Earth, planets and orbits
1846
A planet found on paper
Urbain Le Verrier, John Couch Adams; seen by Johann Galle
Can you discover a planet without looking through a telescope?
Uranus kept drifting off its predicted path. Le Verrier and Adams each calculated where an unseen planet must be to pull it. On 23 September 1846 Galle pointed a telescope near Le Verrier’s position and found Neptune within an evening.
Positions come from published orbital elements and start at today’s date. Planet sizes are not to scale, and distances are squeezed unless you choose true scale.
How they figured it out
Uranus, found in 1781, refused to follow its predicted path; by the 1840s it was off by about two arcminutes. Two young mathematicians independently guessed why. In Cambridge, John Couch Adams calculated the position of an unseen planet in 1845, but the Astronomer Royal was slow to act, and when a search began, the observer James Challis recorded the planet twice without noticing.
In Paris, Urbain Le Verrier published his calculation in 1846 and wrote to Johann Galle in Berlin. Galle and his student Heinrich d’Arrest searched the very night the letter arrived, 23 September 1846, and found Neptune within about a degree of the predicted spot.
The equation
Uranus feels the Sun, plus a small extra pull. Work backwards from the extra pull to find what is pulling.
See it in real life
The detective’s method
You hear footsteps upstairs in an empty house and know someone is there before you see them. Geologists find oil from tiny changes in gravity, and doctors locate a tumour from scanner data, by the same reasoning: work back from the effect to the hidden cause.
Where engineers use it
Inverse problems
Working back from effects to causes: medical scans, finding oil underground, locating faults in cables.
Navigation
Interplanetary probes are steered by modelling every small pull on them.
Where it came from
- Universal gravitation 1687
Led to
Newton’s law, trusted completely, said something unseen was pulling Uranus.
- Finding a lost world 1801
Same technique
Both turned scattered positions in the sky into a prediction of where to point a telescope.
Where the comparison stops: Gauss fitted the orbit of an object people had already seen. Le Verrier inferred an unseen planet from its pull on another, a much harder problem.
What it led to
- Mercury misbehaves 1859
Led to
After Neptune, Le Verrier used the same reasoning on Mercury and predicted a planet, Vulcan, that does not exist.