Clark's Latitude

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Where was Fortunate Camp?
by Robert N. Bergantino


Fortunate Camp, as Lewis and Clark described it, was at the forks of the Jefferson River, for them the farthest point of navigation on the Missouri. It was there that they laid up their canoes and, having had the good fortune to meet the Shoshone and barter for horses, proceeded on more than 300 miles overland, crossing several mountain ranges, until they reached the forks of the Clearwater River in present-day Idaho.

The names that the captains gave these streams – East Fork and West Fork Jefferson River – do not appear on modern maps. Where were the forks of the Jefferson River?

Several means are available to resolve the question. The earliest known "investigators" were several perceptive fur trappers in the late 1820s and early 1830s, such as Warren Ferris, John Work, and Alexander Ross. Their method of identifying features that Lewis and Clark described was simply to read the 1814 edition of the journals and match the captains' descriptions with the geography they saw for themselves. By the 1830s the Jefferson River above its junction with the present-day Big Hole River and Ruby River already was called the Beaverhead River, and fur trappers had given the east fork of the Beaverhead River its modern name, Red Rock River. The west fork was first called Horse Plains presumably because Lewis and Clark traded for horses there with the Shoshones; its name was changed around 1865 to avoid confusion with another Horse Plains in northwestern Montana.

But how certain can we be that the identifications made by the mountain men were correct?

The simplest and surest way to identify these forks is to review Clark's maps and compare them with modern maps. Another way is to plot the expedition's river and route-survey data and compare the plot with modern maps. Both methods have been used, and they confirm the general location of Fortunate Camp. Additionally, by using maps and aerial photographs that pre-date the reservoir now covering the campsite, it appears that the coordinates for Fortunate Camp most probably were at or near 44°59'36"N, 112°51'43"W (1927 datum). Nevertheless, an important cross-check of the camp's location comes from an evaluation of the data that Lewis and Clark obtained from their celestial observations for latitude and longitude.

Clark's "Latitude" for Fortunate Camp

In his journal entry for August 18, 1805, Clark recorded, "The Latd of the forks agreeable to observation is 43°30'43" North." This is a surprising entry not only because the latitude that Clark recorded is nearly one and a half degrees (100 miles) too far south of the actual site, but also because none of the other journals mention a latitude being obtained at Fortunate Camp before the 19th – one day after Clark had left on his reconnaissance of the Salmon River. Considering the intense level of activity that must have occurred after the captains were reunited on the 17th, and their parleys and trading with the Shoshone, it is most unlikely that either captain found time to make a meridian observation. But, supposing Clark did make an observation of the sun's altitude on the 17th (or on the 18th before he set out for his Salmon River exploration), wouldn't Lewis have mentioned it, recording at least the observed altitude of the sun? And later, when Lewis made his own observations and found Clark's latitude to be in error, couldn't he have made a comment about it similar to the one he made about Clark's observation on August 15th at Rattlesnake Cliffs?

Clark's entry for the 18th clearly states that the latitude of the forks came from an observation. But what if an observation was not actually made at Fortunate Camp. Suppose Clark estimated it, taking the distance he traveled south from Rattlesnake Cliffs, converting it to degrees, minutes and seconds and then subtracting that from the latitude he calculated for Rattlesnake Cliffs? Although calling the derived latitude an "observation" would clearly be stretching the meaning of the word, it is still a justifiable usage.

The latitude Clark had calculated for the Rattlesnake Cliffs, unfortunately, was too far south by about one degree (69 miles), as Lewis pointed out – probably after making his own observations at Fortunate Camp. If Clark's latitude for Rattlesnake Cliffs had been correct, he could have calculated the straight-line distance he had traveled south from the Rattlesnake Cliffs, converted that to degrees, minutes and seconds, and subtracted them from the latitude at Rattlesnake Cliffs. This means, however, that Clark would have needed to plot his river survey data before making the calculation, and it is unlikely that he had time to do so by the morning of the 18th. But supposing that the unlikely happened, and that Clark had indeed plotted his survey data, he would have found that the forks lay only 15Ω miles south of the Rattlesnake Cliffs. Converting this distance into latitude would have placed the forks only about 13'30" of latitude south of Rattlesnake Cliffs, not 30 minutes, and his calculated latitude for the forks of the Jefferson would have been about 43° 47' 18". Therefore it is highly improbable that the latitude Clark recorded in his journal for the 18th came from a calculation like that described above.

