Le Verrier, Adams, and Galileo: the discovery of Neptune

Residents of the Montparnasse Cemetery on the Left Bank of the Seine in Paris include such household names as Charles Baudelaire, Samuel Beckett, and Camille Saint-Saens. Not quite so well-known outside of scientific circles, but certainly no less revered, is the astronomer and mathematician Urbain Jean-Joseph Le Verrier.

The tomb credits him with the discovery of Neptune in September 1846, making him only the second person ever to discover a planet – and the first to do so by purely mathematical means, unaided by a telescope. But does Le Verrier deserve sole credit, or should it be shared with the British mathematician John Couch Adams? Indeed, should Adams be given sole credit? The debate started soon after the discovery was announced, and it has been going on ever since.

The son of a government official, Urbain Jean-Joseph Le Verrier was born in 1811 in Saint-Lô, Normandy and studied at the École Polytechnique in Paris. An able scholar, he pursued an academic career in the first instance as a chemist, but he made the switch to astronomy when a teaching position came up at the École Polytechnique. His strong mathematical expertise made him well qualified for the job. His work on the gravitational influence of Jupiter upon the orbits of certain comets earned him significant recognition. In January 1846, he was elected a member of the Académie des Sciences.

By this time, Le Verrier was working at the Paris Observatory. The previous year, François Arago, director of the Observatory, encouraged him to work on the perplexing anomalies with the orbit of Uranus. The Sun’s seventh planet is just about visible to the naked eye, but it was not until 1781 British astronomer William Herschel identified it as a planet. Suggestions that it might have been noted earlier, but dismissed as a star, proved to be correct. No fewer than 19 ‘precovery’ observations were found, stretching back to 1690, when John Flamsteed, the first Astronomer Royal, recorded it as a star and gave it the designation 34 Tauri. The problem was that the orbit as computed from these old observations did not agree with that actually observed after 1841. By Le Verrier’s time, Uranus had completed about three-quarters of an orbit around the Sun since its discovery, and a new orbit had been worked out by French astronomer Alexis Bouvard – but the problems had persisted. Up to 1822, the planet seemed to be moving faster than predicted by Newton’s Law of Gravity; but subsequently it was moving too slowly. Attempts to explain the discrepancy included a massive (and somehow unseen) satellite, an impact from a comet, or the existence of a resisting cosmic medium. It was even possible that the fault lay with the Law of Gravity itself.

In 1834, the Rev. Thomas Hussey contacted astronomer George Airy with the suggestion that the gravitational pull of an undiscovered planet was affecting the orbit of Uranus and that the observed orbital data might make it possible to locate the disturbing planet. Hussey was certainly on the right lines, but Airy did not believe that there was any hope of tracking down the planet with the data available, even assuming that it existed at all. Airy – who became Astronomer Royal in 1835 – was equally unforthcoming when Alexis Bouvard’s nephew Eugene contacted him with a similar proposition in 1837.

The problem was next taken up in 1841 by John Couch Adams, an undergraduate studying mathematics at the University of Cambridge. After completing his degree in 1843, he began working on the Uranus question in earnest, and by the end of that year he had a preliminary solution based on the assumption that the new planet obeyed the Titius-Bode Law, an empirical rule which states that the mean distance from the Sun in astronomical units a for planet m in order from the Sun is given by the numerical sequence a=0.4+0.3 x 2m. Although there was (and still is) no theoretical justification for the law, it had been used four decades earlier to successfully predict the existence of Ceres (which at the time was still recognised as a planet but in an episode foreshadowing the recent Pluto controversy was later downgraded to an asteroid). The result obtained by Adams was sufficiently encouraging to convince him that the unknown planet hypothesis was correct, and by September 1845 he had refined his calculations to the extent that he had an approximate position for the planet.

What he lacked was access to a telescope. Accordingly, he communicated with James Challis, director of the Cambridge Observatory, who suggested he contact George Airy. To this end, in October 1845, Adams twice turned up unannounced at the Royal Greenwich Observatory. On the first occasion, Airy was in France and on the second he was having dinner, and his butler refused to disturb him. Adams left Airy a synopsis of his calculations, to which Airy later raised a query concerning the radius vector (i.e. distance from the Sun at a given time) of Uranus, but for reasons unknown Adams failed to reply (it has often been suggested that Adams regarded the query as ‘trivial’, but some sources dispute this).

