Did you know that the planet Neptune was spotted mathematically even before it was first observed? That implies that the one who observed Neptune knew exactly where to look for it. That someone was German astronomer Johann Gottfried Galle. Not that no one before him had spotted the planet. Only that, he was the first one to know he was looking at the planet Neptune when he made his observations.
If we are discussing the discovery of Neptune, then we will have to start it from Uranus. We would actually have to start with Galileo, who sketched the movement of what he thought were the changing positions of a fixed star, in 1613. But since he never bothered to publish these findings, we might skip over 150 years and get to 1781. Uranus was discovered in that year and it didn’t go unnoticed that the planet was being slightly pulled away from its expected orbit, when compared to that predicted by Newton’s laws of gravitation.
The irregularities in the orbit of Uranus suggested that there might be gravitational interference from another heavenly body. Alexis Bouvard, a French astronomer, published astronomical tables of the orbit of Uranus in 1821 and noted these deviations, which he attributed to a hypothetical body.
Another Frenchman, Urbain Le Verrier, used these anomalies to work out a set of mathematical predictions. British mathematician John Couch Adams also reached similar conclusions, working independently. While Le Verrier made his predictions public, Adams kept his work to himself and colleagues at the University of Cambridge.
By 1846, Le Verrier knew the mass and orbital path of the eighth planet, having completed his calculations. But as he got nowhere with members of the French astronomical society, Le Verrier passed on his information to Galle in the Berlin Observatory. On September 23, that year Galle, assisted by his student Heinrich Louis d’Arrest, spotted Neptune within a degree of where Le Verrier predicted it would be, thereby becoming the first to observe and confirm the existence of the eighth planet. He had received Le Verrier’s data that very day!
While Adams gave full credit to Le Verrier for the information that led to Galle’s discovery, it wasn’t equally well received by everyone. Many in England strongly believed that Adams had also predicted the existence of the planet, leading to a tense international dispute over priority.
The Royal Society, however, gave the Copley medal in 1846 to Le Verrier for his achievement, with no mention of Adams. While the proposal to name the planet Le Verrier fell flat, Le Verrier followed traditional nomenclature to name the world Neptune.
Neptune’s discovery was one of those fine moments when science and math worked in tandem, further confirming Newtonian gravitational theory. Francois Arago, a French physicist, mathematician and astronomer, probably had the best words to sum it all up. He referred to Le Verrier as the man who “discovered a planet with the point of his pen”.
Write to the author at ganesh.a.s@thehindu.co.in
