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Home , Berlin Observatory, Discovery of Neptune, James Challis, Johann Gottfried Galle, John Couch Adams, Urbain Le Verrier

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On September 25, 1846, two German ! worked through the night at the Observatory. They were searching a small square of sky for a hazy dot. The All astronomers acknowledged that ' at the , , had departs from that predicted by Newton's been driven to his telescope after receiving a short letter gravitational law. Note, however, that they from the French . This differed in the way they accounted for that letter contained Le Verrier's prediction for the location of accepted anomaly. an undiscovered beyond the orbit of Uranus. Le Verrier had been unable to convince his fellow French astronomers to search for a planet predicted by his The well illustrates important mathematical calculations. The two Germans took a aspects of how science works. Foremost, it exemplifies chance on Le Verrier and knew well the challenge ahead of the power of previously well-established knowledge and them. Galle sought an eighth fuzzy dot against how scientists react to anomalous data. In the mid- a background of thousands of brighter . All he had nineteenth century, primarily dealt with was Le Verrier's prediction, one night of observation time observations and had very little use for hypothetical work. on the observatory's smallest telescope, and the young The men who made the predictions of Neptune, graduate student Heinrich d'Arrest as his only help. and Le Verrier, were actually not actually astronomersthey were with little experience behind a Little did they know that across the English Channel telescope. They had to convince the nation's best another young astronomer, , had stargazers that their work mattered and could be compiled five years of work into a prediction almost observationally tested. Secondly, it shows the interplay matching Le Verrier's. The head of between technology and science. One of many reasons Observatory, , respected his good friend why Adams and Le Verrier encountered troubles Adams' work. However, his duties were teaching and convincing astronomers of their work's testability lay in the finishing the observatory's usual workload calculating astronomer's view that existing were trajectories. But every few nights Challis would inadequate for such a search. Lastly, this story illustrates spend time watching for the predicted planet's movement. how science involves efforts and collaboration around the world. Early that September morning at the , ! the Germans tired of watching stars pass through their viewfinder. Their telescope time running out, they decided Many people dislike the thought of a science to hunt and peck. Grabbing the observatory's most recent career, seeing it as a solitary undertaking. As you map, Galle shouted positions as d'Arrest checked read this story, consider how it illustrates that each position with the telescope. After less than ten stars science is a social endeavor. he shouted, “That star is not on the map!” They awoke the observatory's director for confirmation. The story of Neptune's discovery began with William We now know the object that inspired a multinational hunt Herschel's discovery of Uranus. While working on his star in 1846 as the planet Neptune. Following the discovery of catalog in mid-March 1781, Herschel came across a Uranus in 1781, astronomers learned that it 'wobbled' peculiar hazy star. It moved across the sky, so at first he along its orbit. The precise reason why this occurred, thought it to be a comet. Even with his precise telescope it however, was a mystery. Some thought the law of appeared fuzzy. Curious to get a closer look he upped the was different at the distance of Uranus. Others thought magnification power of his telescope from its standard 227 maybe a comet had collided with the planet. But a few to 460. The object got bigger and sharper. Herschel knew astronomers suggested the irregularities in Uranus' orbit, he found something important. Stars do not get bigger with called “perturbations,” were caused by another distant and magnification because they are so far away, and as yet unseen planet. have a noticeably fuzzy tail. Only get bigger and

Accounting for Anomaly: The Discovery of Neptune 1 www.storybehindthescience.org sharper when magnified. An international search to verify ever. By New Years Day of 1801 the Italian astronomer the object as a planet commenced, and in late spring 1781 found a moving star in his catalogue of Russian astronomers confirmed the discovery of a new 6,000, but nobody else could find it. Hearing of the planet - Uranus. problem, the young , at the age of 23 already called the 'prince of ,' calculated an Herschel's discovery energized the astronomical orbit from established data. On December 1, 1801, von communitymaybe more planets existed in our solar Zach located his 'planet' between and . Soon system! Johann Bode thought the orbital radius of planets thereafter another moving object appeared nearby. could be predicted. In 1772, about ten years before the Puzzled, calculated their sizes. He discovery of Uranus, he published a mathematical series related the disappointing news to the astronomical based on the older work of Johann Titius. Titius argued that communitythe new planets were too small to be planets. if the distance of the earth to the was considered to be Astronomers realized they were small 'planetoids.' Many 1 unit, then the mean radial distance to the other planets relished the discovery of the string of rocks orbiting could be estimated with the equation 0.4+0.3(2n). Here are between Mars and Jupiter, known today as the his results (Table 1), updated with modern measurements belt. But others continued to wonder if a full planet for clarity: remained undiscovered.

