What the work claims
That terrestrial and celestial motion obey identical mathematical principles. That gravity governs tides, comets, precession, and planetary orbits. That white light is composed of colours with distinct refrangibility.
How it was done
Newton derived universal gravitation and laws of motion in the Principia. He solved the two-body problem. He used prisms to observe differential refraction of colours in 1666. He built the first functional reflecting telescope. He calculated Earth’s age experimentally.
What holds up
Newton’s derivation of Kepler’s laws from gravity holds. His inference of Earth’s oblateness holds — confirmed by Maupertuis, Clairaut, and La Condamine. His conclusion that colour is intrinsic to light holds — based on prism dispersion in 1666.
What does not
The Principia does not prove gravity is a force acting at a distance; it models its effects. It does not explain why mass generates gravitational attraction. It does not resolve the three-body problem — only introduces it.
Why it matters beyond the lab
It replaced qualitative cosmology with quantitative prediction. It enabled precise navigation, satellite trajectories, and geodetic surveying. It made physics a discipline of testable, universal law.
Is it worth your time
Yes. Newton’s work established the first unified, mathematical framework for physical explanation — one that still underpins engineering, orbital mechanics, and geodesy today.