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On April 26, 1786, Thomas Jefferson left London, bound for Paris, with a newly-purchased £10 telescope in his baggage. It was nearly midnight when he reached Dover, because he stopped in Greenwich to view the Royal Observatory, home of some of the finest astronomical instruments in the world.1 He was accompanied to Greenwich by his scientific friend John Paradise, who apparently introduced him to Nevil Maskelyne, Astronomer Royal since 1765.2 Maskelyne, like Paradise a Fellow of the Royal Society, may have given his visitors a tour of Christopher Wren's main building, with its octagonal Great Room, and the New Observatory containing a time-seeking transit instrument and the eight-foot mural quadrant first used by Edmond Halley. Jefferson may have been eager to climb to the top of Flamsteed House to see the camera obscura Maskelyne had installed in the turret room.

If their guide was Maskelyne's assistant, Joseph Lindley, he may have described his daily routine. One of his successors recalled the lonely night watches, with only hungry mice for company, and the time-consuming mathematical "reduction" of his observations: "Here forlorn, he spends days, weeks, and months, in the same long wearisome computations, without a friend to shorten the tedious hours, or a soul with whom he can converse."3

Since the founding of the Observatory in 1675, the Astronomers Royal had been engaged in mapping the heavens, fixing the positions of thousands of stars. Maskelyne's policy gave precedence to the motions of the moon. Tracking its daily passage over the meridian was part of the mission of the Royal Observatory as outlined by Charles II a century before to consult the heavens "so as to find out the so much desired Longitude of Places for perfecting the art of Navigation."4

Jefferson's own scrutiny of the moon and other heavenly bodies had begun early. William Small, his professor of natural philosophy at the College of William and Mary, had given him his "first views of the expansion of science and of the system of things in which we are placed" — the beautiful and rational order of the laws of nature revealed by Sir Isaac Newton.5 Astronomy, for Jefferson the "most sublime of all sciences," was his favorite window to the harmonious Newtonian universe.6 He and his classmate John Page spent hours stargazing from the roof of Rosewell, the Page residence near Williamsburg. Chiding his friend in 1770 for his delinquency as a correspondent, Jefferson guessed that Page was "always in the moon, or some of the planetary regions." In this letter, sprinkled with astronomical allusions, Jefferson facetiously predicted that, if he and Page spent too much time together, the gods might fear that they would "pull down the moon or play some such devilish prank with their works."7

In forty years of public service, Jefferson rarely found time to pursue his favorite study. When he did, he read the heavens to learn about the earth. As he wrote in 1816, it is only by "interrogating the sun, moon, and stars" that we can know the "relative position of two places on the earth."8 He repeatedly searched the sky to fix his own position on the ground and to contribute to one of his national goals — a "true geography" of his country.9

European nations had a different concern — the safety of their ships on the ocean. Finding latitude posed few problems, but longitude was more elusive, and longitude at sea was critical — whole fleets had been lost because of errors of navigation. A prize of £20,000 offered by the Board of Longitude in 1714 inaugurated the century's most celebrated scientific quest. Nevil Maskelyne was a champion of the method of finding longitude by lunar distances, and Newton too had thought the ultimate solution would be astronomical rather than mechanical. But it was a method that consulted a clock rather than the moon that was finally awarded the prize for "discovering the longitude" in 1773. John Harrison spent a lifetime perfecting a timepiece that maintained its accuracy even on long sea voyages.

Because of the earth's rotation, time is distance — an hour is fifteen degrees of longitude. If a clock marking the time at the Greenwich Observatory were carried to a point in the Atlantic Ocean, the longitude could easily be found by comparing it to local time. Since the marine chronometers developed from Harrison's timepiece were not yet being produced in large and affordable quantities, two methods for finding the longitude were in use on the oceans during Jefferson's lifetime: by chronometer and by lunar observations.