TABLE 1 Meanwhile, astronomers realized unusual motions in Bode-Titius Series (modernized) Uranus' orbit. Looking at past star maps, Johann Bode realized William Herschel was not the first to observe Observed Titius’ Uranus. In 1690 made the first recorded Planet AU Radius Prediction observation of Uranus, but thought it to be a star. Using this 0.38 0.4 century old data, in 1820 the French astronomer Alexis Venus 0.72 0.7 Bouvard calculated Uranus' orbit by combining it with his Earth 1.0 1.0 own data. No matter how he worked the numbers, he Mars 1.52 1.6 couldn't match Uranus to its current orbit. Bouvard couldn't 2.8 2.8 Jupiter 5.2 5.2 answer why Flamsteed's observations were so far off. His 9.55 10.0 words hinted that a deeper mystery might lie beneath the Uranus 19.2 19.6 data: “I leave to the future the task of discovering whether Neptune 30.06 38.8 the difficulty of reconciling the two systems results from the 38.4 77.2 inaccuracy of the ancient observations, or whether it depends on some extraneous and unknown influence which may have acted on the planet.” The series came to be associated with Bode. Because of the accuracy it gave to Uranus' orbit, it drove astronomers Two possible explanations surfaced. The first proposed to search for another missing planet between Mars and that 's law of gravity might not hold at Jupiter. distances as great as Uranus. While astronomers occasionally fudged the inverse square law to compensate for their technological inadequacies, it 1. The Bode-Titius Series resides in a hazy realm certainly had never been to the point of entirely altering the of science. On one hand, it predicts quite laws of . Newtonian mechanics explained so many accurately the distance to planets (with the aspects of the world that few scientists were willing to give exception of Neptune and Pluto, the latter which it up. is no longer considered a planet). On the other hand, it gives no hint of explanation for why the planets should be organized as such, nor does it 2. Note that the orbit of Uranus appears not to be account for objects other than planets (like behaving as Newtonian physics says it should. comets). Moreover, the Series does not apply to Scientists knew of this for decades, but few newly discovered solar systems around other questioned Newtonian mechanics. Many non- sun-like stars. What value should scientists scientists think that scientific ideas should be assign to such predictive methods that have abandoned when disconfirming evidence exists. such limitations and no underlying explanation? Why would quickly giving up on well-established scientific knowledge not be appropriate? In the autumn of 1800, the Baron von Zach got the itch to search for the predicted planet between Mars and Jupiter. Most astronomers favored the other possibility: that He gathered twenty-four astronomers of all nationalities to another planet lay beyond Uranus. However, some partake in the most coordinated search of the night sky observational astronomers feared their telescopes to be

