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Observations of Lunar Distance for Longitude

While at Clearwater Canoe Camp, the captains took four separate observations of Lunar Distance for longitude, one with the sun and three with stars. Only at their principal camps did the captains ever take that many observations for longitude. It's not surprising, though, for they had traveled several hundred miles without an observation for longitude. Not only that, this junction is where a west-bound traveler could resume travel by water, and where an east-bound traveler would begin the difficult overland trip. It was essential that they determine the longitude of this camp.

September 30

Less than 15 minutes after taking the afternoon set of Equal Altitude observations, the captains began taking observations of the angular distance between the west edge of the moon, then just past first Quarter, and the east edge of the sun. A simple average of the times gives 4h 28m 55s PM; the average of the distances (after correcting for an obvious error of 10 arc minutes on the 10th observation is 91°59'23".

Although the chronometer's error at the time of the observation is not known owing to its uncertain rate of loss, the Equal Altitudes observation showed that it was 29m 33.7s slow on Local Apparent Time at noon. If one uses the same rate of loss that can be established from the Equal Altitude observations taken on October 3 and 6, the chronometer was losing 15 seconds in 24 hours. If true, in the nearly 4 hours between noon and the Lunar Distance observation it would have lost an additional 2.5 seconds, making the Local Apparent Time of the mean of the observation 3:59:18 PM.

By taking 1) the chronometer error at noon on the 30th, 2) the calculated time of the PM Equal Altitudes obervation, and by 3) projecting this trend to the time that the chronometer showed for the Lunar observation on the 30th, one derives a time of 3:59:24 PM. Averaging this with 3:59:18 gives 3:59:21. The equivalent Greenwich Apparent Time is 11:44:40 PM.

October 3

The captains measured the angular distance between the west (far) edge of the moon and the star Hamal (Arietis).1 There was a gap of more than nine minutes betrween the first two observations, so the first data set should not be included in the averages. A plot of the data sets shows that some of them diverge widely from the smooth line that should result from the observations. In addition, there is an error in the distance of 77°59'00" for the sixth data set; that distance probably should be 78°00'00". Nevertheless, using 78°00'00" for the sixth data set, the average distance is 78°00'10.0"; the average time is 8:30:10 PM.

October 6

On this date the captains made two Lunar Distance observations. The first was with the star Altair (Aquilae)2 and the other with a star they identified as Aldebaran (Tauri).3 For their first observation the average chronometer time of the 10 sets of observations was 8:36:55 PM and the average angular distance was 58° 58' 09.0". The first three and last two data sets, however, seem to follow another trend. Using the five most consistent data sets gives an average chronometer time of 8:38:37 and the average angular distance 58° 58' 27".

For the second Lunar Distance observation on the 6th (from eight data sets) the average chronometer time was 9:16:42.4 PM. The angular distances, however, contain several problems (probably transcribing errors) and at least one error in distance by 1 arc minute. The plot, however, reveals that the first two and last three data are consistent enough to provide a useable average time and distance of 9:17:09 PM and 65° 25' 51". The 1805 Nautical Almanac, however, shows that at the time of the observation, the moon and Aldebaran should have been about 75° apart, not 65°. Additionally, Aldebaran, at the time of the observation, would not yet have risen above the tops of the adjacent mountains. It seems that the captains misidentified the star they observed, rendering their observation unuseable.


Canoe Camp and Camp Chopunnish

Clark's 1806 map showing the location of Canoe Camp and Camp Chopunnish on the Clearwater River

The captains never calculated the longitude, nor did Ferdinand Rudolph Hassler, to whom some of the Corps' data was given after the expedition's return. The longitude of Clearwater Canoe Camp, as determined from present geographic features, places it at 116°19'46" West. On the map that Clark created at Fort Clatsop in February of 1806 and completed sometime after the expedition, the camp is located at about 116°40' West, as is shown in this redrafting of the section of the 1806 map from Travelers' Rest to River la Page (John Day River). (Moulton's Atlas Map 123).

Clark's original map was made at a scale of about 1:3,000,000 (1 inch = approximately 50 miles). His "hybrid" latitude-longitude reticule has been replaced with one based on the Equidistant Conic Projection, and the sites of Clearwater Canoe Camp and Camp Chopunnish have been added.


1. The Greek symbol for the letter Alpha () identifies Hamal as the brightest star in the constellation Aries (The Ram).

2. Altair—Arabic for "flying eagle"—stands out brightly in the constellation Aquila (The Eagle).

3. Aldebaran, meaning "follower," dominates its constellation, Taurus—the Bull—near the bull's left eye. It is in fact the 13th brightest star in our sky.

4. At their camp near Potlatch River ("Colters Creek") on May 6, 1806, Lewis and Clark met three men of the "Skees-so-mish" people (Skitswish, later Coeur d'Alene), who told them of "a large lake in the mountains" near which they lived. Neither the source nor the meaning of "Wayton" is known.

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