Geographic Information

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Geographic Information from Lewis and Clark's Celestial Observations

In the early 1800s, the agglomeration of Mandan-Minitari villages was the most important trade and population center between the Mississippi River and the Cascade Mountains north of Santa Fe. Not even St. Louis could boast having so many citizens nor such an exchange of goods. The Mandans, a Siouxan-speaking Nation, once lived near Lake Michigan, possibly in southern Wisconsin or northern Illinois before moving westward to the Missouri. After reaching the Missouri they had moved their villages many times—always farther up the Missouri River. The Mandans and the Minitari (also Siouxan-speaking, but distantly related) eventually gathered their villages near each other for mutual protection against another linguistically related nation, the Dakota or Sioux proper. The western Dakota (Tetons) also were an important trading nation, but because of their numerical superiority and their alliances, they were able to influence or control trade coming up the Missouri. The Mandan and Minitari, consequently, traded almost exclusively with the British-Canadian posts. In 1804, however, these villages were within the area comprising the Louisiana Purchase. Meriwether Lewis thought that trade with these nations now should belong to traders from the United States. The British-Canadians saw it differently. Not only that, the natural boundary (not geographically well defined) between Louisiana and British claims lay only a score of miles or so northeast of the villages. Who was going to stop them from such a lucrative trade?

In December 1797, the Canadian trader-surveyor-mapper David Thomson had stopped at the Mandan villages. In January 1798, Thompson took celestial observations at the villages and determined them to be at a latitude of 47°17'22"N and a longitude of 101°14'24"W. Why then did Lewis and Clark think it necessary to take twenty-six observations during the bitter cold winter that dragged on until late March? Lewis's persistent Anglophobia may have had much to do with the many celestial observations he took. He may have suspected that Thompson's coordinates had been altered for political purposes before they reached Jefferson. On the other hand, Lewis may just have wanted to see if he could verify the site's location with independent data of his own taking. At any rate, the location of these villages was as important as any river junction or mountain pass. Cold weather or no, the observations had to be taken!

Summary of the Celestial Observations Taken at Fort Mandan
1804
Dec
22 Equal Altitudes of the Sun AM (3 observations of the sun’s altitude + the time)
22 Equal Altitudes of the Sun PM (3 observations of the sun’s altitude + the time)
1805
Jan
07 Lunar Distance from the Sun (6 sets of the sun-moon distance and time)
07 Lunar Distance from the Sun (6 sets of the sun-moon distance and time)
13 Meridian altitude of the Sun (altitude of the sun at noon for latitude)
14 Eclipse of the Moon for Longitude (observation of eclipse elements sextant’s telescope and time)
15 Eclipse of the Moon for Longitude (observation of eclipse elements sextant’s telescope and time)
15 Equal Altitudes of the Sun AM (3 observations of the sun’s altitude + the time)
15 Equal Altitudes of the Sun PM (3 observations of the sun’s altitude + the time)
26 Meridian altitude of the Sun (altitude of the sun at noon for latitude)
28 Equal Altitudes of the Sun AM (3 observations of the sun’s altitude + the time)
28 Equal Altitudes of the Sun PM (3 observations of the sun’s altitude + the time)
Feb
06 Equal Altitudes of the Sun AM (3 observations of the sun’s altitude + the time)
06 Equal Altitudes of the Sun PM (3 observations of the sun’s altitude + the time)
06 Lunar Distance from the Sun (12 sets of observations of sun-moon distance and time)
23 Lunar Distance from the Sun (12 sets of observations of sun-moon distance and time)
23 Equal Altitudes of the Sun AM (3 observations of the sun’s altitude + the time)
23 Equal Altitudes of the Sun PM (3 observations of the sun’s altitude + the time)
Mar
25 Magnetic Declination with Sun (3 sets of the sun’s altitude and bearing + the time)
28 Equal Altitudes of the Sun AM (3 observations of the sun’s altitude + the time)
28 Equal Altitudes of the Sun PM (3 observations of the sun’s altitude + the time)
30 Equal Altitudes of the Sun AM (3 observations of the sun’s altitude + the time)
30 Equal Altitudes of the Sun PM (3 observations of the sun’s altitude + the time)
Apr
03 Equal Altitudes of the Sun AM (3 observations of the sun’s altitude + the time)
03 Equal Altitudes of the Sun AM (3 observations of the sun’s altitude + the time)
03 Lunar Distance from the Sun (7 sets of observations of sun-moon distance and time)

Funded in part by a grant from the National Park Service, Challenge-Cost Share Program