enrolled at Trinity College, Cambridge

recorded naked-eye observations of a comet in his Waste Book, or journal, under a section headed "Quaestiones quaedam Philosophicae" (Certain Philosophical Questions) set aside for his musings on natural phenomena (heat, light, sound...)

bought and read books on mathematics and astronomy including Galileo's Dialogue on the Two Chief World Systems and Descartes' numerous philosophical works, most notably the Principia Philosophiae

learned about Kepler's "first" and "third" laws of planetary motion from Thomas Streete's new book, Astronomia Carolina (1661)

recorded his early views on the nature of "Gravity and Levity" in the Quaestiones:

Newton's early thoughts on the nature of gravitational attraction are reminiscent of Descartes' vortical theory. 

In Newton's view of gravity, ethereal matter is contantly rising.  When it reaches the upper atmosphere it condenses and begins to "rain down" on the Earth.  As it falls, it presses down on all substantive matter.

Newton analysed the circular motion of a ball on a string.  Armed with Descartes' law of inertia, he focused his thoughts on the ball and its resistance to change in either its rate or direction of motion. 

Constrained by the string to move in a circle, the whirling ball is constantly being forced to change its direction.  At every instant, because of its natural tendency to move forward without constraint, the ball resists that change.  Newton referred to this resistance as "centrifugal endeavor", a calculatable quantity (= v²/r, where "v" is the speed of the ball and "r" is the length of the string).

August.  With a deadly outbreak of bubonic plague spreading through England, Cambridge University sent students home as a precaution.  Students were invited to return the following March, but a resurgence of the epidemic sent them home again in June.  Classes did not resume until spring 1667.

Newton spent the duration of Cambridge's hiatus at his home, Woolsthorpe, in Grantham.  While there he applied what he had learned about the motion of a ball on a string to the problem of planetary orbital motion (assumed here to be circular).

The situations are similar, but not precisely identical: 

• The ball must remain firmly attached to the string in order to continue moving in a circle, but the planets are not attached in any visible way to the Sun. 

• The ball can be whirled about at a wide range of desired speeds, but each planet moves around the Sun at a rate that is unique to itself. 

When Newton calculated the "centrifugal endeavor" of each planet and compared their values, he found them to be proportional to 1/r², where "r" is a given planet's distance from the Sun. 

Note that Newton is merely describing the planets' inertial response to an external constraint on their motion.  He has not characterized the force at work.  That is because he still thought of planetary motion in a very Cartesian way:  Planets are swept along by the swirling solar vortex.  Indeed, he continued to think about planetary motion in those terms for many years. 

That is not the way Newton's views on planetary motion are commonly presented in historical accounts.  According to biographers who knew Newton personally in his later years, the young scholar was "pensively meandering" in the garden at Woolsthorpe one day when a falling apple caught his attention.  He is reported to have wondered if the force that pulled the apple to the ground extended as far as the Moon.  Could this be the same force that keeps the Moon tethered in its orbit around the Earth?  How strong would that force be at such a great distance?

An apple tree in the garden outside Newton's apartment at Trinity College, Cambridge as seen in May 2009.  The tree has been grown from a graft of an apple tree reputed to have been "the" tree whose falling apple inspired Newton to ponder the relative strengths of Earth's gravitational pull on the apple and the Moon.

These accounts are based on Isaac Newton's own recollections of events.  It is tempting to give great weight to such an authoritative eyewitness, but it is important to keep in mind that Newton was describing his earliest thoughts on the question of gravity long after Principia had been published, read and acclaimed.  There is no contemporaneous evidence to verify that Newton was even thinking about gravity as an attractive force at this early stage in his career. 

