From the October 2002 issue

Neptune’s discovery

British astronomers had Neptune in the bag. But then they let it slip away...
By and | Published: October 27, 2002 | Last updated on May 18, 2023

In a classic example of misunderstanding, Adams regarded Airy’s question as having little importance. Adams felt that Airy was attempting to evade him. Feeling rebuffed for having been turned away at the door, Adams failed to respond to Airy, effectively terminating their correspondence for almost a year. Adams’s failure to pursue the matter further cannot be laid solely at Airy’s doorstep. One of his Cambridge colleagues later commented, with some justification, that Adams “acted like a bashful boy rather than like a man who had made a great discovery.”

The older Le Verrier, unlike Adams, was already an established figure in science due in part to his brilliant work on Mercury’s orbit. He was, moreover, a forceful personality. And whereas Adams’s results were known only to Airy and Challis, Le Verrier published his planet prediction in June 1846.

On seeing the remarkable coincidence between Le Verrier’s and Adams’s predictions (they agreed to within half a degree — the diameter of a Full Moon) Airy was at last stirred into action. He confidently announced on June 29, 1846, at a high-level meeting in Greenwich that included Challis and John Herschel, “the extreme probability of now discovering a new planet in a very short time, provided the powers of one observatory could be directed to search for it.” Airy exchanged letters with Le Verrier. Le Verrier politely answered Airy’s technical question, even though he, like Adams, found it to be irrelevant.

Airy quickly drew up a plan for a search, though interestingly, he said nothing about the search or Adams to Le Verrier. Airy claimed that none of the Royal Observatory’s telescopes were large enough to detect the planet (the largest was the 6.7-inch Sheepshanks refractor), so he asked Challis to conduct a search with the Cambridge Observatory’s 11.7-inch Northumberland refractor. (Perhaps the real reason for Airy’s reluctance is that he did not want to bog down the Royal Observatory in time-consuming observations.) Airy advised Challis to map the stars in the region between Aquarius and Capricornus where Adams’s and Le Verrier’s calculations had placed the planet.

Poor Challis didn’t stand much of a chance. But he resolved to do his duty, and on July 29 he finally got underway. Clearly Challis was expecting a long siege. He went about his work in a rather perfunctory manner, not bothering to compare his observations from night to night. He was also preoccupied with observations of a new comet, which in his mind took higher priority. Challis later wrote, “It was so novel a thing to undertake observations in reliance upon merely theoretical deductions; and that while much labor was certain, success appeared very doubtful.”

While Challis was plodding along, now came the missed opportunities that Herschel would later regret. In August 1846, Herschel paid a visit to his friend William Rutter Dawes, an amateur astronomer based near Liverpool. Herschel leaked information about the planet search. Herschel suggested that Dawes search for the planet himself with his 6-inch refractor, but conveyed the impression that it would not appear “until around Christmas.” This might explain why Dawes failed to seize the initiative. Another possibility is that Dawes believed that the planet, if it existed, would only be visible in large telescopes. (In fact, it can be seen in binoculars, if one knows where to look.) Dawes did not search himself but instead passed the information along in a letter to his friend, William Lassell.

Lassell, a brewer by profession, had just completed work on a splendid 24-inch equatorially mounted reflector at his private observatory near Liverpool. Recognizing Lassell’s superior equipment, Dawes confided to Lassell the probable position of the new planet and told him to search for a star with a disk.

Again, however, bad luck plagued the English. The story of Lassell’s role in the events leading up to the discovery of Neptune comes from accounts by two astronomers, Edward S. Holden and Albert Marth, who interviewed Lassell’s daughter 30 years after Neptune’s discovery. This is perhaps the least-known sidelight of the Neptune story.

Apparently, Dawes wrote to Lassell only two weeks before the planet’s actual discovery. When his letter arrived with the predicted position of the planet, a sprained ankle was confining Lassell to a sofa. The letter, unfortunately, was misplaced through the carelessness of a maid-servant (shades of Airy’s butler). Lassell attempted to find the letter the following night, but was unsuccessful. By then, events were moving too quickly for him to keep pace. Before he could follow up with a letter to Dawes requesting the position that had been lost, the planet was within Galle’s grasp, and England’s last, best chance for a discovery had slipped away.

On reading the Times announcement on October 1, John Herschel penned a letter to the journal Athenaeum. This letter made the first public mention of Adams’s name and his earlier but unpublished results. Herschel’s belated announcement naturally angered the French, who felt the English were trying to steal Le Verrier’s glory. The controversy over “Le Verrier’s planet,” or Neptune as it was soon being called (after the Greek and Roman god of the sea), was played out against a long background of Anglo-French tension and hostility. The two countries had been at odds for centuries — the Napoleonic Wars had ended only in 1815, and in recent times they had been involved in serious disagreements over affairs in Egypt and Spain. On October 17, Challis published a candid account of his search in the Athenaeum — confessing that he had recorded the unknown planet twice, on August 4 and 12, but had not gotten around to comparing the observations. Challis would have found the planet easily if only he had possessed the star chart that Galle used. Challis’ account described Adams’s work going back to 1843, but failed to even mention Le Verrier’s name. The French were outraged, and newspapers in Paris vilified Airy, Challis, and Adams.

