The Wayward Red Planet —the Mars problem

   The Red Planet and the motions challenged every astronomer from antiquity onward. By the late 1500s, the Danish astronomer Tycho Brahe (1546-1601) had amassed decades of observations of Mars and the other planets. His observations were by far the most accurate to date, yet he failed to weld them into a coherent system of the universe. Then Tycho hired a young Austrian mathematician-astronomer named Johannes Kepler (1571 -1630) and put him to work on the orbit of Mars. When Tycho died shortly afterward, Kepler inherited his position and, more importantly, his observations.
   Kepler battled with Mars for much of a decade, finally emerging with the, first two of his three laws of planetary motion. These include the conclusion that planets orbit in ellipses, with the Sun at one focus. (Every prior astronomer had insisted on combinations of circles.)
   Kepler's work paved the way for Isaac Newton's Principia (1687), the cornerstone of all modern physical science.

Retrograde motion of Mars