Jens Asbjørn Bøgen
It might seem unfathomable to us living today, but 179 years ago on this very date, our galaxy, The Milky Way, actually grew by one planet.
It might seem unfathomable to us living today, but 179 years ago on this very date, our galaxy, The Milky Way, actually grew by one planet.
Okay, okay, we know, that that is not what actually happened, but on September 23rd 1846, telescopic observations finally confirmed that there was actually an eighth planet orbiting the Sun.
But wait a minute – confirmed? Wasn’t it discovered on this very date?
Yes, it was. But the existence of the eighth planet, now known as Neptune, had actually been predicted more than a decade before the discovery.
Not by a prophet, though – by mathematics.
The early observations
The mystery of Neptune begins with earlier observations by astronomers who saw it but did not recognize it as a planet
Galileo Galilei observed Neptune in 1612 and 1613, but mistook it for a fixed star. Later observations by Jérôme Lalande in 1795 and by John Herschel in 1830 similarly saw the object but did not register it as moving.
Then in the early 19th century, astronomers noticed that Uranus was not behaving quite as predicted by Newtonian gravitational theory. Deviations or “irregularities” in its orbit suggested that something else was tugging on it.
Alexis Bouvard published tables of Uranus’s motion which began to show this mismatch, prompting speculation that a more distant, unseen planet was influencing it.
Two men, John Couch Adams in England and Urbain Le Verrier in France, independently worked on the perturbation problem. They used mathematical modelling to estimate where such a perturbing body might lie, its mass, and its orbital parameters.
The search begins
By 1845-46 both had done calculations; Le Verrier presented his results to the Académie des sciences in Paris.
When Le Verrier’s prediction reached Germany in autumn 1846, Johann Gottfried Galle at the Berlin Observatory, assisted by Heinrich Louis d’Arrest, set out to look for the predicted planet.
Using Le Verrier’s location, they compared star charts to what they saw through the telescope.
One hour later
On the night of September 23-24, 1846, Galle and d’Arrest used a Fraunhofer refractor to scan the skies.
Within about an hour of searching, and less than one degree from where Le Verrier had predicted it to be, they found a “star” not on the chart.
That turned out to be Neptune.
After verifying its motion over a couple more nights, Galle telegraphed confirmation to Le Verrier: “the planet whose place you have computed really exists.”
It was a striking example of prediction followed by observational verification, and a triumph of mathematical astronomy.
Dispute over credit
The announcement of Neptune’s discovery triggered debate over priority.
Adams had worked out a similar prediction independently, but his findings were not published in time, nor followed by observational confirmation.
In Britain there was some criticism of the Royal Observatory and its leadership for delays.
Le Verrier was awarded the Copley Medal by the Royal Society in 1846 primarily for his prediction. Adams received recognition too, though somewhat later, once his role became more widely known.
Both are often now credited, though many historians argue Le Verrier’s contribution was decisive.
This article is made and published by Jens Asbjørn Bogen, which may have used AI in the preparation