Meanwhile, as noted above, Le Verrier had been tasked with the Uranus problem by Arago at the Paris Observatory, and in November 1845 he published his first memoir on the subject. A second memoire followed in June 1846, and on 31 August of the same year he published a predicted position for the disturbing planet in a third paper. Word of the second memoire reached Airy, who wrote to Le Verrier posing the same radius vector question he had asked of Adams. Le Verrier replied promptly, and like Adams, requested Airy’s help in locating the planet.

Airy did not respond, and he also kept quiet about Adams’ work, which he was now inclined to take more seriously. On 9 July, he wrote to Challis at Cambridge, asking him to search for the predicted planet. The 12-inch Northumberland refractor at Cambridge, which Airy himself had designed, was one of the biggest telescopes of its day, and it was far superior to anything at Greenwich. Challis began observing on July 29, but he was hampered by a lack of star charts for the zone of interest, and he was therefore forced to undertake a laborious program of observation and chart the positions of all the stars within it. Essentially, his approach was the same as that used to discover Pluto in 1930: comparing star fields over a period of days in order to find a ‘star’ that moved from night to night. Clyde Tombaugh was able to take photographs of the star fields of interest and use a blink comparator to find the moving dot of Pluto, but in the 1840s astrophotography was still in its infancy.

Le Verrier meanwhile had sent his results to the Paris Observatory, and given that he had been working on Arago’s instructions, it might have been expected that the matter would have been given some urgency. But it was not; a brief search was abandoned early in August. On 18 September, Le Verrier wrote to Johann Galle, assistant director of the Berlin Observatory, asking him to look for the planet at the position he predicted. The letter reached Galle on the evening of 23 September, and after getting approval from his boss Johann Franz Encke (of Encke’s Comet fame), he started a search without further ado. Encke did not take part, possibly because 23 September was his birthday. One of Galle’s students, Heinrich d’Arrest, suggested the use of the new Carta Hora XXI (map for Hour 21, i.e. the portion of the sky between R.A. 21h 00m and 22h 00m), a high-resolution star chart that was so recent it had yet to be sent to the publishers.

Galle took charge of the telescope and described the positions and magnitudes of the stars he could see, while d’Arrest checked them off against the chart. It did not take long to find an eighth-magnitude star that did not appear on the charts; and the object also showed a small disk. Encke was hastily dragged away from his birthday celebrations, and he agreed that the object had a resolved disk. A repeat observation the following night confirmed that it had moved in relation to the other stars, and that it was indeed the predicted planet. It was less than a degree away from the predicted position. Galle then wrote to Le Verrier confirming that his planet did indeed exist.

There was understandable enthusiasm in France, and the fact that the actual sighting had been made in Germany was conveniently forgotten. Le Verrier’s achievement was described by Arago as “…of the most magnificent triumphs of theoretical astronomy, one of the glories of the Académie and one of the most beautiful distinctions of our country.” Then came a nasty surprise for the French in the form of a letter from Sir John Herschel (son of William Herschel) to the Athenaeum Club, making reference to the work of Adams. Shortly afterwards, it emerged that Challis had recorded Neptune four times, with the last observation being made on 4 August. On one occasion, he had even noted that one of the ‘stars’ he had observed “seems to have a disk”. Had Challis compared his observations more thoroughly, he would certainly have made the discovery.

To the British, it was an embarrassingly missed opportunity; to the French it was Perfidious Albion up to its usual tricks. Arago made it clear that Adams had “…no right to figure in the history of the new planet, neither by a detailed citation, nor even by the slightest allusion”. Airy and Challis came in for considerable stick on both sides of the Channel. But neither Le Verrier nor Adams took any part in the rumpus. Adams was happy to acknowledge Le Verrier’s priority, and he did not join in with the criticism heaped on Airy and Challis. When he and Le Verrier finally met face to face, they are said to have struck up an immediate friendship and they remained on good terms for the rest of their lives.

Le Verrier suggested the name ‘Neptune’ for the new planet, but then proposed to have it named after himself. This was not entirely unreasonable, as at the time, Uranus was still widely referred to as ‘Herschel’ or ‘The Georgian Planet’ (after Herschel’s patron King George III). However, the name ‘Neptune’ soon became widely adopted, and at Adams’ suggestion the variant names for Uranus were finally dropped.

So, who really deserves the credit – and the blame?

