LearnHistory & discoverySome maths
The discovery of Neptune
A planet found in 1846 by mathematics rather than by searching — predicted from discrepancies in Uranus's orbit, then located within one degree on the first night of looking.
Why it matters
It is the clearest demonstration that Newtonian gravity was not merely descriptive but predictive, and remains the standard against which theories are judged.
What people usually get wrong
Nobody had seen Neptune before 1846.
Galileo drew it in 1612 while observing Jupiter, and recorded that it appeared to move. He took it for a star and did not follow up.
Rests on
- GravityEvery mass attracts every other mass with a force that falls off as the square of the distance between them.
- Kepler's lawsOrbits are ellipses with the Sun at one focus; a planet sweeps equal areas in equal times; and the square of the period is proportional to the cube of the semi-major axis.
3 concepts in the full chain back to first principles.
Leads to
Nothing here builds on it yet.