Please explain the whole paragraph:

For example. when Johannes Kepler (1571-1630) examined the extensive data on planetary motion collected by Tycho Brahe (1546-1601), the planetary circular orbits in heliocentric theory (sun at the centre of the
solar system) imagined by Nicolas Copernicus (1473—1543) had to be replaced by elliptical orbits to fit the data better. Occasionally, however, the existing theory is simply unable to explain new observations. This causes a major upheaval in science. In the beginning of the twentieth century, it was realised that Newtonian mechanics, till then a very successful theory, could not explain some of the most basic features of atomic phenomena.

Dear Student,
                           According to this  paragraph earlier  it was assumed that  planetary  motion  was circular 
                         but later Kepler proved it is elliptical and in which sun exists at one of focus of ellipse .
                         and it was  not explainable by old theory . but Newtonian mechanics later explained accurately except some aspects .

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