Both Challis and Airy came in for searing criticism in England as well for what seemed, in retrospect, their rather half-hearted efforts to follow up on Adams’s promising lead. Even one of Airy’s Cambridge friends exclaimed, “Oh! curse their narcotic Souls!” Adams and Le Verrier, however, did not participate in the nasty international priority dispute. They actually met in Oxford the following June without rivalry or bitterness and parted as lifelong friends.

In addition to the Adams bombshell, Herschel wrote another important letter on October 1. He wrote to Lassell, with whom he had been in correspondence about the satellites of Saturn. Herschel must have felt guilty that he had not directly informed the brewer-astronomer about the unknown planet. Now he wrote with considerable urgency: “Look out for satellites with all possible expedition!” Something might yet be salvaged for England. If there were Neptunian satellites to be discovered, Lassell with his powerful telescope would be the man to do so.

Lassell indeed turned his reflector on the new planet, and on October 2 recorded that its small disk was surrounded by a ring. Then on October 10 he noticed a “star” whose “close situation…and minuteness…occasioned my strong suspicion that it may be a satellite.” The ring proved to be illusory — it had nothing to do with the actual rings of the planet, which are far too wispy and faint to have been seen in Lassell’s telescope. But Lassell had found a satellite, later named Triton, which he confirmed in July 1847.

One intriguing question remains. If Herschel had informed Lassell sooner of the position of the planet or if Dawes’s letter had arrived at a more opportune moment, would Lassell have made the discovery by sighting its small disk among the stars? We can never know for certain, but the answer seems very likely to be yes.

Galle at first had been unable to make out the disk, but he was sweeping with low power. After he and d’Arrest had recognized it, he is quoted as having said on examining it with higher power, “My God in heaven, this is a big fellow!” D’Arrest too, the night after the discovery, remarked on its disk: “There it is! There is the planet, with a disk as round, bright, and beautiful as that of Jupiter!”

Neptune’s disk was well within range of Lassell’s 24-inch reflector, and given his skill and diligence, he would have recognized it if only he had known where to look. You can just make out the disk with a 6-inch telescope, using magnifying powers of 300x to 400x. It is slightly larger than the disks of Jupiter’s four Galilean satellites, which are another exacting test of small-telescope optics and seeing.

In a large instrument, such as the Lick 36-inch refractor, with a magnification of 1200x, Neptune appears satis — fyingly round and planetary — like a cold-blue moon. It is distinctly bluish, in contrast with Uranus, which appears greenish. In recent years comparing the two planets has been convenient due to their proximity in the sky — they lie within a few degrees of each other, where in 1993 they passed their first mutual conjunction since 1821-22.

But it’s sobering to realize that Neptune has yet to complete one circuit of the Sun since it was discovered; its period of revolution — the Neptunian “year” — is 165 Earth years. The first “Neptunian anniversary” of its discovery will not occur until 2011, when the planet will finally return to the same star field where Johann Galle found it in 1846.

Even with a small telescope, it is always exciting to spot this portentous little disk among the stars, and to recall that as small as it seems, Neptune is a planet with a diameter four times that of Earth; a giant world massive enough for its influence to have been felt by mathematicians before it was seen in the telescope. Inevitably, one’s mind returns to the thrilling days of a century and a half ago when Challis, Dawes, or Lassell might well have found it, but it fell instead to the deserving — and fortunate — Galle in Berlin. And d’Arrest’s exclamation “That star is not on the map!” remains among the most ringing words uttered in the history of astronomy. The London Times announced “Le Verrier’s Planet Found,” Thursday, October 1, 1846. This was startling news to most people of the day, and is still one of the most memorable events in the history of astronomy. For the first time in the 19th century and only the second time in history, astronomers had discovered a new major planet in the solar system.

The name Johann Gottfried Galle, a German astronomer at the Royal Observatory in Berlin, will forever be etched in history for first recognizing the planet. But the lion’s share of the glory goes to French mathematical astronomer Urbain Jean Joseph Le Verrier. It was Le Verrier’s prediction of the planet’s position among the stars that led Galle to his finding.

The discovery of the planet made a profound impression on astronomers of the day, who hailed it as the “Zenith of Newtonian mechanics.” But to the lay mind, the discovery seemed nothing short of miraculous. One of Le Verrier’s colleagues commented that he had “discovered a star with the tip of his pen, without any instruments other than the strength of his calculations alone.” Meanwhile, back in England, news of the planet caused more embarrassment than jubilation. While reading the Times, Sir John Herschel, one of England’s leading astronomers, reacted in shocked disbelief. On September 10, just two weeks prior to Galle’s discovery, he had remarked presciently to the British Association for the Advancement of Science: “We see it [the new planet] as Columbus saw America from the shores of Spain. Its movements have been felt trembling along the far-reaching line of our analysis with a certainty hardly inferior to that of ocular demonstration.”

