Arrival at the Forks

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Celestial Observations at the
Three Forks of the Missouri
1805 July 28-29

From the Falls to the Forks

On July 15, 1805, after having spent a month in the area near the Great Falls of the Missouri, the canoes that had been portaged around the falls and the new ones built at Canoe Camp, finally headed upstream; they were laden to the gunnels. The next day the explorers passed a meander bend bristling with about forty willow lodges; an experienced eye determined that these lodges had been abandoned only 10 to 12 days earlier. Inasmuch as this willow-lodge camp was only about 12 miles on a direct line southwest of the expedition's White Bear Islands Camp it is possible that the people who camped here had observed the activities of the expedition at that camp or had seen its hunters. The captains concluded that these lodges had been built and occupied by Shoshone, but this is not necessarily so.

As the expedition continued upstream, the captains daily expected to meet the Shoshone. They found well-traveled trails, more abandoned lodges, even a smooth-bore trade musket, but no Shoshone. Perhaps the firing of the hunters' guns induced the cautious Shoshone to keep out of sight. The captains held council: If a small scouting party was sent ahead on foot it might be able to make contact with these people whose horses certainly would be needed when the expedition crossed to the Columbian waters.

Clark drew the assignment, and after the expedition passed the mouth of Dearborn River on July 18, he set out on foot with a few men to seek the Shoshone. Clark's party saw a horse and found a bow. They also found a multitude of prickly pear cactus spines, but no Shoshone.

At the Three Forks

Seven days later, on July 25, Clark and his party reached the Three Forks of the Missouri. Although the south fork (Madison) and the west fork (Jefferson) were essentially the same size, Clark—even before examining these rivers upstream—left a note for Lewis recommending the west fork. Not surprising. . . . Clark had had enough of traveling south; it was time to head west!

Meanders more than doubled the distance that the canoe party had to struggle upstream compared to Clark's overland route. Nevertheless, on the morning of July 27, just two days behind Clark, the canoe party, after traveling several miles through a narrow canyon, reached a valley that opened wide before them. Lewis, with no less geographic instinct than Clark, knew that he had arrived at the Three Forks of the Missouri. He halted the canoes opposite the mouth of the east fork (Gallatin) and, while most of the others began to eat their breakfast, he had himself ferried across to the east side of the river. He then hiked up along the east fork until he found a place where he could ascend a limestone promontory. From that vantage point he studied the country ahead and sketched the meanderings of the rivers. After he returned to the canoe party they all continued a short distance upstream to the junction of the Madison and the Jefferson. There Lewis found Clark's note and, following Clark's advice, directed the canoes up the Jefferson. About a mile farther up this river, on a wedge of land between it and the Madison, Lewis made camp, calling this island Camp Island. The expedition's camp here, commonly called Three Forks Camp, was at or near 45°55'44" N, 111°30'40" W as determined from the expedition's river survey, early and modern maps and from aerial photos.

About noon Clark rejoined the canoe party at this camp and gave Lewis an overview of the geography along both the southwest and west fork as far as he had scouted. That evening Lewis wrote: "beleiving this to be an essential point in the geography of this western part of the Continent I determined to remain at all events untill I obtained the necessary data for fixing it's latitude Longitude &c."

On July 28, while a fever-ridden Clark drew maps, Lewis began his celestial observations. By late night on July 29, Lewis had completed ten discrete celestial observations consisting of a total of forty-eight separate angular measurements. Two of these were for the sun's noon altitude; eleven were to check his chronometer, thirty were for longitude and five for magnetic declination.

The only calculations that Lewis made from the celestial observations he took at Three Forks Camp were for latitude, and he made these using an index error for his octant of 2°40'; it should have been 2°11'40". While at Fort Clatsop Lewis discovered that he had used the wrong index error for the octant during 1805 and recalculated some of the latitudes. The latitude of the Three Forks, however, was not among them.

Suppose, however, that Lewis had recalculated the latitude of the Three Forks Camp using the correct index error; and suppose further that Lewis, while at Fort Clatsop, had determined the chronometer's error on Local Time and its daily rate of loss from his Three Forks observations. He then could have calculated the longitude and magnetic declination from the observations he made there. What longitude and magnetic declination might those calculations have yielded if Lewis had used 111° W for his "dead-reckoned" longitude (the Three Forks are shown at about 110°50' W on the Lewis and Clark map of 1806; Moulton, Atlas, map 123).

At 111° W the time difference from Greenwich would be 7 hours and 24 minutes. Thus, for example, when Lewis made his meridian observation of the sun at Local Noon, the Greenwich Apparent Time would have been 19:24:00 or 7:24 p.m.

The following are the separate observations that the captains made at their camp near the Three Forks; all data are from Lewis:

Celestial Observations Made at Three Forks
28–29 July 1805
Date Type of Observation Number of Measurements and Description
28 Equal altitudes of the Sun AM 3, altitude of the sun + the time
28 Meridian (noon) altitude of the Sun 1, sun’s altitude at its highest daily point in the sky
28 Equal altitudes of the Sun PM 2, altitude of the sun + the time (clouds blocked no. 3)
29 Magnetic declination of the Sun 2, sun’s altitude and bearing + the time
29 Equal altitudes of the Sun AM 3, altitude of the sun + the time
29 Meridian (noon) altitude of the Sun 1, sun’s altitude at its highest daily point in the sky
29 Equal altitudes of the Sun PM 3, altitude of the sun + the time
29 Lunar distance from the Sun 1 10, angle between the moon and sun + the time
29 Lunar distance from the Sun 2 10, angle between the moom and sun + the time
29 Magnetic declination of the Sun 2, sun’s altitude and bearing + the time
29 Lunar distance from Antares 10, angle between the moon and Antares + the time
29 Magnetic declination of Polaris 1, bearing of Polaris + the time

The results of these observations are given in the following sequence:

  1. Latitude
  2. Local Time
  3. Longitude
  4. Magnetic Declination (Variation of the Needle)

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