mars, using the geometrical rate law and assuming an egg-shaped ovoid orbit. After approximately 40 failed attempts, in early 1605 he at last hit upon the idea of an ellipse, which he had previously assumed to be too simple a solution for earlier astronomers to have overlooked. Finding that an elliptical orbit fit the Mars data, he immediately concluded that all planets move in ellipses, with the sun at one focus—Kepler's first law of planetary motion. Because he employed no calculating assistants, however, he did not extend the mathematical analysis beyond Mars. By the end of the year, he completed the manuscript for Astronomia nova, though it would not be published until 1609 due to legal disputes over the use of Tycho's observations, the property of his heirs.[Caspar][Koyré]


[1] Caspar M., Kepler, pp. 131–140;

[3] Koyré A., The Astronomical Revolution, pp. 277–279