Four Hundred Years Ago Today Galileo Discovered Neptune – Almost

The discovery of the planet Neptune – the eighth and most distant major planet from our sun – nearly took place on December 27th and 28th, 1612 – four hundred years ago today. Most text books record that it was formally discovered over 230 years later.

On that day, Galileo Galilei turned his telescope toward Jupiter, and made detailed notes on the four moons now called the Galilean moons. He also recorded a “fixed star in his notes.” He was actually looking at the planet Neptune. He saw it again a month later, recording its position accurately in meticulous diagrams in his notebooks.

Of course, Galileo doesn’t need any additional discoveries to add to his fame, but historians are always looking over the notes of notable scientists to see what they actually recorded. Standish and Nobili (1997, below) believe that Galileo actually recorded the position of Neptune a total of four times while making notes about Jupiter and the four moons that, as a group, are named after him

What happened two hundred years later was a more complex story than just a recording of an observation. By 1821 astronomers had compiled tables of the motions of all of the planets out as far as Uranus, the seventh planet in the solar system, Between 1844 and 1846 both British and French astronomers made calculations based on the orbital information of Uranus and started a hunt for an unknown, but predicted, 8th planet. Credit for actually spotting Neptune and realizing what he was seeing goes to Johann Gottfried Galle in Berlin in 1846. He was working with predictions from Urbain Le Verrier in France (who was right about where a new planet should be to within about 1° in the sky) and John Couch Adams in Britain (who was a little further off, at 12°, but both predictions were amazing considering the times).


© 2012, David Allan Galbraith