Eleven days of starving, freezing and struggling over the Lolo Trail had worn the Lewis and Clark Expedition to exhaustion. Though well met and given food by the Nez Perce at Weippe Prairie, the food so generously given did not agree with the travelers and many of them became sick. Barely able to travel, they continued a few days west from Weippe Prairie and arrived at the junction of two rivers. Here the captains made a camp and set the men to fell trees. From these they began to form their canoes, a task made easier when the Nez Perce showed them how to "burn out" the logs.
The captains clearly understood the need to obtain the latitude and longitude of this key place where travelers, once again, could take to river travel. As soon as they were well enough to make celestial observations, they did so. During their sojourn at this camp they made: 1) three sets of Equal Altitudes observation to determine the chronometer's error at noon and its rate of loss; 2) two meridian altitude observations of the sun for latitude; 3) two observations for magnetic declination (variation of the compass); and 4) four observations of lunar distance for longitude.
No archaeological evidence has yet surfaced to identify the exact site of Clearwater Canoe Campsite, and the junction of the Clearwater River and North Fork Clearwater River certainly has changed some since 1805. Nevertheless, the coordinates of that camp would have been near 46°30'05" North latitude, 116°19'46" West longitude (Lewis and Clark's map of 1806 – Atlas Map 123 – shows this junction at about 46°43'N, 116°40'W); the river bank there is at about 900 feet above sea level.
Celestial Observations from Fortunate Camp
to Clearwater Canoe Camp
After concealing their canoes in a pond near Fortunate Camp on the east side of the Continental Divide, the expedition traveled more than 330 miles overland to their canoe camp on the Clearwater River. Clark's survey between the two points (not including his exploration of the Salmon River) totals 400 miles. In all this distance the captains recorded taking celestial observations at just three places.
The first place was at the second or "upper" Shoshone camp on August 29, 1805. On that date Clark recorded that they took observations, but gave no latitude, longitude, or observational data.
The second place was in the northern part of the Bitterroot Valley, along the Indian road, about eight miles south of Travelers' Rest; the date was September 9. Rain and cloudy weather had prevailed since the expedition took leave of the Salish on September 6, but on the ninth the sky had cleared. Lewis, probably learning from Toby that they were about to start heading west over Lolo Trail, took advantage of the opportunity to make a meridian (noon) observation of the sun. Clark, on his map of their route (Atlas Map 68) misplaced this point of observation, locating it at the expedition's camp for September 4 and 5.
The third place that the captains took celestial observations was at Travelers' Rest. During the evening of September 9 the sky remained clear, the moon was just past full; it was a ready-made occasion for Lewis to make a Lunar Distance observation. This he did, but after that observation he realized that the moon's western limb already was imperfect and noted that the observation could not be depended on. The moon's western limb, indeed, was imperfect, but (unknown to Lewis) the point on the moon's limb to which he brought the star's image into contact was still whole, and the observation could have provided valuable data if the captains had taken an Equal Altitudes on the 10th to determine the chronometer's error.
Strangely, although the weather was again fair, the captains took no Lunar Distance observation on the 10th, either. Lewis did take a meridian observation of the sun, but recorded no altitude observed nor did he or Clark record taking an Equal Altitudes observation. A possible explanation might be that Lewis needed to use the sextant to make the noon observation because one of the octant's limbs already had been broken1 (see Lewis, February 16, 1806), and he did not trust using the octant until he had checked its error.2
1. "By several trials made today in order to adjust my Octant and ascertain her error in the direct observation, I found that it was 2° 1' 45" + or additive beyond the fracture; this error was ascertained by a comparison with my sextant the error of which had been previously ascertained. the error of Octant in the direct observation on the broken limb next to 0 or 55° 20' inclusive is 2° additive only." Lewis, February 16, 1806.
2. Lewis, when taking an Equal Altitudes observation, always clamped the sextant's index arm at the angle he had measured during the AM observation. To ensure that the angle he measured in the afternoon was exactly that measured in the forenoon, he left the index arm clamped until he had completed the PM observation. Consequently, if he took an Equal Altitudes observation he could not use the sextant for a Meridian Observation of the sun.
Funded in part by the Idaho Governor's Lewis and Clark Trail Committee