I guess now they use a mix of observational data and supercomputer calculations, but how did they figure out the orbits of Uranus, Neptune and Pluto (no arguments about Pluto’s status please!) on the discovery of those?
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Kepler’s laws (which can pretty easily be derived from Newton’s law of gravity) tell that the orbit is an ellipse and planets move with certain speeds along their orbits. So, to describe the planet’s movement you only need to know two numbers: axis length (size) of the ellipse and its eccentricity (how stretched it is).
Do some measurements, gather a few points of data (you can describe pretty accurately at what angle the planet is seen from Earth) and then you have a system of equations with only two variables to find. Which can be solved with some trigonometry.
Uranus’ orbit was calculated from observations of the planet in 1783. The observed orbit was slightly different from the calculated orbit which led to the thought that this might be caused by another planet. Calculations were made to predict an orbit of a planet that could cause these effects on Uranus and that led to the discovery of Neptune.
I *believe* they calculated Pluto’s orbit *first* and then looked where it was supposed to be before ever finding it…
But really, it’s not difficult math, once you understand equal area sweeps (which I still do not, so don’t ask me), and some gravitational interactions. You can actually measure the perturbations of a much closer mass to predict the orbit and location of a celestial mass that you can’t see (yet) because the much closer body will wobble in ways its not supposed to *unless* there’s an additional (unseen) mass.
It’s kind of like knowing that there’s a car accident ahead of you where you can’t see it based on the cars slowing down ahead of you that you *can* see…
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