Themes
Everything is a conic
One force law, four curves, and the discovery that a comet and a planet differ only in how fast they were going.
Measuring the unreachable
Nothing in this subject can be picked up, weighed or visited. Every number in it was inferred from an angle or a brightness.
Two bodies, and no more
The two-body problem is solved exactly and the three-body problem is not solved at all. Almost everything since has been about living with that.
The sky as a clock
Periods that repeat to the second, used to build calendars, find longitude, and detect planets nobody can see.
Light is the only messenger
One thin stream of photons carries distance, temperature, composition, motion and age — and everything visible is already old.
Small perturbations, large consequences
The corrections that were supposed to be negligible: Neptune found in the residuals, Mercury's extra arcseconds, and orbits that are stable only for a while.
Scales that defeat intuition
Distances, times and densities so far outside experience that the only reliable instrument is arithmetic.