NOTES FOR LECTURE IX
| The Principia published | 1687. |
| Newton died | 1727. |
The Law of Gravitation.—Every particle of matter attracts every other particle of matter with a force proportional to the mass of each and to the inverse square of the distance between them.
Some of Newton's Deductions.
1. Kepler's second law (equable description of areas) proves that each planet is acted on by a force directed towards the sun as a centre of force.
2. Kepler's first law proves that this central force diminishes in the same proportion as the square of the distance increases.
3. Kepler's third law proves that all the planets are acted on by the same kind of force; of an intensity depending on the mass of the sun.[19]
4. So by knowing the length of year and distance of any planet from the sun, the sun's mass can be calculated, in terms of that of the earth.