That’s what happened, or seemed to, when the British scientists who measured the path of starlight around the sun reported to a meeting of the Royal Society that they had observed a number that matched Einstein’s prediction and contradicted Newton. A single unequivocal observation had spoken: Light swerves along the contours of spacetime, and just like that, the 200-year-old Newtonian cosmos came crashing down.

There’s only one problem: It didn’t happen that way.

Albert Einstein had no need to wait four years for confirmation of his theory. From at least a week before he completed general relativity in its final form, he already knew that nature agreed with him. When he did his sums, what had seemed a tiny error in an obscure measurement could be completely accounted for by his theory. For him, that was enough: The general theory was the real thing.

At first glance, that’s just another example of how Feynman said science ought to work. But actually, the mystery that convinced Einstein had gone unsolved for over half a century—and no one, not even Einstein himself until the very end, had recognized the phenomenon for what it was: a decisive challenge to Newton’s whole approach. Instead, decades were spent in pursuit of a planet that by every reasonable measure should have existed, but didn’t.

The story of that missing planet begins with one that was and is very much present. A definitive analysis of the orbit of Mercury in 1859 had revealed a glitch. A tiny wobble, less than one part in 10,000 of the innermost planet’s track around the sun could not be explained by any known source of gravity within the solar system. Within the framework of Newtonian gravitation, the explanation was obvious: If every recognized body had been accounted for, then Mercury’s misbehavior could only be explained by something yet to be discovered, a planet between it and the sun.

First sight of the expected body, captured in transit across the face of the sun, came almost immediately, in December 1859. The new planet was so obviously necessary that there was no hesitation in naming it: Enter Vulcan. Astrophotography—the technique of attaching cameras to telescopes—was in its infancy, so this first observation was drawn and described, but to be confirmed, it would have to be repeated by someone else. No one did, but no matter. Professional and serious amateur astronomers would glimpse their version of Vulcan at least a dozen times over the next 20 years.

The final “Eureka!” came at the great American eclipse of 1878, when James Watson, the director of the Ann Arbor Observatory, recognized Vulcan in a small reddish object within a few degrees of the limb of the shadowed sun. Unfortunately, none of the other professional astronomers at the eight stations set up by the federal government to observe the eclipse saw anything out of the ordinary.