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—— ON THIS DAY ——
SEPTEMBER 23, 1846
Berlin, Prussia
180 years ago
Neptune, the eighth planet from the Sun, identified at the Berlin Observatory on the night of September 23, 1846, after its position was predicted mathematically.
On the night of September 23, 1846, the astronomer Johann Gottfried Galle at the Berlin Observatory, assisted by the student Heinrich d'Arrest, pointed a telescope at a patch of sky and compared what he saw against a newly printed star chart. Within about an hour he found an object of the eighth magnitude that was not on the chart. The following night it had moved. It was a planet, and it lay roughly one degree from where a letter he had received five days earlier said it would be.
The letter was from Urbain Le Verrier, a French mathematician who had calculated the position of an unknown planet from the irregularities in the orbit of Uranus. Uranus, discovered in 1781, had persistently failed to follow the path Newtonian gravity predicted. Something was tugging on it. Le Verrier worked backwards from the deviations to the mass and position of the body causing them, and asked Galle to look.
—— MARQUEE EVENT ——
Urbain Le Verrier, the French mathematician who calculated Neptune's position from irregularities in the orbit of Uranus and directed astronomers where to look.
The discovery set off an immediate and bitter priority dispute. In England, the young Cambridge mathematician John Couch Adams had done comparable calculations somewhat earlier but had struggled to persuade British astronomers to conduct a serious search. When news of Galle's observation arrived, English astronomers announced that Adams had reached the same answer first. The resulting argument between the two countries ran for years. The modern assessment generally credits Le Verrier and Adams jointly for the mathematics, and Galle and d'Arrest for the observation.
The planet had been seen before without being recognized. Galileo recorded it in 1612 and 1613 while observing Jupiter's moons and noted that one apparent star seemed to have shifted, but he did not pursue it. Others catalogued it as a fixed star in the intervening centuries. Recognizing a planet requires noticing motion, and Neptune takes about 165 Earth years to complete one orbit, so it moves slowly against the background.
Neptune is an ice giant, about seventeen times the mass of Earth, with a deep atmosphere of hydrogen, helium, and methane that gives it a vivid blue colour. Only one spacecraft has visited it: Voyager 2, which flew past in August 1989 and revealed a violently active atmosphere with the fastest winds recorded in the solar system, along with a large storm system and the moon Triton, which orbits backwards and has geysers of nitrogen. Almost everything known about the planet in detail still comes from that single encounter, supplemented since by telescopes.
—— WHY THIS MATTERS ——
It was the first planet found by prediction rather than survey. Locating a body by calculating its gravitational effect on another planet, then pointing a telescope at the answer, was a dramatic confirmation of Newtonian mechanics and of the power of mathematical physics.
It produced one of science's most famous priority disputes. The competing claims of Le Verrier in France and Adams in England, and the failure of British astronomers to act on Adams's work in time, became a lasting case study in credit, nationalism, and institutional inertia.
It remains the least visited planet in the solar system. A single spacecraft flyby in 1989 supplied most of what is known in detail about Neptune, its winds, and its strange moon Triton, and no mission has returned in the decades since.
—— THE TAKEAWAY ——
On September 23, 1846, an astronomer in Berlin opened a letter containing a predicted position, aimed a telescope at it, and found a planet within the hour. Neptune had been staring at observers for centuries, including Galileo, without anyone noticing it moved. One spacecraft has been there since.
—— QUOTE OF THE DAY ——
“The planet whose position you have indicated really exists.”
— Johann Galle, writing to Urbain Le Verrier the day after the discovery, September 1846
—— OUR QUIZ OF THE DAY ——
How much do you know about the discovery of Neptune, the calculations of Le Verrier and Adams, the priority dispute that followed, and the Voyager 2 flyby?