2 Accounting for Anomaly: The Discovery of Neptune www.storybehindthescience.org far too weak for the task. The search for the planet beyond orbit, Adams took Bouvard's data and assumed two Uranus revolved mostly around two people, then: the features: first, that the planet had a circular orbit, and British John Couch Adams and the French Urbain Le second, that its distance agreed with Bode's prediction of Verrier. Near opposites in character and unaware of each 38.8 astronomical units. An offer by Cambridge head other's work until 1846, both used the universal law of astronomer James Challis helped significantly. If Adams gravity to calculate the exact position of Uranus' neighbor. helped Challis with a comet observation, Challis would If successful, it would be the first application of an already give Adams access to all modern observation data on existing conceptual framework to precisely locate an Uranus. Adams agreed. unknown astronomical body. By 1845 Adams concluded that two reasons accounted for ! the perturbations in Uranus' orbit. The first reason was an While many people think that scientific unseen planet that definitely did exist. The second reason knowledge is simply an end result of research, was that the calculations by Alexis Bouvard were wrong. note how that knowledge may also be used to After fixing Bouvard's calculations, Adams concluded the make predictions and direct future research. This missing planet must have an elliptical orbit and be located is the case in all fields of science. at double the distance of Uranus. He fashioned a page long equation to describe the orbit of Uranus and the missing planet. He then applied it to twenty-one John Couch Adams was born in 1819 to a family of tenant measurements between 1780 and 1840. Many pages of farmers. A quiet and reserved man, he earned his way into calculations later, Adams presented a new model of Cambridge through diligent hard work. Earning the highest Uranus' measurement. He concluded that Flamsteed's honor of on the notorious mathematics 1690 measurement had been right, but in 1820 the examination, his excellence garnered him a teaching missing planet interfered with Bouvard's data. position after graduation. He first became aware of the problem with Uranus' orbit in 1841, when he chanced upon Adams decided to personally take his work to George Airy, an astronomical report in a bookstore. He had just enough Astronomer Royal and the Head of the Royal money to live on following graduation, but a contest put on Observatory. The reserved and timid Adams forgot to by a German university in 1844 incited him to work day and make plans, however. In September 1845, holding an night on the Uranus problem. introductory letter from James Challis that would get him a personal meeting with Airy, Urbain Le Verrier was born in France in Adams arrived in Greenwich and learned 1811. His father worked in the State Airy had left the country. A month later, Property Administration and could Adams again visited without announcement afford to send his son to the prestigious and arrived at Airy's dinnertime. The maid École Polytechnique. Le Verrier never told Airy of Adams' arrival. managed to land a stable and well- Disenchanted, Adams left behind his paper paying job at the Administration of of equations and sulked home. After Tobacco as an experimental chemist. hearing that the young astronomer had Excelling in mathematics, he turned to twice made the trek, on November 5, 1845, astronomy and quickly earned a name Airy wrote a letter of thanks to Adams, for his prowess with numbers. Little though he offered him no help. Adams could be said for his demeanor, though. worked furiously on refining the data and In the words of a colleague, “I do not didn't try to contact Airy until a year later. know whether M. Le Verrier is actually the most detestable man in France, but I While Adams cloistered himself, the French am quite certain that he is the most John Couch Adams mathematician Urbain Le Verrier publicly detested.” announced his search for the missing planet. Encouraged by the director of the Adams started his work on Uranus just before Le Verrier. Observatory, Le Verrier started his search from Adams began with the premise that another planet beyond scratch, unaware of Adams' work. In December 1845 Le the orbit of Uranus explained the perturbations better than Verrier published a short manuscript on the orbit of the a variable law of gravity. He knew the gravitational pull of missing planet and pointed out the many errors of Alexis Jupiter and Saturn could be calculated and their effects Bouvard. Six months later Le Verrier published a more subtracted from the total perturbations of Uranus. The complete treatise on the orbit of Uranus, concluding the remaining anomalies could then be attributed entirely to planet must be somewhere between two and three times the missing planet. But he still didn't know where in the the orbital distance of Uranus. He would not realize he had it might be. To estimate the mystery planet's competition in England until June 1846.