The lunar-distance method, finding the longitude by observing the angular distance between the moon and the sun or certain stars, was practiced well into the nineteenth century. As Jefferson noted in 1822, comparing the times of the same "celestial phenomenon" at two different points provided the difference in longitude. He considered the moon as the best heavenly body for the job, as her motions (for Jefferson the moon was female) were rapid and daily.10 From 1767, the moon's position eight times a day, based on the time kept by the clock at the Royal Observatory at Greenwich, could be known from the Nautical Almanac and Astronomical Ephemeris. Nevil Maskelyne was responsible for this compilation of the world's collective astronomical wisdom, and Jefferson may have purchased some almanacs at Greenwich, for he began sending them to American friends on his return to Paris.11 The Nautical Almanac, and its companion Requisite Tables, reduced the lunar distance method from a four-hour to a half-hour operation. Nevertheless, the complexity of the calculations made "taking the lunars" a dreaded component of naval training.

Whatever method Captain Wyatt St. Barbe used on his transatlantic crossings in the summer of 1784, Jefferson was probably an observer. He certainly came up on the deck of the Ceres to witness the ritual of the noon sight, since he kept his own log of the voyage in his memorandum book. He noted the daily latitude, longitude, temperature, and wind direction, also noting the shearwaters and petrels, sharks and whales, and other ships that came into view on the three-week passage from Boston to the Isle of Wight.12 The ship had "a most favourable run," he told John Adams,13 while his daughter Patsy provided a more lyrical account of their "lovely passage," with "a fine sun shine all the way, with the sea which was as calm as a river."14

On the Ceres Jefferson sailed through nearly seventy degrees of longitude, moving ever closer to the zero of the Royal Observatory. Before the publication of Maskelyne's nautical almanacs, the meridian at Greenwich was just one of many prime meridians. Afterwards the Greenwich meridian became the reference point for more and more mariners and mapmakers until, in 1884, it was officially accepted as the basis for the world's system of time.

Jefferson set up his own meridians wherever he went. Monticello's north-south line had been calculated in November 1778,15 and in 1789 he asked the director of the Paris Observatory for precise times to compute his meridian at the Hôtel de Langeac.16 Establishing a meridian took on a political dimension fifteen years later, when he found himself the president of a new nation. Inspired by the launching of the Lewis and Clark expedition, Jefferson began to contemplate a suitable geography for an independent nation. "[W]e have done too little for ourselves, & depended too long on the antient & inaccurate observations of other nations," he wrote.17 American maps deserved an American prime meridian. He commissioned surveyor Isaac Briggs to lay out a meridian through the middle of the President's House in Washington.18

This project lapsed, and the mysterious "Jefferson stone" near the Washington Monument is its only relic. But Jefferson, still in a patriotic mood, realized that the situation of the United States, with its enormous expanse of unexplored land, demanded renewed thinking about longitude. For Europeans, dwelling on a continent long settled and mapped, longitude was vital largely to their commercial and naval fleets at sea. A small band of American explorers moving steadily west over mountainous land, "changing their longitudes rapidly and at every step," had different requirements. New circumstances called for new resources.19

Jefferson then "sat himself" down and came up with an American method of finding the longitude. It differed from the usual methods in not requiring a timekeeper, for in a western wilderness a chronometer would be subject to "a thousand accidents." Its novelty consisted in the use of a meridian, because land observations permitted the calculation of this north-south sight line, an operation impossible at sea. The speedy and dependable motions of the moon, and their Greenwich times revealed in the nautical almanacs, could then be observed in relation to the meridian.20

All of this depended, as Jefferson admitted, on the use of a very expensive and sophisticated instrument — the jewel of his own "Mathematical Apparatus" — a Universal Equatorial.21 Jesse Ramsden had perfected the equatorial mounting for a telescope in the 1770s. Its "combined motions" in three planes (horizon, equator, and circle of latitude) made it extremely versatile. Jefferson had acquired his Ramsden equatorial in 1792, and sang the praises of "this noble instrument" for the rest of his life.22

Although his American method required an English instrument, English almanacs, and an English prime meridian, Jefferson elaborated it in letter after letter in succeeding years, to all the astronomers of his acquaintance and the leaders of later exploring parties. Most were skeptical, and some took the time to explain why, but his efforts did stimulate a number of counter-proposals.23 The lukewarm reaction may have been partially due to the fact that no one else had used an equatorial before — Jefferson's was the only one in the country. He was correct in his prediction about Lewis and Clark's chronometer, which failed early in the expedition. Still, it is hard to imagine the bulky but delicate equatorial surviving a journey over the Rocky Mountains.

