Chapter 13, second method

This animation corresponds to Kepler's second method for determining the inclination of the plane of Mars's orbit from the latitudes of Mars as observed from the earth (pp. 226-229). In this method, Kepler asserts that when the earth (or, what is the same, the Sun) is in one of Mars' nodes (16 or 17 degrees Taurus or Scorpio) than the latitude percieved by the earth, corresponds to the latitude which the Sun would percieve were Mars in the same constellation. This animation is meant to demonstrate the coincidence of the latitudes by providing the line of the earth's perception and that of the Sun's were Mars in the same constellation. The moving red and purple lines are parallel, that is, they point to the same constellation. The green and purple tracks left behind are the earth seeing Mars and the sun seeing Mars were it in the same constellation. If you rotate the view so as to stand on the line of the nodes, you can see that the two lines have the same inclination to the plane of the ecliptic.