Mid-continental magnetic declination: A 200-year record starting with Lewis and Clark Robert E. Criss, p. 4 North-Central Section Meeting, p. 44 Call for Committee Service, p. 46 Call for Nominations, GSA Awards and Medals, p. 49 VOLUME 13, NUMBER 10 OCTOBER 2003 Cover: Part of the map resulting from the Lewis GSA TODAY publishes news and information for more than and Clark expedition of 1803-1806, showing 17,000 GSA members and subscribing libraries. GSA Today the Rocky Mountains, the confluence of the lead science articles should present the results of exciting new Yellowstone and Missouri Rivers, and other research or summarize and synthesize important problems or geographic features (map copied in 1814 by issues, and they must be understandable to all in the earth science community. Submit manuscripts to science editors Samuel Lewis from William Clark’s original Keith A. Howard, [email protected], or Gerald M. Ross, drawing). Instrument is an octant. Right: [email protected]. 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Criss, Department of Earth & Planetary Sciences, Washington University, “variations of the compass” in different St. Louis, Missouri, USA locations, and the requisite data were indeed acquired in cases where both compass and sextant were used to make ABSTRACT requirement that they make, with “great observations. These data seem likewise Compass and sextant observations pains and accuracy,” astronomical mea- not to have been reduced before, even by Meriwether Lewis and William Clark surements to determine the latitude and though they contain a valuable record of are combined to provide the oldest longitude of all remarkable points along magnetic declination across the United determinations of the magnetic declina- the way (e.g., Preston, 2000). To ac- States in the early 1800s. This paper de- tion in the continental interior of the complish this, Lewis received training in tails several different methods whereby United States. Over the past 200 years, the methods of celestial navigation and this record may be reduced. the magnetic declination near St. Louis acquired a sextant, an octant, a “circum- has changed from an azimuth of 7.7° ferentor” or large-diameter surveyor’s Importance of Magnetic Declination east to 0° today. The 1803–1806 decli- compass, and a chronometer that was Earth’s magnetic field approximates a nations are essential to interpreting the somewhat troublesome but good for dipole, though the magnetic north and travel legs made by Lewis and Clark on its day (Moulton, 1986–1993). Most of south poles are neither fixed nor do their historic journey, and could be used the original instruments have been lost, they coincide with Earth’s geographic to test and improve existing magnetic but all were ably described by Lewis in poles. Nevertheless, the utility of mag- models. his 22 July 1804 journal entry (Moulton, netized needles in aiding travelers, 1986–1993, v. 2, p. 410–413). For histori- mariners, and miners to determine north INTRODUCTION cal reasons, the data for longitude were has probably been known for millennia, Included in Jefferson’s ambitious not reduced until recently (Preston, although the earliest written reference charge to Lewis and Clark for their his- 2000), and the calculations for latitude to the compass may be by Alexander toric expedition of 1803–1806 was the that were made in the field by Lewis Neckam ca. 1187 A.D. (Hoover and 4 OCTOBER 2003, GSA TODAY Hoover, 1950). Recognition that mag- 1882) are oriented relative to magnetic netic north deviates from true north and north at the time, as a compass is es- that the magnitude of this deviation var- pecially handy when it is impossible ies both geographically and temporally to see the Sun or the stars. Similarly, led to the establishment of several mag- knowledge of the magnetic declination netic observatories to facilitate global during the Lewis and Clark expedition is exploration. Nearly continuous records needed to understand their sketch maps of the secular variation of declination and the individual travel legs that they have been available since ca. 1570 report in terms of compass bearings (see A.D. for selected sites such as London below). Figure 1. The “astronomical triangle” has legs of great circle arcs on the celestial sphere (Malin and Bullard, 1981). Numerous Magnetic models for the past 400 that connect the observer’s zenith (Z), Earth’s ship captains augmented this record and years are available but their accuracy rotational pole (P), and the Sun or star of hundreds of thousands of observations depends on both the distribution and interest (X). The angular arc lengths are the at sea have been compiled (Jackson et quality of available historical data respective complements of the observer’s al., 2000), but before ca. 1850, little data (Jackson et al., 2000; U.S. Geological latitude (φ), the star’s altitude (a) relative to the existed for the North American interior. Survey, 2003). In this regard, Lewis and observer’s horizon, and the star’s declination δ Accurate knowledge of the declina- Clark’s observations yield the first deter- ( ) relative to the celestial equator as given in tion and its variations has numerous minations of declination in the interior the ephemeris tables.