The Royal Greenwich Observatory was a publicly-funded institution, the purpose of which was the production of accurate tables of star positions for navigators at sea. As Astronomer Royal – basically a senior civil servant – George Airy would not have believed it appropriate to interrupt the Observatory’s program to go hunting for a planet. In any case, there was at that time no suitable telescope at Greenwich: the 28-inch Great Equatorial Telescope (still the seventh largest refractor in the world) did not see first light until 1893. By that time, though, the ‘mission’ of the RGO had been expanded to include astrophysics and astronomical photography. Airy’s decision to ‘outsource’ the search for the new planet to Challis at Cambridge and the Northumberland refractor was entirely justifiable. Airy could perhaps be faulted for his initial scepticism at the possibility of locating Neptune through its effects on the orbit of Uranus, but he acted quickly enough when he realised that two independent researchers had arrived at very similar solutions.

As noted, James Challis was hampered by a lack of star charts for the region, and therefore faced an extremely laborious task. However, it is inescapable that he recorded Neptune on four occasions and failed to recognise it. Challis apparently worked in secret, keeping knowledge of the search from his fellow British astronomers. One can but speculate as to his motives for so doing, but had he recruited one of his students as an assistant (as had Galle), then it is highly likely that he would have made the discovery.

After the row over priority had died down, a consensus emerged that Le Verrier and Adams should be jointly credited as the discoverers of Neptune, although recently it has been suggested that Adams’ predictions were significantly less accurate than those of Le Verrier.

Although Neptune is too faint to be seen with the naked eye, the most basic telescope or even a good pair of binoculars will show it as a bluish eighth-magnitude star. ‘Precovery’ observations were made by Sir John Herschel in July 1830; the French astronomer Jérôme Lalande recorded it twice in May 1795; and the Scottish-born astronomer Johann von Lamont recorded it least three times between 1845 and 1846, with his last observation on 11 September coming just days before the actual discovery. But none of these observers thought it was anything other than a star.

The best-known precovery observation of Neptune was made by Galileo at the very dawn of the telescopic era, more than two centuries before its ‘official’ discovery. The conventional view is that Galileo – as others would do later – mistook Neptune for a star. The first record of a telescope dates to 1608, when the Dutch spectacle-maker Hans Lippershey attempted unsuccessfully to patent it. Hearing of this, Galileo built his own telescope in 1609 and, as is well-known, used it to discover Jupiter’s four major moons. Other discoveries include the craters and mountains of the Moon, the phases of Venus, and the ‘triple’ nature of Saturn (the rings, as seen through his primitive telescope, appeared as a pair of large moons flanking the planet).

In 1980, the American astronomer Charles Kowal and Canadian science historian Stillman Drake found that during the course of his Jovian observations, Galileo had recorded Neptune as an eighth magnitude object on 28 December 1612 and again on 28 January 1613, when it is shown close to the seventh magnitude star SAO 119234. Accompanying the drawings is a note that suggests that Galileo observed (but did not record) the pair the previous night and noticed that they had then seemed further apart.

In 2009, the Australian physicist David Jamieson noted a possible further observation of Neptune. Galileo’s observations on 6 January 1613 show an unlabelled black dot, which is in the right position to be Neptune. Jamieson believes that it is possible that the dot was actually added on 28 January. He suggests that Galileo went back to his notes to record where he had previously seen Neptune. It had then been even closer to Jupiter, but he had initially ignored it, thinking it to be just another unremarkable star. The implication is that on 28 January, Galileo realised that one ‘star’ was moving with respect to the others, and that he had had it under observation since at least 6 January. It suggests that Galileo thought, to paraphrase Obi Wan Kenobi, “that’s no star”.

If so, why did Galileo not follow it up? Kowal and Drake suggested that the lack of a suitable mount for his telescope made it impossible to keep track of Neptune once Jupiter had moved away. Jamieson suggests bad weather prevented further observations. However, he also notes that Galileo sent cryptic anagrams to his correspondents to establish priority for his discoveries. Jamieson believes that Galileo’s literature might include a coded reference to Neptune, although as of a decade later it has still not come to light.

Jamieson, D., 2009. Galileo’s miraculous year 1609 and the revolutionary telescope. Australian Physics, 46(3), pp. 72-76.
Kowal, C. & Drake, S., 1980. Galileo’s observations of Neptune. Nature, 25 September, Volume 287, pp. 311-313.
Krajnović, D., 2016. The contrivance of Neptune. Astronomy & Geophysics, October, 57(5), pp. 5.28-5.34.
Moore, P., 1993. New guide to the planets. London: Sidgwick & Jackson.
Smart, W., 1946. John Couch Adams and the Discovery of Neptune. Nature, 9 November, Volume 158, pp. 648-652.