Since 1610 Galileo had been tracking Jupiter's four large moons, drawing their positions night after night. On December 28, 1612, his sketch included a faint object close to Jupiter. By chance, Neptune was then in conjunction with Jupiter, appearing near it in the sky, and it was too faint and moved too slowly for Galileo's small telescope to reveal it as a planet. He recorded it as a fixed star. He saw it again on January 27, 1613.
Neptune moves so slowly against the stars that over a few nights its drift is barely detectable, and in Galileo's telescope it showed no disc.
Yet there are hints that Galileo sensed something odd. On January 28, 1613, he noted in his notebook that a background star appeared to have moved, and he marked Neptune's position with a dot in a different ink from the fixed stars. That suggests he compared it against an earlier sketch, perhaps from January 6, and was systematically checking.
He never followed it up. Neptune was not identified as a planet until 1846, when Johann Galle, working from Urbain Le Verrier's mathematical prediction, found it on the night of September 23-24 at the Berlin Observatory. Galileo had seen it almost 234 years earlier and filed it among the stars. He was not the last: Michel Lefrançois de Lalande in 1795 and John Herschel in 1830 also recorded Neptune without recognising it.
Key people: Galileo Galilei