There is another possible source of Clark's erroneous latitude, but it involves some compounded errors plus the fact that the captains didn't always record events on the date that they happened. On Clark's map, east of the indicated site of Fortunate Camp is the notation: No. 43 Latd. 43°30'43" (the figure in minutes can be interpreted variously as 30', 35', or 38', but most likely it is 30).1 A line through it indicates that Clark wished to delete it. Underneath the deleted latitude he wrote: 44°35'28.1", which is the average of the four observations that Lewis made between August 19th and August 21st.

Based on the drafting style and the notations, it seems reasonable to suggest that Clark plotted Atlas Map 65 (from the three forks of the Missouri to three forks of the Jefferson), Atlas Map 66 (from the three forks of the Jefferson to Fortunate Camp) and Atlas Map 67 (at least that part to the divide) while waiting at Fish Weir Camp for Lewis. On August 27th, Clark received a message from Lewis to meet him at the "upper" Shoshone camp. In view of the language barrier, Lewis must have written a note for the young Shoshone messenger to deliver. Did that note contain the latitude of Fortunate Camp from one of Lewis's observations? Lewis's observation for the 20th yielded a latitude of 44°39'43". The 9 of 39' easily could have been misread for a 0, making it 30', but the 4 of 44 would be difficult to mistake for a 3. Even if the minutes of latitude were misread from a poorly written numeral, it still takes another condition to produce the change in the degrees. One answer might be that Clark, thinking of his own observation of August 15, with its latitude of nearly 44° north, may have thought that Lewis meant 43° and changed it. Not knowing of his error, he then may have added the 30'43", thus recording 43°30'43" on his map and in his journal, but back-dating the journal entry to the 18th because that's where he was at the time. This is a difficult interpretation to defend, but it could have happened.

When Lewis and Clark rejoined on August 29th Lewis corrected Clark's understanding of the latitude, and Clark dutifully changed it on his map. Why didn't he also change it in his journal? It is likely that the journals already had been put away for safe-keeping in preparation for the resumption of the journey, and Clark forgot to change it later. At any rate, despite the uncorrected latitude in Clark's journal, Clark's map bears Lewis's calculated average latitude of Fortunate Camp from four observations.

Now, what about those observations?

Lewis's Observations at Fortunate Camp

On August 19th, Lewis dutifully made and recorded a series of celestial observations of the sun (sun) and moon (moon) for longitude and a meridian2 altitude observation of the sun's lower limb (L.L.) at noon for latitude:

today I observed time and distance of the sun's and moon's nearest limbs with sextant3 sun East. it being the Point of Observation No. 43

Observed Meridian Altitude of sun's L.L. with Octant4
by the back observation 69°15'00"
Latitude deduced from this observation. N. 44°37'57.4"

And, on the 20th, LewisÖ

Öobserved time and distance of the sun's. and moon's nearest Limbs with Sextant. sun East.

Longitude deduced from this observation West of Greenwich [blank – not calculated]
Latitude N. deduced from the Hor angle5 of the P.M. Observation of sun's center 44° 33' 50.5"

Observed Equal Altitudes6 with Sextant of the Sun.

Observed Meridian Altitude of sun's. L.L. with Octant by the back observation 70° ' "
Latitude deduced from this observation N. 44° 39' 43"

Finally, on August 21st, LewisÖ

Öobserved Equal Altitudes of the sunwith Sextant.

Also observed Meridian Altd of sun's L.L. with Octant by the back observation 72°00'00"

Latitude deduced from this observation North. 44°30'21.7"

Mean Latitude of the Forks of Jefferson's river deduced from three observations of the Meridian Altd of the sun's L.L. with Octant, and one calculation by means of the hor: angle of the sun's center in the P.M. observation for equal Altitudes on the 20th Instant N. 44°35'28.1"


1. The "No. 43" stands for "Point of Celestial Observation Number 43," their forty-third such point of observation since leaving Fort Mandan.

2. On the earth a meridian is any line that passes through the north and south poles and is perpendicular to the equator. The half of the meridian circle that is on the observer's side of the globe is called the upper meridian. A meridian altitude is a celestial observation taken when the object observed (usually the sun) is on the observer's upper meridian and thus at the highest point that it reaches above the plane of the equator that day.

