Observations

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On their westward trek the captains dutifully took celestial observations at important locations. Now that they were headed back, the captains may have thought there was less need to acquire celestial data. After all, their 1806 route and campsites could be approximated from their 1805 celestial observations and the bearings and distances they took every day they traveled—no?

Perhaps that was the captains' thought, for despite the accident to the chronometer on the 17th and despite fair weather on May 22, 23 and 24, Lewis made no effort to check and see how this important timepiece was running—at least, there is no mention of any such effort in the journals.

A. May 25, Equal Altitudes of the Sun, Lunar Distance from the Sun

Clouds brought a light rainfall during the night but they cleared away a few hours after sunrise. Was it seeing the sun break through those parting clouds that finally prompted Lewis to make an Equal Altitudes observation of the sun. Whatever it was, the observation was long overdue and the captains put aside their "doctoring" duties that morning to make the observation. Fair weather prevailed the remainder of the day and Lewis was able to complete the afternoon part of the Equal Altitudes observation.

Lewis made no calculation to find the chronometer's error with respect to local time. He may be excused in this, however, because he did not have the 1806 Nautical Almanac with him and he may have thought that, without the data from it, his calculations would have been incorrect. This is true, but only to a minor extent. He could have used the data from the 1804 and 1805 Almanacs to project the 1806 data sufficiently close to make this calculation.

But . . . even without that calculation—in fact, even without the Equal Altitudes observation—Lewis should have been aware that the chronometer was losing time at a much higher rate than it ever had before. Lewis's standard procedure was to wind chronometer every day about noon, determining that time either from his pocket watch or by estimating it from the sun's position. In doing so, Lewis (or whoever was designated to wind the chronometer) must have noted the discrepancy between solar noon and the time that chronometer showed then. By May 25, Lewis ought to have known that the chronometer had lost about 2 hours since the 17th. Additionally, Lewis certainly must have looked the chronometer times recorded for his forenoon and afternoon observations and seen how "off-balanced" they were—5:40 AM and 1:19 PM; the average should have been 12 noon, not about 9:30 AM.

Just eight minutes after the captains finished the afternoon Equal Altitudes observation, they began taking an observation of the Lunar Distance from the sun. The moon, then at its First Quarter, stood pale-white against the blue sky, about 32° high, bearing to the southeast. The sun at that time was nearly due west, over the hills west of Clearwater River, and just a little lower than the moon. Lewis would have to hold the sextant nearly horizontally to make this observation; a difficult position made more difficult by the large angle of separation of the two bodies.

Less than fifteen minutes after they began the observation, the captains had finished it and were putting away the instruments. This was the only Lunar Distance observation the captains made during the 27 days the expedition stayed at this camp.

B. May 26—June 4, 1806

Although the weather was fair again after sunrise on May 26 and remained so all day, Lewis made no Equal Altitudes observation to find the chronometer's error on local time. This was a substantial omission on his part because from two observations a day apart it would have been a simple matter to find the chronometer's daily rate of loss. Then, with chronometer's error at noon and its daily rate of loss it was an equally simple matter to find the "true" time of the Lunar Distance observation taken on the 25th.

Additionally, because the time for an observation could be calculated from the latitude and the sun's altitude and declination, the time for the observation that was derived from Equal Altitudes observations, taken a day apart, could be used to check the calculated time and be averaged with it if necessary.

From May 27 the afternoon of June 4 the weather remained mostly cloudy with occasional rain. There were several afternoons, especially that of June 1, when the sky cleared, but the moon, during this period, was too far from the sun for observations. Clouds, even if intermittent, might have thwarted chances for taking a Lunar Distance observation with a star, but the real reason for not taking the distance from a star was that Lewis didn't have the 1806 Nautical Almanac, and he just wasn't that certain of his star identifications without the information in that almanac.

C. June 5, Equal Altitudes and Magnetic Declination

Finally, a clear morning after so many cloudy ones! Bartering for food would have to wait while the captains took another Equal Altitudes observation. There would be plenty of time for bartering between the forenoon and afternoon observations.

If (unlikely as it may be) Lewis hadn't already known about the chronometer's error with respect to local time, he certainly would have with this Equal Altitudes observation. The mid-time for the forenoon observation by the chronometer was about 3:09 AM and the mid-time for the afternoon observation was 11:41 AM! Even a quick mental calculation would reveal that the chronometer was about 4 hours and 35 minutes slow on Local Apparent Time.

