of uniform circular motion for the planets. During the 17th century, several further discoveries eventually led to the wider acceptance of heliocentrism: Using detailed observations by Tycho Brahe, Kepler discovered Mars's orbit was an ellipse with the Sun at one focus and its speed varied with its distance to the Sun. This discovery was detailed in his 1609 book Astronomia nova along with the claim all planets had elliptical orbits and non-uniform motion, stating "And finally... the sun itself... will melt all this Ptolemaic apparatus like butter". Using the newly invented telescope, in 1610 Galileo discovered the four large moons of Jupiter (evidence that the Solar System contained bodies that did not orbit Earth), the phases of Venus (more observational evidence not properly explained by the Ptolemaic theory) and the rotation of the Sun about a fixed axis as indicated by the apparent annual variation in the motion of sunspots; With a telescope, Giovanni Zupi saw the phases of Mercury in 1639; Isaac Newton in 1687 proposed universal gravity and the inverse-square law of gravitational attraction to explain Kepler's elliptical planetary orbits. Modern views Substantially correct From a modern point of view, the Copernican model has a number of advantages. Copernicus gave a clear account of the cause of the seasons: that the Earth's axis is not perpendicular to the plane of its orbit. In addition, Copernicus's theory provided a strikingly simple explanation for the apparent retrograde motions of the planets—namely as parallactic displacements resulting from the Earth's motion around the Sun—an important consideration in Johannes Kepler's conviction that the theory was substantially correct. In the heliocentric model the planets' apparent retrograde motions' occurring at opposition to the Sun are a natural consequence of their heliocentric orbits. In the geocentric model, however, these are explained by the ad hoc use of epicycles, whose revolutions are mysteriously tied to that of the Sun's. Modern historiography Whether Copernicus' propositions were "revolutionary" or "conservative" has been a topic of debate in the historiography of science. In his book The Sleepwalkers: A History of Man's Changing Vision of the Universe (1959), Arthur Koestler attempted to deconstruct the Copernican "revolution" by portraying Copernicus as a coward who was reluctant to publish his work due to a crippling fear of ridicule. Thomas Kuhn argued that Copernicus only transferred "some properties to the Sun's many astronomical functions previously attributed to the earth." Historians have since argued that