Johann Galle needed under an hour on September 23, 1846, to find Neptune within a degree of the spot Urbain Le Verrier had calculated from irregularities in Uranus's orbit and mailed to Berlin sight unseen; it became the first planet found by math before anyone saw it.

Neptune should have been found the ordinary way: a patient astronomer sweeping a telescope across the sky, night after night, until one point of light refused to sit still against the stars behind it. It wasn't.

It was found by a mathematician who never looked up.

What was wrong with Uranus?

By the 1820s, Uranus was misbehaving. The French astronomer Alexis Bouvard had drawn up tables predicting its path using Newton's laws, and the planet kept drifting off them anyway, first ahead of schedule, then behind. Bouvard suspected some outside force was tugging at it and said so in print, but he died in 1843 without finding the culprit.

The problem passed to Urbain Le Verrier, a mathematician at the Paris Observatory with a reputation for finishing what other people started. Working through 1845 and 1846, he treated Uranus's wobble as a puzzle in reverse: instead of predicting where a known planet would go, he asked what mass, at what distance, moving at what speed, would produce exactly this deviation. He solved for a planet nobody had ever recorded.

Did he tell anyone?

He did, and almost nobody in France listened. Le Verrier's colleagues at the Paris Observatory showed little urgency about pointing a telescope where he said to look, so in the summer of 1846 he tried a different address. He wrote to Johann Gottfried Galle at the Berlin Observatory, a young astronomer he had never corresponded with before, and enclosed his calculated coordinates.

The letter arrived on September 23, 1846. Galle asked his observatory's director for permission to search that same night, and got it. He recruited a student, Heinrich Louis d'Arrest, who suggested they work against a freshly printed star chart of that region of sky, the Hora XXI sheet, engraved by Carl Bremiker and only just delivered to the observatory.

They found an eighth-magnitude object within the hour. D'Arrest ran to check it against Bremiker's chart and reportedly called out that the star wasn't on the map. It sat less than one degree from where Le Verrier said it would be: a difference smaller than the width of a fingertip held at arm's length, for a body so far from the sun that its light takes about four hours to arrive.

Was Le Verrier really first?

Not entirely, and this is where the story gets ugly. An English mathematician named John Couch Adams had been chasing the same problem since 1843 and reached broadly similar numbers well before Le Verrier published his. George Airy, the Astronomer Royal, knew about Adams's work and, after Le Verrier's success became public, pushed hard to have Adams recognized as a co-discoverer.

The trouble is that Adams never nailed down one clean answer. He circulated at least half a dozen shifting estimates between 1845 and 1846, some of them twenty degrees apart in the sky, and he never got a British telescope pointed decisively at the planet before Berlin did. James Challis at Cambridge began his own search on July 29, 1846, and, it later turned out, had actually recorded the planet twice (on August 8 and August 12) without recognizing it as new; his star maps were too out of date to show that the object had moved.

Adams himself was gracious about it. In November 1846 he told the Royal Astronomical Society plainly that Le Verrier's work "led to the actual discovery of the planet by Dr. Galle," full stop. Airy and the British press were considerably less gracious on his behalf, and a nationalist tug-of-war over credit ran for years afterward.

Modern historians, working from Royal Greenwich Observatory papers that resurfaced only in the twenty-first century, have leaned toward Le Verrier. A file of Adams's key working papers went missing from the observatory's archive for over a century and turned up again in 1998; when the science historian Nicholas Kollerstrom examined it, he concluded that Adams's 1845 predictions were far less complete than Airy later claimed. That reassessment is not universally accepted. What is not in dispute is who mailed usable coordinates to an astronomer willing to act on them that same night.

Why isn't it called Le Verrier?

For a few weeks, it nearly was. The French astronomer François Arago proposed naming the new planet after its discoverer, and some French almanacs briefly printed it that way. Le Verrier does not seem to have objected.

International astronomers wanted a name that fit the pattern set by Mercury, Venus, Mars, Jupiter, and Saturn, and by 1847 the convention had settled on Neptune, for the Roman god of the sea, a nod to the planet's blue color as seen through a telescope. Uranus's own name had been an argument a few decades earlier for the same reason; astronomers were not eager to repeat it.

What happened to it after 1846?

For 143 years, Neptune was a point of light and a set of orbital calculations, nothing more. That changed on August 25, 1989, when NASA's Voyager 2 became the only spacecraft ever to fly past it, sending back images of a deep blue globe wrapped in the fastest winds recorded anywhere in the solar system, gusts clocked well above 1,000 miles per hour.

No mission has been back since. NASA and the European Space Agency have both studied proposals for a dedicated Neptune orbiter, sometimes called Neptune Odyssey, that would arrive sometime in the 2040s if funded; nothing has been formally approved. Two centuries after Bouvard first noticed Uranus drifting off its tables, the planet that explained the drift is still mostly known through a single flyby and a handful of equations.

Sources

• Nicholas Kollerstrom, "Neptune's Discovery: The British Case for Co-Prediction," University College London, and his subsequent analysis of the recovered Royal Greenwich Observatory "Neptune File."
• Encyclopaedia Britannica, "Urbain-Jean-Joseph Le Verrier" and "Johann Gottfried Galle."
• Wikipedia, "Discovery of Neptune," for the timeline of Le Verrier's letter, the September 23–24, 1846 search, and the Bremiker star chart.
• NASA Jupiter/Neptune mission pages and Voyager 2 flyby records, National Aeronautics and Space Administration.
• Royal Astronomical Society, Monthly Notices, November 1846, for John Couch Adams's own account of the discovery.