Herschel’s uneasy state of mind was due to his knowledge that the planet’s position had been first predicted not by the Frenchman Le Verrier, but by a young English mathematician, John Couch Adams, a recent graduate of Cambridge University.

Like Le Verrier, Adams had immersed himself in an analysis of Uranus’s motion. Uranus had been wandering slightly off its predicted course ever since its discovery by Herschel’s father, Sir William Herschel, in March 1781. In the early 1800s Uranus traveled too fast, as if it were being pulled forward by another body. But after 1822, it began to slow down, as if another body was holding it back. As we now know, Uranus and Neptune were in conjunction in 1821-22, meaning Uranus was overtaking its more distant cousin.

Both Le Verrier and Adams diagnosed the problem as an unknown planet orbiting outside Uranus. By using the best available mathematical methods and painstaking hand-computation, they independently determined the unknown planet’s position – and their results agreed to within half a degree. However, Adams reached his initial conclusion in September 1845, nine months before Le Verrier’s first published prediction, and a full year before Galle had even sighted the mystery planet. Adams’s theoretical position was within two degrees of where Neptune was lurking in Aquarius.

Unfortunately, Adams did not publish his results. He was a brilliant mathematician, but he was painfully shy and sensitive, and he failed to make a strong impression on his superiors. A fellow Cambridge student remembered him only as “a rather small man, who walked quickly, and wore a faded coat of dark green.”

Adams had been obsessed with the problem of Uranus since July 1841. In February 1844, using James Challis, director of the Cambridge Observatory, as an intermediary, Adams wrote to obtain observations of Uranus from Sir George Biddell Airy, the Astronomer Royal at the Royal Greenwich Observatory. In September 1845, Adams shared his preliminary position of the planet with Challis. But neither Airy nor Challis gave him more than polite encouragement. Moreover, neither encouraged Adams to publish his results.

Airy proved to be the crucial actor on which the whole drama came to turn. A good mathematician and a capable if rather despotic and rigid administrator, he has, largely because of his role in the Neptune affair, acquired the reputation of being an unimaginative and irascible bureaucrat. He seems to have felt that the role of the Royal Observatory was to precisely determine the positions of the sun, moon, and planets. Possibly he felt that other observing projects, including planet-hunting, should not interfere with this task. Airy was also deeply suspicious of theoretical predictions, and he distrusted young people, often treating his junior assistants badly.

As a result Airy has taken most of the heat for the Neptune fiasco. He did, however, express an interest in what Adams was doing (which is more than can be said of most of Le Verrier’s French colleagues), and he sent Adams the Greenwich observations of Uranus that he requested. In the fall of 1845, Adams attempted to visit Airy on two occasions while traveling between Cambridge and his home in Cornwall. However, he failed to announce his visits. On the first occasion Airy was away in France. On the second, Adams actually stopped by twice; in the morning Airy was out, and in the afternoon Airy was at dinner (Airy always ate punctually at 3:30 p.m., an unusual time even for that era) and his butler turned Adams away at the door.

Had Adams announced his visits, and had he actually met Airy in person, matters might well have taken a different turn. Though Adams left a summary of his calculations after his second visit, he went away feeling that he had been rebuffed. Two weeks later, Airy wrote Adams thanking him for leaving his calculations. Airy also asked Adams a technical question regarding Uranus’s orbit that he felt was paramount in solving the discrepant orbital motion. Neptune’s discovery seems almost inevitable in retrospect, but when Le Verrier first published his new planet prediction in June 1846, he found no one willing to look for it. Apparently a brief search was undertaken at the Paris Observatory, but it was soon abandoned, and Le Verrier knew nothing of searches that were going on at the time in England. At last, losing patience, he wrote a letter to Galle. The idea instantly seized Galle’s imagination, and he sold it to his director, Johann Franz Encke, who remarked, “Let us oblige the gentleman in Paris.”

On the night of September 23, 1846, just hours after Galle received Le Verrier’s letter, Galle took his place in the observing chair of the Royal Observatory’s Fraunhofer refractor. At first he attempted to find the planet by locating its small disk among the stars of Aquarius, as Le Verrier had suggested. A German student and volunteer observer, Heinrich d’Arrest, waited patiently as Galle’s discouragement mounted. Finally, d’Arrest suggested that they try using a star chart. Rummaging through a drawer, they found a chart so fresh that it had not yet been mailed to observatories elsewhere. After about half an hour of searching, at just about midnight, Galle sighted an 8th-magnitude star that d’Arrest could not find on the map — it was only 55 arcminutes (about two Moon diameters) from Le Verrier’s calculated position. D’Arrest immediately cried out, “That star is not on the map!” The following night, Galle noticed that the object had moved, proving that it was not a fixed star but rather an object orbiting the sun.

“Monsieur,” Galle wrote to Le Verrier on September 25, “the planet of which you indicated the position really exists.” Le Verrier savored his moment of supreme triumph, and replied to Galle, “I thank you for the alacrity with which you applied my instructions. We are thereby, thanks to you, definitely in possession of a new world.”