Accounting for Anomaly
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ehind t b h y e r s c o c Accounting for Anomaly t t i i e e s s n n e e The Discovery of Neptune c c h h e e T T On September 25, 1846, two German astronomers ! worked through the night at the Berlin Observatory. They were searching a small square of sky for a hazy dot. The All astronomers acknowledged that Uranus' astronomer at the telescope, Johann Gottfried Galle, had orbit 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 mathematician Urbain Le Verrier. This differed in the way they accounted for that letter contained Le Verrier's prediction for the location of accepted anomaly. an undiscovered planet 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 discovery of Neptune 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 magnitude fuzzy dot against how scientists react to anomalous data. In the mid- a background of thousands of brighter stars. All he had nineteenth century, astronomy 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, Adams graduate student Heinrich d'Arrest as his only help. and Le Verrier, were actually not actually astronomersthey were mathematicians 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, John Couch Adams, 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 Cambridge between technology and science. One of many reasons Observatory, James Challis, 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 telescopes were comet 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 Berlin Observatory, ! 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 star 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 gravity 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 comets as yet unseen planet. have a noticeably fuzzy tail. Only planets 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 Giuseppe Piazzi 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 Carl Friedrich Gauss, at the age of 23 already called the 'prince of mathematics,' 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 Mars and Jupiter. 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, William Herschel 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 sun 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 asteroid 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 John Flamsteed made the first recorded Planet AU Radius Prediction observation of Uranus, but thought it to be a star. Using this Mercury 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 Asteroid Belt 2.8 2.8 Jupiter 5.2 5.2 answer why Flamsteed's observations were so far off. His Saturn 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 Pluto 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 Isaac Newton'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 physics. 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.