Fourteen Data Sets

Page 1 of 6

Lewis and Clark's Celestial Observations
at the mouth of the Kansas River
27-29 June 1804

On a rainy Monday afternoon, 14 May 1804, the "Detachment destined for the Expedition through the interior of the Continent of North America," embarked in a keelboat and two pirogues from the mouth of Wood River in the Illinois country, and headed up the Missouri River. William Clark, in sole command—until joined by Meriwether Lewis a few days later in St. Charles—logged a mere four miles before making camp for the night. Six weeks and about 350 river miles later, in the afternoon of Tuesday June 26, these three teams of boatmen, duly seasoned by six weeks of practical experience, arrived at the mouth of the Kansas River.

At various stops along the Missouri River's nearly east-west general course up to that point, Lewis and Clark, in accordance with President Thomas Jefferson's instructions, had taken four different types of celestial observations. These were:

  1. Equal Altitudes of the sun to check the chronometer (seven separate observations, each consisting of six sets of measurements of the sun's altitude, and the times of the observations);
  2. Meridian (noon) Altitude observation of the sun for latitude (twelve measurements);
  3. Bearing and altitude of the sun and the time (five observations to determine the magnetic declination1); and the
  4. Lunar Distance from the sun or a star for ultimately calculating longitude. These observations were made at four places and consisted of a total of ninety-four separate angular readings. The captains did not calculate longitudes themselves, but after returning home sent the data to Ferdinand Hassler, the mathematician then at West Point.

Altogether, these four types of observations added up to a total of at least 153 different angular measurements.

The Kansas Meets the Missouri

To see labels, point to the image.

Interactive map with an overlay of present geographic names

Library of Congress, Geography and Map Division, Washington, D.C.

The coordinates at the mouth of the Kansas River today, at the mid-point between its right and left banks where it touches the Missouri's right bank, is 39°06'56" N; 94°36'36" W. The point where the centers of the two rivers intersect is at about 39°07'N.

From maps that the captains had studied during the winter of 1803-04, and from information given them by knowledgeable people around St. Louis, the captains recognized the importance of this river junction. The Kansas River reached westward nearly to the Rocky Mountains and, along it, there lived Indians whose trade and commerce could provide impetus to cultural exchange, the benefits of which could be important both to those native nations and to the fledgling United States. Equally important, but from a geographic point of view, at this confluence the Missouri, having flowed generally toward the southeast for more than 650 miles from the vicinity of the Mandan Indian villages, abruptly turned eastward. Unmistakably, this was one of those "remarkable points" where Jefferson wished Lewis to take celestial observations in order to refine the sets of day-to-day travel measurements they had estimated.2

Celestial observations at this junction began shortly after 8 a.m. on June 27. The first observation would provide the data necessary to calculate the magnetic declination at this junction.1 One observer used the six-inch diameter magnetic compass (circumferentor) and took the sun's bearing while another observer measured the sun's altitude using the sextant and an artificial horizon. A third person recorded the chronometer time for each set of bearings and altitudes.

Moments after completing the observations for magnetic declination, the captains began taking the a.m. set of observations of Equal Altitudes of the sun. Following the p.m. set of Equal Altitudes observations, they could make calculations that would show how fast or slow their chronometer had been at noon compared with Local Time. Between the a.m. and p.m. sets of observations, Lewis took an observation of the sun's noon altitude with the octant and artificial horizon to determine the latitudeof the mouth of Kansas River.

Observations continued until late at night on the 27th, and more followed the next day. The captains took their last celestial observations at this junction in the afternoon of 29 June just before continuing upriver. These are the fourteen sets of observations they took:


Date   Type of Observation

27       Magnetic Declination with the Sun
27       Equal Altitudes of the Sun, a.m.
27       Meridian Altitude of the Sun
27       Equal Altitudes of the Sun, p.m.
27       Magnetic Declination with the Sun, p.m. #1
27       Magnetic Declination with the Sun, p.m. #2
27       Magnetic Declination with Polaris
28       Equal Altitudes of the Sun, a.m.
28       Meridian Altitude of the Sun
28       Equal Altitudes of the Sun, p.m.
29       Lunar distance from the Sun observations 1—6 (8 sets of distance and time for each)
29       Equal Altitudes of the Sun, a.m.
29       Meridian Altitude of the Sun
29       Equal Altitudes of the Sun, p.m.


1. Magnetic declination, also called variation of the needle, is the difference in degrees between Magnetic North, which is the direction the needle of their magnetic compass pointed, and True North. The magnetic declination calculated from this and the other magnetic observations at this junction, together with those from other magnetic observations made along the expedition's route, would allow the bearings in the captains' river survey to be corrected from Magnetic North to True North.

2. Jefferson had instructed Lewis to base their estimates of distances traveled on waterways "by the compass the log-line & by time."Lewis listed "Log line reel & log ship" among the equipment he purchased for the trip, but nowhere in the journals is their use confirmed. Donald Jackson, ed., Letters if the Lewis and Clark Expedition with Related Documents, 1783-1854, 2nd ed., 2 vols. (Urbana: University of Illinois Press, 1978), 1:61-62, 96.

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