Hindsight always colors even the best of memories.  Priority disputes -- such as the one that Newton became embroiled in with Robert Hooke over who was the first to enunciate gravity's inverse-square law -- further skew participants' memories of what they did and when they did it.

created his first working model of a reflecting telescope

It is to be supposed ... that there is an aethereall Medium much of the same constitution with air, but far rarer, subtiler & more strongly Elastic....  But it is not to be supposed, that this Medium is one uniforme matter, but compounded partly of the maine flegmatic body of aether partly of other various aethereall Spirits....  For the Electric & Magnetic effluvia and gravitating principle seem to argue such variety.  Perhaps the whole frame of Nature may be nothing but various Contextures of some certaine aethereall Spirits or vapours condens'd as it were by praeciptiation, much after the manner that vapours are condensed into water or exhalations into grosser Substances, though not so easily condensible....  At least the electric effluvia seem to instruct us that there is something of an aethereall nature condens'd in bodies.... [S]o may the gravitating attraction of the Earth be caused by the continuall condensation of some ... aethereall Spirit, not of the main body of flegmatic aether, but of something very thinly and subtily diffused through it ... [I]f such an aetherall Spirit may be condensed in fermenting or burning bodies, or otherwise coagulated, in the pores of the earth and water, into some kind of humid active matter for the continuall uses of nature ... the vast body of the Earth ... may continually condense so much of this Spirit as to cause it from above to descend with great celerity [speed] for a supply.  In which descent it may beare down with it the bodys it pervades with force proportionall to the superficies [surface area] of all their parts it acts upon... ...For nature is a perpetuall circulatory worker....  And as the Earth, so perhaps may the Sun [drink in] this Spirit copiously to conserve his Shineing, and keep the Planets from recedeing further from him.

correspondence with Robert Hooke prompted Newton to think more deeply about the nature and cause of free fall and of planetary motions

began to describe such motions, not in terms of a moving body's inertia, but as a natural and dynamic response to a center-seeking attractive force

January.  Christopher Wren (1632-1723), Robert Hooke (1635-1703) and Edmund Halley (1656-1742) met in a coffeeshop.  The conversation turned to an interesting puzzle:  what shape would planetary orbits have "supposing the force of attraction towards the Sun to be recipocal to the square of their distances from it"? 

Recall that as early as 1603, Kepler had declared all planetary orbits to be elliptical.  But this was a conclusion based on observational evidence.  An ellipse was the best fit Kepler could find to match Tycho's data.  What if you tackled the problem the other way around and started from unassailable mathematical principles?  Would you get the same answer? 

Hooke was certain that orbital paths must be ellipses and claimed he could demonstrate it mathematically.  Wren offered a prize to the first of the three who brought in a proof for all to see.

August.  Months passed and Hooke had yet to produce his evidence.  Edmund Halley traveled to Cambridge to find out what Isaac Newton had to say on the matter. 

When Halley put the question to Newton, Newton promptly replied, "It would be an Ellipsis."  When Halley asked Newton how he could be so sure, Newton replied "Why ... I have calculated it."

November.  Newton sent Halley a nine-page manuscript titled De Motu Corporum in Gyrum (On the Motion of Orbiting Bodies).

In De Motu, Newton finally abandoned all remnants of the Cartesian vortical system and turned instead to one in which a planet's orbital motion is governed by a Sun-centered attractive force that decreases in strength with the square of the planet's distance from the Sun.

Philosophiae Naturalis Principia Mathematica [Mathematical Principles of Natural Philosophy] published with Halley's financial assistance

Newton developed a new and revolutionary view of falling bodies -- a view that brought the physics of the terrestrial and celestial realms together into one simple package.

All bodies, regardless of size, shape, constitution, color, texture.... generate and respond to gravitational forces in the same way.  The Earth pulls on and is pulled by an apple (a terrestrial body) in the same way it influences and is influenced by the Moon (a celestial body).

Although it took a while for Newton's ideas to become known, understood, and accepted, the days of the Earth-centered universe were numbered.  As perfect circles gave way to ellipses and the distinction between terrestrial and celestial matter evaporated, Aristotle's physics became more and more of a philosophical fossil:  a relic from which students could learn "what people used to think."

died at London; buried in Westminster Abbey

This famous Newton, this destroyer of the Cartesian system, died in March, anno 1727.  His countrymen honoured him in his lifetime, and interred him as though he had been a king who had made his people happy.

-- Voltaire, from Letters on the English, XIV