Over two decades would elapse from Newton’s awakening to his remarkable phase of study in the mid-1660s and his eventual commitment to writing the Principia. This chapter of his life is often filled with myths and folklore in popular history.
The entire era begins with a mix of myths. One prevalent myth suggests that Newton had already conceived the idea of universal gravitation, the core aspect of his Principia, during the mid-1660s. Integral to this narrative are the stories of the apple, both mythic and legendary, along with the Annus mirabilis myth. I have thoroughly examined the apple tale here and will not reiterate my points. As I detailed comprehensively, the Annus mirabilis claims that during a single year of the plague in 1665, the young Newton, who turned twenty-three that year, essentially discovered everything—calculus, optics, universal gravity—for which he would later gain fame. My analysis shows this to be complete nonsense, but the myth endures. In all of this, Newton himself bears responsibility due to statements he made fifty years later:
At the onset of the year 1665 … [claims regarding mathematics and optics]
And in that same year I began to contemplate gravity extending to the moon’s orb ] (having discovered how to gauge the force with wch [a] globe moving within a sphere impacts the sphere’s surface) based on Kepler’s law regarding the periodic times of the planets being in sesquialterate proportion to their distances from the centers of their orbs, I derived that the forces wch maintain the planets in their orbs must [be] inversely proportional to the squares of the distances from the centers about wch they revolve: & thus compared the force necessary to keep the moon in her orb with the gravitational force at the earth’s surface, & found them to be quite similar.[1]
In his Waste Book, a substantial notebook passed down from his stepfather, during this time Newton, motivated by Descartes, recorded three geometrical analyses of circular motion, none of which hold particular significance. Notably, he did not accept the law of inertia at this juncture. However, these analyses led to his juxtaposition of the moon’s “endeavor to move away from the center of Earth” with the gravitational force at the earth’s surface. He discovered that gravity was somewhat over 4,000 times greater. He also integrated Kepler’s third law (which states that the cubes of the mean distances of the planets are proportional to the squares of their periods) into his formula for centrifugal force [derived from Huygens]: “the attempts to move away from the Sun [he established] will be inversely proportional to the squares of their distances from the Sun.” This laid the groundwork for the inverse-square relationship firmly based on Kepler’s third law and circular motion mechanics.[2]
In his later years, Newton’s reflections on his actual accomplishments during the 1660s were intended to quell his critics and affirm his precedence regarding everything, driven by his conflict with Leibniz concerning calculus. His remarks regarding gravity were, posthumously, aimed at Robert Hooke (1635–1703) and Hooke’s assertion that Newton derived the concept of universal gravity from him. This traces back to an exchange from 1679, during which Newton was otherwise engaged in teaching, mathematics, alchemy, and theology, having advanced little further on