Accounting for Anomaly: The Discovery of Neptune 3 www.storybehindthescience.org For an unknown reason, Airy took a much better liking to In Britain, Challis made sweeps on June 29, June 30, Le Verrier's work, exclaiming about the paper, “I cannot August 4, and August 12. In comparing the August 12 data sufficiently express the feeling of delight and to that of June 30, the forty-ninth star in satisfaction which I received from it.” One the August data was missing from the possible explanation could be that Le Verrier June data. Challis marked it and came from high class and had already continued his comet calculations. garnered an international reputation. Meanwhile, Adams sent another Nevertheless, Airy announced to the Board refinement of his calculations to Airy. of Visitors at Greenwich Observatory that a Another miscommunication, Airy was planet should be discovered very soon, on vacation when the paper arrived in although he stopped short of calling British Greenwich. observatories into action. He convinced James Challis of Cambridge Observatory to Le Verrier's letter to Galle asked “to find head up the only British search. Challis went a persistent observer, who would be about the work slowly, already inundated willing to devote some time to an with calculating comet trajectories and examination of a part of the sky in which disgruntled by Airy's rebuff of his friend there may be a planet to discover.” The Adams. young Galle had sent Le Verrier his dissertation a few years earlier and The British telescopes would 'sweep the sky' Urbain Le Verrier worked a subordinate job at the Berlin by a method known as transiting. To do this, Observatory. The observatory's senior the telescope remained mostly stationary as astronomer, Johann Encke, felt slighted stars passed the field of view. The telescope operator by Le Verrier's appeal to the young Galle. Encke allowed called out the positions of each star as it passed through Galle one night's use of the observatory's smallest the crosshairs while an assistant marked positions on a telescope. By chance Heinrich d'Arrest, a graduate star map. Every night the process repeated, searching for student, walked by and offered help. The night sky the appearance of a new star. Historian Martin Grosser awaited. remarked, “It was like searching for a particular bright pebble on a beach by removing, one by one, thousands of Hours passed on September 25, 1846, as Galle and other pebbles from a large area around a point where (one d'Arrest searched for a fuzzy disk. They spotted Neptune had been told) the desired object was lying.” The in the early morning, and after waking Encke, they kept disgruntled Challis went weeks watch on the speck until 2 AM. Le Verrier's between checking his nightly data and prediction was almost perfect. Galle proudly instead calculated comet trajectories. wrote the next day to Le Verrier, “The planet whose position you have pointed out actually Meanwhile, Le Verrier continued exists.” refining his work, conscious that the British had started searching. ! Announcing his plotted orbit in August Note here the context of “discovery.” It 1846, he had calculated over 10,000 applies to the initial idea that an pages of equations and used 279 unknown planet might account for the measurements of Uranuswhereas perturbations in Uranus' orbit, to the Challis had used only twenty-one mathematical work that predicted the measurements. Le Verrier predicted location of the planet, and to a lesser the planet would appear at 326 extent to the actual observation of the degrees longitude on January 1, planet. This exemplifies why giving 1847. Le Verrier also claimed that the sole credit for scientific discovery to planet would be easily visible by most one individual distorts how science telescopes, and that “the planet's disk really works. has a large enough diameter to Johann Gottfried Galle preclude its being confused with the stars.” He was clearly trying to convince astronomers to It took until October 1 for word to reach England. Challis begin the search, but nobody in France wanted to chase rechecked his work and realized he had located the Le Verrier's planet until his predicted date in January. missing planet over a month earlier. Disheartened, he Impatient after all his work, Le Verrier wrote Johann wrote, “It is useless now to regret my having missed the Gottfried Galle at the Berlin Observatory. planet when it was so possible to detect it. All that remains to do is to make the best of the observations that I have

4 Accounting for Anomaly: The Discovery of Neptune www.storybehindthescience.org succeeded in getting.” The dispirited Adams followed his Neptune was the first planet to be predicted by first advice and began calculating an orbit for the new planet. applying previous knowledge, in this case Newtonian mechanics, and then confirming it through observation. After the more accurate orbit of Neptune was calculated in The discovery of Neptune illustrates that anomalous data 1847, astronomers realized how lucky they had been. The do not necessarily result in the rejection of well-accepted first surprise recalled the incident of William Herschel and scientific knowledge. Instead of rewriting the law of gravity, John FlamsteedNeptune had been unwittingly observed in astronomers accounted for the discrepancies of data and 1795 by the Frenchman Michel Lalande, but thought at the fashioned a testable explanation for the perturbations in time to be a faint star. The crucial second stroke of luck Uranus' orbit. Their predicted result, a planet, was came in Bouvard's timely examination of Uranus. For most established through observational evidence with of the century Neptune's orbit strayed far away from remarkable precision. Bolstered by the discovery of Uranus, but in 1822 Neptune passed by at its closest point Neptune, theoretical astronomy became more popular and accelerated Uranus along its orbit. Bouvard had and worked alongside observational astronomy to set the mistaken the fast Uranus for its standard orbit, thus stage for twentieth century astronomy. explaining the perturbations. Despite his own inaccuracies, had Bouvard not discovered the perturbations in Uranus' orbit, it might have been a long 4. Science textbooks often attribute the time until Neptune's discovery. development of scientific knowledge to a particular person on a specific date. How does this story show that portrayal to be wrong? 3. How does this story illustrate that science is a social endeavor?

Accounting for Anomaly: The Discovery of Neptune written by Blair Williams, Michael P. Clough, Matthew Stanley, & Charles Kerton

ehind Partial support for this work was provided by the National Science Foundation's b th y e Course, Curriculum, and Laboratory Improvement (CCLI) program under

r s Award No. 0618446. Project Principal Investigator: Michael P. Clough. Any

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