3. Lewis's sextant was in the form of an arc encompassing one-sixth of a circle, hence the name sextant. Because a sextant uses an index mirror to reflect objects to a second mirror and then into the eyepiece, the angles (degrees) marked on its graduated arc are double the actual. That is, a sextant actually could measure an angle of 120°, but this compresses the angle (degree) marked on the graduated arc very close together even though the radius of that arc on Lewis's sextant was ten inches. Each of the degree marks, in turn, was equally divided into four parts of 15' each. The instrument also had a vernier (Lewis called it a nonius), which allowed him to interpolate the angle to 15". Finally, there was a micrometer screw that allowed him a further interpolation down to 7-1/2". To read this angle, even with the attached magnifying glass (Lewis called it a microscope) was no easy task, especially when it had to be done in a short time and under difficult conditions. Errors sometimes did occur. Like all precise scientific instruments, a sextant could become out of adjustment, especially when it had been banged about on a storm-tossed ship or in a canoe on the Missouri or Jefferson River. Lewis recorded that his sextant had an index error of 8' 45". What he actually meant was that the sextant read too high by 8' 45" and, to correct for this error the person doing the calculations had to subtract 8' 45" from the angle measured. Usually an instrument's index error is corrected before any other calculations are made.

A vernier is a scale for measuring smaller subdivisions of a scale or circle than can be done by estimation; Lewis called this device a nonius, but that term is obsolete now. On a sextant or octant the vernier measures an of arc of about 20° and is so graduated as to have one line less (or more) than the equivalent distance or angle on the main scale or arc. Lewis said that with the vernier he could read his sextant to 15 seconds (15") of arc. Therefore, its vernier contained 60 equally spaced lines that encompassed an arc of 14° 45' or (less likely) 15° 15'; that is, 60 lines on the vernier spanned the same arc as 59 (or 61) spaces of 15' each on the main scale. The 60 lines on the vernier were blocked into groups of four. The first line was marked with a symbol to indicate the 0 mark or starting point. The 20th, 40th and 60th lines were marked 5, 10 and 15, respectively.

4. The octant also was a device to measure the angle between two objects. Although its graduated arc spanned 45° (an eighth of a circle, hence octant) it could measure an angle of 90° (when used in the fore sight method) because it utilized the principle of reflection of a ray of light from two mirrors. Lewis's octant, with a radius of 14 inches, was fitted with an additional peep sight and mirror on the "back" limb. Using the octant in the "back sight method" Lewis, with the help of geometric principles, could thus "measure" an angle of 180°.

5. It appears that Lewis uses the abbreviation "Hor" to mean "horizon." Combining it with the geometric symbol for angle creates the expression "horizon angle." It refers to the altitude of the sun above the horizon at the time of the PM observation for Equal Altitudes.

6. An equal altitudes observation is a two-part process. The first set of measurements is taken when a celestial object is east of the observer's meridian and the second set is taken when the object is west of the observer's meridian. The equal altitudes observation provides information from which calculate several types of navigational information may be calculated. Lewis and Clark, however, used the observation exclusively to check the time of their chronometer.

The procedure for making the equal altitudes was quite simple. On land the instruments required were a sextant, an artificial horizon (see next page, Note 7) and a chronometer. The observer would pick a convenient time in the morning when the sun was high enough above the horizon so that effects of refraction were reduced, then would set out the artificial horizon. Using a sextant, the observer, while looking at the sun reflected from the artificial horizon, would swing the index arm until the upper limb (tangent) of the sun's reflection from the index mirror touched the upper limb reflected from the artificial horizon. At that instant the observer would call "Time!" (or words to that effect), and the person watching the chronometer would record the time. The observer locked the setting on the sextant and continued to observe until the two images of the sun were superimposed (the center observation) and call "Time!" Finally, when the lower limbs of the two reflected images matched, the observer again would call "Time!" and the time was recorded. The sextant then was put away carefully until the afternoon. Knowing approximately how many hours before noon the morning observation had been taken, the observer waited until about the same time had elapsed since noon, then got out the instruments again. The sequence of limbs making contact, however, was reversed. First the observer would wait until the lower limbs matched, then the center, then the upper limbs. At each contact the times as shown by the chronometer were dutifully recorded. The equal altitudes observation was complete except for the calculations. If the observations were properly made the calculations would reveal, usually to within about one second or so, how fast or slow the chronometer was with respect to the time it should have showed at that location.

This page is supported in part by a grant from the National Park Service CCSP Program.