Ada: Le Verrier, frustrated at home, wrote to a young astronomer in Berlin — Johann Galle. The letter is dated the eighteenth of September, eighteen forty-six.
Theo: And what did it say?
Ada: In essence: point your telescope near the border of Capricorn and Aquarius, and look for a small disk. Not a point of light — a disk, the tell of a planet. The letter reached Berlin on the twenty-third.
Theo: So Galle just gets this letter and races to the telescope?
Ada: First he needs permission. He goes to the observatory's director, Johann Encke. And Encke says, reportedly, 'Let us oblige the gentleman in Paris.'
Theo: I love that line. Very gracious.
Ada: And it happened to be Encke's birthday, which may be why he was in a generous mood. That evening Galle takes the Fraunhofer refractor, with a student named Heinrich d'Arrest alongside him.
Theo: Two of them at the eyepiece.
Ada: At first Galle can't spot any disk — the sky's just full of stars. Then d'Arrest suggests something simple. They had a brand-new star chart, Bremiker's Berlin Academy map, so fresh it hadn't even been mailed out to other observatories yet.
Theo: Ah — so anything not on the map is a suspect.
Ada: Precisely. Galle calls out the position of each star through the eyepiece, d'Arrest checks it against the chart. And within about half an hour, near midnight — Galle names a star, and d'Arrest says: it isn't on the map.
Theo: No.
Ada: An eighth-magnitude object, sitting under one degree from Le Verrier's prediction — about fifty-five arcminutes. Accounts differ on the exact time; sources say late on the twenty-third, just after midnight, spilling into the twenty-fourth.
Theo: But how do you know it's a planet and not just a star nobody charted?
Ada: The next night, it had moved. Stars don't. The telescope had confirmed the pen — and immediately the question became: whose pen?