Eleven minutes after the captains completed the afternoon Equal Altitudes observations, they began taking an observation of the sun to determine the Magnetic Declination (Variation of the Compass Needle). A little more than six minutes later they took a second observation. The chronometer at that time showed it to be noon, in reality, it was about 4:35 PM.

D. June 6, Equal Altitudes of the Sun

Afair morning. The captains took advantage of this opportunity to take a next-day Equal Altitudes observation and discover the chronometer's true rate of loss! After the forenoon observation clear skies continued, allowing the captains to complete the afternoon observations. Lewis left no record of any calculations made to determine the chronometer's error and rate, but it is likely that he made at least a rough calculation.

To make this calculation all he would have needed to do is: 1) take the average of the mid-time of the forenoon and afternoon observations for the 5th; 2) take the average of the mid-times for forenoon and afternoon observations for the 6th and 3) subtract the average time on the 5th from that of the 6th. Doing so, he would have discovered that the chronometer's daily rate of loss—at least for the interval between those two observations—would have been about 1h 17m!

E. June 9, Meridian Observation of the Sun

Why did Lewis wait until just one day before the expedition left this camp for Weippe Prairie to take a Meridian Observation for latitude? He certainly knew how critical it was to have a "good" latitude upon which to base almost all other astronomical calculations at a site.

Perhaps Lewis was uncertain about his octant, especially its index error. On February 16, 1806, Lewis recorded that there was a fracture on one of the octant's limbs and gave its index error when used in the "fore" method. On June 9 he finally noted the octant's index error when used in the "back" method, but only for that part of the limb "beyond" the fracture. Did Lewis know, for sure, what the index error was for that part of the limb "below" the fracture? Was it the same as he recorded on July 22, 1804? Did he trust using the octant? It seems, at least, that he was reluctant to do so.

In Lewis's defense it is well to remember that the previous October, he and Clark, despite being sick, took many celestial observations at a site which, by now, they knew to be only 20 or 30 miles north of this camp. Lewis may have figured that the latitude at this camp could be determined fairly closely by using both: 1) the latitude they had determined with the sextant for their canoe camp and 2) the courses and distances they recorded while traveling to this site from where they left Clearwater River on May 7 and would record on their way back to Weippe Prairie.

Lewis could not use the sextant to take the Meridian Altitude of the sun while at Camp Chopunnish because the sun's noon altitude exceeded 60°, but he made an important statement about it—also on June 9. On that date Lewis recorded that the sextant's index error was 6'15" "subtractive"1 instead of the 8'45" it had been previously. It should be noted that Lewis, on February 4, 1806, recorded that the sextant's index error was 5'45", but this value, most likely, is a transcription or copy error for 8'45".

F. Summary

Weather, lack of an 1806 Nautical Almanac, the need to stay atop a multitude of pressing daily situations and the logistics of command all interfered with Lewis's opportunities to take celestial observations while at Camp Chopunnish. Still . . . he seems to have missed a number of excellent opportunities to take needed observations.

Clark's "doctoring" may have taken a large amount of his time, and he did draw several maps from Nez Perce sources while at Camp Chopunnish, but he never made a detailed plot of the expedition's route from the Columbia to this camp, nor did he plot it on any of the detailed maps he had made previously. The only surviving map Clark made that shows the location of this camp is that in his field notebook (Moulton, Journals, 7:317). Using the map in his field notebook and projecting it onto the Lewis and Clark Map of 1806 gives the approximate coordinates of Camp Chopunnish as 46°15'N, 116°30'W. The coordinates of the camp, determined from map interpretation and information from Olin D. Wheeler in 1902,2 are 46°14'31"N, 116°02'04"W.

Considering the problems with the chronometer and the octant, and the changed index error for the sextant, can anything useful be made of the few celestial observations the captains made while at Camp Chopunnish?


1. That is, the sextant read too high by that amount. Thus, that value had to be subtracted from any observation made with the sextant. What caused this change of 2'30"? When?

2. Olin D. Wheeler, The Trail of Lewis and Clark, 1804-1806 (New York: G. P. Putnam's Sons, 1904), 268-71.

Funded in part by the Idaho Governor's Lewis and Clark Trail Committee