Sideways Orion

Neptune Completes One Orbit July 1, 2011

On September 23, 1846, the planet Neptune was observed and recognized as a planet for the very first time. This month, it will complete it’s first orbit since its discovery.

That’s right. Neptune is so far away from the Sun that it takes longer than any human’s lifetime to complete one orbit.

That’s very, very far away.

Neptune’s discovery is also an interesting story, and one that really shows just how human Science really is.

Galileo observed it several times, but since it always appeared to be in the same position, he thought it was a star. I’m sure many other astronomers also observed it and thought the same thing. Part of that was surely because they weren’t prepared for there to be more than 5 planets. Sometimes, discoveries are missed because scientists aren’t prepared for the discovery, so it goes unrecognized.

Then in 1781, Herschel discovered Uranus, which was also observed by others but not recognized as a planet. Over the next 20 – 30 years, astronomers mapped its motions and generated tables of its positions. They soon realized its orbit did not perfectly obey Newton’s and Kepler’s laws based on what was already known. However, the variations could be explained if there was another massive planet even farther from the Sun. The first Planet X was born!

Two mathematicians, Adams in Britain and Le Verrier in France used the variations in Uranus’ orbit to predict the location of an 8th planet out beyond Uranus. Adams published his data first, but observers in England showed little interest in taking up the chalenge of finding a dim dot among hundreds of dim dots. Le Verrier published his data, then immediately sent a letter to Galle, director of the Berlin Observatory, asking him to look for the planet. Thinking that astronomers in England would already be searching for it, Galle immediately took up the challenge. Meanwhile, back in England, the astronomer Royale, Airy, noticed the similarity between the two sets of predictions. He wrote to Challis at the Cambridge observatory, encouraging him to take up the search.

On September 23, 1846, Galle identified object in his telescope as the planet Neptune, in a position very close to where Le Verrier predicted. He published his observation, along with an earlier star map that included Neptune. The two maps together showed Neptune’s change in position, showing it was definitely the planet Galle predicted, even if the position wasn’t perfect.

After looking at Galle’s work, Challis realized that he had observed the planet earlier that year, but failed to recognize it, because it wasn’t exactly where Adams predicted it would be. Sometimes, even when you are prepared for a discovery, you miss it because the details aren’t perfect.

And here is where it gets really interesting.

France and England have always been a bit competitive. There was no doubt it was a French astronomer who discovered Neptune, but who predicted it first wasn’t as clear. The competition to be first was just made for the media, and newspapers on both sides took up the challenge to ensure that their countryman was the one who got credit for being first. The fight got downright vicious, with name calling and possible libel on both sides. To this day, people will argue over who got it right. Some credit Adams for publishing first. Others credit Le Verrier for being closer. Most people now credit both. But no matter which side you take, there’s no denying that the competition helped speed the discovery.

The real irony to the vitriol is that the data on Uranus wasn’t really complete, so neither one of them really had a precise calculation. If Challis started observing a few months earlier, Adams’s prediction would have been much more accurate, and Challis probably would have recognized the planet. If Galle had waited a few months more, Le Verrier’s  predictions would have been as bad as Adams’s predictions, and Galle might have missed the planet too. Because of the flaws in the data from Uranus, both sides depended a little on luck, and a little on open-mindedness to actually make the observation.

But best of all (in my opinion) is this: because of the flaws in the data for Uranus, Neptune’s position couldn’t explain the discrepancies in the orbit of Uranus. This caused a  new burst of activity, as mathematicians and astronomers sought to be the first to predict the new planet X.

But that’s another story.