The Victorian Global Positioning System by Trudy E
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The Victorian Global Positioning System by Trudy E. Bell TARS GLEAMED OVERHEAD in a velvet-black With this vivid account, published immediately in the sky on a crisp October night in 1848 as six astrono- Cincinnati Observatory’s periodical The Sidereal Messen- mers entered the darkened dome of the three- ger, Mitchel affords an eyewitness’s glimpse into one of the year-old Cincinnati Observatory. Seating them- great untold stories in the histories of telegraphy, as- s selves around a table laden with several large tronomy, and geodesy: the telegraphic method of deter- batteries and clocks, Ormsby McKnight Mitchel, the mining longitudes. observatory’s director, was keenly aware of the moment’s Within a few short years, the telegraph had transformed historic significance. About 9 p.m., the men were joined by a both positional astronomy and geodesy. The telegraphic telegraph operator who, Mitchel wrote, method of determining longitudes held sway for eight decades not only in the . perfected the necessary arrange- United States but also in Europe—be- ments for communicating directly by ing replaced only in the 1920s by radio telegraph between the Cincinnati The telegraph (wireless) positioning techniques. Yet Observatory and the Philadelphia its history appears to have been largely [Central High School] Observatory. was as revolutionary overlooked, including in two recent At ten o’clock the way offices along for determining popular bestsellers on the measure- the line were closed, and the line of ment of longitude and on the telegraph.1 the telegraph given up to the use of longitude on land the astronomers. The novelty of the as the marine WHY A DEMAND FOR operations excited the deepest inter- chronometer was LONGITUDES? est among all who were present. … In the fewer than four years between At a quarter past ten, the com- for finding longitude the installation of the first electromag- pliments of the astronomers in the at sea. Along the netic telegraph (May 1844) and the cry two observatories were passed. Cin- of “Gold!” from Sutter’s Mill (January cinnati then asked Philadelphia, way, the telegraph 1848), the already-vast continental “Are you all ready?” The answer also reformed United States grew another 50 percent came back instantly, “Aye! Aye! by several fast, enormous gulps of terri- O.K., go ahead.” Cincinnati then di- observational tory. By the time of the Cincinnati-Phila- rected Philadelphia to send her clock astronomy and delphia longitude determination, the na- signals. The answer was—“All right, nationwide tion spanned from Atlantic to Pacific look out for gong signal”—the sig- with essentially the same boundaries it nal well understood by both parties, time-keeping. has today—most of it trackless wilder- and which precedes each important ness. communication. … All was instantly Now, it’s tough to develop, defend, quiet. In about half a minute, the well mine, govern, or tax millions of square known gong signal sounded, and after the lapse of miles of uncharted land. So the U.S. Army, state governmen- half a minute more, we heard the swing of tal agencies, and commercial concerns hired astronomers, to- [Philadelphia’s] sidereal clock pendulum, every vi- pographers, and geologists to mark legal boundaries, map bration giving us a tick. And thus did it swing, beat- natural resources, ascertain military routes, divide arable ing seconds for fifteen consecutive minutes. lands into salable township squares, and survey routes for … Now, Philadelphia did not undertake to trans- the tracks of the new-fangled “iron horse” locomotives. More- port her sidereal clock to Cincinnati, that its beats over, with all the new territories came thousands of miles of might be compared with our mean solar chronom- uncharted coastlines, whose collective hazards annually eter, but with the potent (I almost said omnipotent) wrecked scores of ships with uncounted losses of cargo and aid of magnetism, this comparison is virtually and human life. Accurate maps were badly needed of harbors, absolutely effected at the distance of seven-hundred shoals, and inland waterways—the mission of the U.S. Coast miles. Survey, chartered by President Thomas Jefferson in 1807. 