Chapter IX

UNIVERSAL GRAVITATION (FROM 1643 A.D. TO 1727 A.D.),
§§ 164-195 210-246

§ 164. Division of _Newton’s_ life into three periods 210

§ 165. Early life, 1643 to 1665 210

§ 166. Great productive period, 1665-87 211

§ 167. Chief divisions of his work: astronomy, optics, pure
mathematics 211

§ 168. Optical discoveries: the _reflecting telescopes_ of
_Gregory_ and Newton: the _spectrum_ 211

§ 169. Newton’s description of his discoveries in 1665-6 212

§ 170. The beginning of his work on gravitation: the
falling apple: previous contributions to the
subject by Kepler, _Borelli_, and Huygens 213

§ 171. The problem of circular motion: _acceleration_ 214

§ 172. The _law of the inverse square_ obtained from Kepler’s
Third Law for the planetary orbits, treated as
circles 215

§ 173. Extension of the earth’s gravity as far as the moon:
imperfection of the theory 217

§ 174. _Hooke’s_ and _Wren’s_ speculations on the planetary
motions and on gravity. Newton’s second calculation
of the motion of the moon: agreement
with observation 221

§ 175-6. Solution of the problem of elliptic motion:
_Halley’s_ visit to Newton 221

§ 177. Presentation to the Royal Society of the tract _De
Motu_: publication of the _Principia_ 222

§ 178. The =Principia=: its divisions 223

§§ 179-80. The _Laws of Motion_: the First Law: acceleration
in its general form: _mass_ and _force_:
the Third Law 223

§ 181. Law of _universal gravitation_ enunciated 227

§ 182. The attraction of a sphere 228

§ 183. The general problem of accounting for the
motions of the solar system by means of
gravitation and the Laws of Motion:
_perturbations_ 229

§ 184. Newton’s lunar theory 230

§ 185. Measurement of the mass of a planet by means
of its attraction of its satellites 231

§ 186. Motion of the sun: _centre of gravity_ of the solar
system: relativity of motion 231

§ 187. The non-spherical form of the earth, and of Jupiter 233

§ 188. Explanation of precession 234

§ 189. The tides: the mass of the moon deduced from
tidal observations 235

§ 190. The motions of comets: _parabolic_ orbits 237

§ 191. Reception of the _Principia_ 239

§ 192. Third period of Newton’s life, 1687-1727: Parliamentary
career: improvement of the lunar
theory: appointments at the Mint and removal
to London: publication of the _Optics_ and of the
second and third editions of the _Principia_, edited
by _Cotes_ and Pemberton: death 240

§ 193. Estimates of Newton’s work by Leibniz, by Lagrange,
and by himself 241

§ 194. Comparison of his astronomical work with that of
his predecessors: “explanation” and “description”:
conception of the material universe
as made up of bodies attracting one another
according to certain laws 242

§ 195. Newton’s scientific method: “_Hypotheses non fingo_” 245