Neptune
Another world
The unseen is a mass. Galle finds a point.
Vulcan
A false world
The same habit names a planet that is not there.
Mercury
Another law
The leftover is geometry, not a body.
Newtonian gravity worked extraordinarily well. Mercury’s perihelion still held a small unexplained advance. The leftover was not a missing planet. General relativity accounted for the anomalous 43 arcseconds per century. A successful model met a stubborn remainder.
NASA / JPL / University of Arizona · PIA01384 · DOCUMENTARY IMAGE
Rømer’s Io is a clock that runs late as Earth recedes: light itself becomes slow. Later seeing of the clock-moon. Not Rømer’s 1676 telescope.
The leftover motion does not name its unseen. The fact of mattering does not name its cause.
World of WorldsOne wandering light becomes a system with moons. Later, resonance, timing, and navigation.
The MessengerThe fastest naked-eye wanderer later exposed a limit in Newtonian prediction and became a test of relativity.