14 SPRING 2002 T H E B ENT OF TAU BETA PI In short, in the burgeoning westward- local time as determined by meridian tran- expanding nation, latitude and longitude sits of the sun or stars. This was not just determinations were big business. a matter of packing an additional pocket Determining latitude was straightfor- watch. Chronometers were bulky and ex- ward. At the instant a star of known ce- pensive precision instruments, carefully lestial position transited (crossed) the lo- cushioned in bread-loaf-sized waterproof cal meridian (imaginary north-south line wooden boxes that had to be hand-carried passing through the observer’s zenith), an when on the backs of spirited horses. As observer measured its altitude above the insurance, field astronomers, surveyors, local southern horizon—or, after the intro- and topographers routinely toted three or Figure 1 duction of the Talcott method in the 1830s, more chronometers—some actually took A typical 19th-century portable its angular distance down from the local more than a dozen. Still, despite every astronomical transit telescope zenith. A lone observer could do with sat- pain, they often found their chronometers for measuring latitudes and isfactory accuracy even in the remote had “tripped” because of temperature longitudes from the sun and wilds with a portable transit telescope (a changes or jostling, jeopardizing the pre- stars was mounted so as to confine its observations to the telescope constrained to move only in the cision of painstaking longitude calcula- meridian—the imaginary line plane of the observer’s meridian [Fig. 1]). tions. running from due north to due Determining longitude, however, was The telegraph transformed all that. south through an observer’s challenging. Because of the rotation of the The seemingly-instantaneous speed of zenith. The main telescope tube earth, a difference in longitude is equiva- what Mitchel heralded as the telegraph’s AA was supported on a lent to a difference in local time: that is, “omnipotent magnetism” effectively an- perpendicular axis BB that the difference between the instant either nihilated distance, allowing two clocks 700 rested in Y’s fixed to the the sun or a star crossed the local merid- miles apart to be compared virtually as if uprights CC as part of a single- ian of an unknown place and the instant it they were sitting side by side, in what to- piece stand of cast iron. Near crossed the local meridian of a reference day would be called “real time.” the right end of the axis was a graduated circle G that turned location. Lured by that beckoning promise, with the axis; the circle’s Now, recall that when the telegraph whither the telegraph went, field astrono- angular measurements, was invented, time zones and standard mers followed, from the first land line to indicating the altitude to which time were still 40 years off in an unknown the first transatlantic cable—just as the telescope was elevated, future. In the 1840s, clocks were set to quickly as arrangements could be made were read by a stationary each town’s own individual local solar and transportation allowed. vernier H. time—that is, local noon was the moment At the telescope’s focus the real sun transited the local meridian MANUAL TIME-COMPARISON were crosshairs [see Fig. 7] that of some astronomical observatory or sig- TECHNIQUES at night were faintly illuminated by the lamp M to make them nificant landmark in town.2 Thus, longi- The telegraphic method of determining visible against the dark sky. tude differences were equivalent to differ- differences in longitudes was actually sev- ences in the time on local clocks separated eral methods, all of which evolved within [Elias Loomis, An Introduction to by some distance east and west. the telegraph’s first five years. There were Practical Astronomy (New York: Until the advent of the telegraph, how- three techniques for measuring differences Harper & Bros., 5th edition, ever, there was no direct, instantaneous in local times, which subsequently became 1863), p. 40] way to compare clocks at places hundreds standard to use in tandem. of miles apart. The only method was indi- rect, cumbersome, and subject to lengthy EXCHANGE OF CLOCK SIGNALS delays: physically transporting a chro- The earliest technique was a direct, nometer reading the local time of a refer- manual comparison of two chronometers ence longitude to a place of unknown set to the local solar time of the cities at longitude and comparing its time with the each end of the telegraph line. It relied on SPRING 2002 T H E B ENT OF TAU BETA PI 15 skilled observers’ eyes, ears, hands . and dard for quickly determining longitude patience. differences to within a second of time. On June 9, 1844—just two weeks after Samuel F.B. Morse’s inaugural Biblical EXCHANGE OF STAR SIGNALS quotation “What hath God wrought?” After Wilkes’ pioneering proof of the con- flashed 40 miles along the first experimen- cept for the U.S. Navy, the U.S. Coast tal telegraph wire between Washington, Survey took the lead. Within months of DC, and Baltimore—Commodore (later Wilkes’ experiment, the survey’s super- Admiral) Charles Wilkes and an associate intendent, Alexander Dallas Bache—who stepped into the same two telegraph of- grew to be a towering figure in 19th-cen- fices. Wilkes, famous for recently leading tury American science—officially indi- the U.S. Exploring Expedition 87,000 cated his interest in the possibilities of miles around the Pacific Ocean (1838-42), the method. But it was another two years was one of the nation’s foremost chrono- before enough commercial telegraph wire metric experts; as former superintendent had been installed to offer actual oppor- of the U.S.