It was with the intention of calculating his own longitude at Monticello that Jefferson made preparations for an annular eclipse of the sun in the fall of 1811. Two timekeepers and two observing instruments (a telescope and the great equatorial) were readied. Three additional observers were drafted, including President James Madison and his stepson Payne Todd. On the morning of September 17 the instruments were brought to the viewing site — probably in the midst of the late plantings of tennisball lettuce and Swedish turnips in the kitchen garden. The garden terrace provided a clear view of Willis Mountain forty miles to the south, the sighting point for Monticello's meridian.24

Letters exchanged after the event tell a story of a nearly bungled opportunity. The critical first contact of the moon with the disk of the sun, which occurred shortly after noon, caught the scientific gentlemen off guard. "Deceived" by their timepiece, they reached their appointed stations only just in time, but too flustered to attend to the instruments. The several lost seconds nullified their first observation, in Jefferson's opinion. They regained their composure for the next three hours and carefully charted the behavior of the "two luminaries." Jefferson was confident, to the "instant," of their observations of the forming and breaking of the ring.25

Jefferson enthusiastically communicated the results to other Americans who had trained their telescopes to the south on September 17. Ten pages of calculations soon arrived from William Lambert, a civil servant and "indefatigable" astronomical calculator in Washington. Using the suspect external contacts, he pronounced Monticello's longitude to be 78° 35' — about 8 minutes (and 8 miles) over the correct 78° 27' west of Greenwich.26 Lambert and Nathaniel Bowditch in Massachusetts, using the allegedly reliable observations of the annulus, calculated longitudes for Monticello that were twenty miles off.27

Preempted by Lambert's exertions, Jefferson never calculated the results himself. Instead, he planned to "amuse myself with the further ascertainment of my longitude by the lunar observations, which have the advantage of being repeated ad libitum, and of requiring less laborious calculations."28 Unfortunately, no records of such an experiment survive, nor does the equatorial that inspired it. Jefferson's final assessment of the idea and instrument that enchanted him for so long may never be known.

One troublesome consequence of his undaunted advocacy of a "method of ascertaining the longitude by the moon's motion without a time piece" was a flood of methods more controversial than his own.29 Among his papers are even more letters from discoverers of longitude than from inventors of perpetual motion machines. Fellow longitude addicts seem to have been particularly hard for Jefferson to rebuff. To one man who also reported on his opium addiction and interlude in an asylum, Jefferson gave fifty dollars and advice to apply his efforts to "the comfort of your family" rather than the will-o'-the-wisp longitude.30 After another discoverer accosted him on the streets of Charlottesville, Jefferson lamented to a friend that his "false reputation" as an expert on scientific matters "has made me a kind of Vortex into which the projects of our country are very much emptied." He protested against "the sacrifice of the remains of my life in the investigation for others of projects which very often require a great deal of consideration, much research, and sometimes elaborate calculations."31 And yet he spent two days considering the proposal and wrote its author a long and gentle critique.32

Jefferson's remaining life was spent in the creation of the University of Virginia. An adjacent hill was purchased for an observatory and instruments "prepared with the highest degree of skill and correctness" were ordered from London.33 While waiting for the fulfillment of his designs, Jefferson drew up a plan for a temporary meridian from the portico of the Rotunda leading south through his "academical village." As he thought of future students hacking the moon over this meridian, he may have recalled the question he posed to that celestial body over fifty years before. Captivated by the imagery of a poet he believed to be the third-century Scottish bard Ossian, he had transcribed into his literary commonplace book the opening lines of James McPherson's Darthula:

Daughter of heaven, fair art thou! The silence of thy face is pleasant. Thou comest forth in loveliness: the stars attend thy blue steps in the east. The clouds rejoice in thy presence O moon, and brighten their dark brown sides. Who is like thee in heaven, daughter of the night?34

- Lucia C. Stanton, 1996. Originally published as "Interrogating the Moon," in Monticello Keepsakes 59 (April 13, 1996).

Further Sources

Bedini, Silvio A. Thomas Jefferson: Statesman of Science. New York: Macmillan, 1990.

The Royal Observatory Greenwich.


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